Tools and Functions of Reconfigurable Colloidal Assembly - American

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Invited Feature Article

Tools and functions of reconfigurable colloidal assembly Michael J Solomon Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03748 • Publication Date (Web): 03 Feb 2018 Downloaded from http://pubs.acs.org on February 4, 2018

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Tools and functions of reconfigurable colloidal assembly Michael J. Solomon, University of Michigan Ann Arbor MI 48109 ([email protected]) Abstract We review work in reconfigurable colloidal assembly, a field in which rapid, back-and-forth transitions between the equilibrium states of colloidal self-assembly are accomplished by dynamic manipulation of the size, shape, and interaction potential of colloids, as well as the magnitude and direction of the fields applied to them. It is distinguished from the study of colloidal phase transitions by the centrality of thermodynamic variables and colloidal properties that are time switchable; by the applicability of these changes to generate transitions in assembled colloids that may be spatially localized; and by its incorporation of the effects of generalized potentials due to e.g. applied electric and magnetic fields. By drawing upon current progress in the field, we propose a matrix classification of reconfigurable colloidal systems based on the tool used and function performed by reconfiguration. The classification distinguishes between the multiple means by which reconfigurable assembly can be accomplished (i.e. the tools of reconfiguration) and the different kinds of structural transitions that can be achieved by it (i.e. the functions of reconfiguration). In the first case, the tools of reconfiguration can be broadly classed as: (i) those that control the colloidal contribution to the system entropy – as through volumetric and/or shape changes of the particles; (ii) those that control the internal energy of the colloids – as through manipulation of colloidal interaction potentials; and (iii) those that control the spatially resolved potential energy that is imposed on the colloids – as through the introduction of field-induced phoretic mechanisms that yield colloidal displacement and accumulation. In the second case, the functions of reconfiguration include reversible: (i) transformation between different phases – including fluid, cluster, gel, and crystal structures; (ii) manipulation of the spacing between colloids in crystals and clusters; and (iii) translation, rotation, or shape-change of finite-size objects self-assembled from colloids.

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With this classification in hand, we correlate the current limits on the spatiotemporal scales for reconfigurable colloidal assembly, and identify a set of future research challenges.

Introduction Colloidal particles self-assemble into ordered and disordered structures as a consequence of Brownian motion and interparticle interactions 1,2. If gelation and glass transitions are avoided or suppressed, colloidal systems explore the ensemble of available configurations; in these situations, free energy is minimized and thermodynamic equilibrium is achieved. Excluded volume interactions, which depend on shape, yield crystalline phases with complex symmetry in both two and three dimensions 3. Patchy interactions – those localized to specific volumetric or surface regions of the colloid – can produce new kinds of structures, including open lattices 4. Combining shape anisotropy with attractive interactions produces additional unit cell symmetries 5. Finally, programmable bonding – as mediated for example by DNA – is an effective tool for structural design 6. The explosion in phases predicted by theory and simulation 7–9 – with many now being identified in experiments – is remarkable. The structures into which colloidal particles assemble are useful for new materials. Crystalline arrays of dielectric structures produce structural color of variable wavelength and polarization state 10,11; metallodielectric structures control the strength and frequency of plasmonic responses 12. Certain structures such as diamond 13, gyroid 14, and the icosahedral quasicrystal 15 can yield a photonic band gap. Ordered arrays of colloids can be processed to produce materials with controlled porosity that are useful for conductive electrodes 16 and membranes 17. Finally, colloids can be assembled into arrays for materials with mechanical properties useful for actuation 18 and acoustic control 19. Compared to other constituents available for self-assembly at similar scales – such as block copolymers, DNA, surfactants, and proteins – colloids display advantages that stem from the range of different compositions and


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material functionalities that can be incorporated either into the volume or onto the surface of the colloids. Control of volumetric composition yields a versatile range of particle dielectric, conductive, magnetic, and elastic properties. Control of surface functionality generates tunable interactions that are well modeled by statistical thermodynamics. These positives are balanced against challenges such as the limited number of crystal structures that have been assembled from colloids to date, the slow kinetics of self-assembly because colloidal diffusion is retarded relative to molecules, and the tendency for self-assembled structures to be defect prone. Materials applications of self-assembled colloids can require crystalline structures that are either static or dynamic. Static structures – such as an iridescent coating or a porous electrode – are first fabricated by self-assembly; a subsequent step – such as drying, sintering, or solvent photo-polymerization – permanently fixes the structure in the state needed for its application. By comparison, dynamic structures are not fixed; critical features such as building block symmetry, crystal structure, and/or lattice spacing can be controlled in time. Dynamic structures are instrumental to applications – e.g. sensing, transduction, and actuation – in which a time-dependent optical, structural, or mechanical response is sought. The purpose of this review is to examine the means by which these dynamic colloidal structures have been achieved, and the functions that such dynamic control yields. This kind of dynamism is called reconfigurability. Reconfigurability – as we define more specifically in this review – can be combined with equilibrium self-assembly in colloidal systems to generate reversible transitions between colloidal states with different functions. There is broad interest in generating this kind of dynamic control of colloidal structures; methods used to produce it have varied considerably to date. Simultaneously, usage of the term reconfigurable colloidal assembly has grown. Here we classify the distinctive features of reconfigurable colloidal assembly and identify a set of research challenges that can motivate future work in this field. The conception of reconfigurable colloidal assembly proposed here


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combines earlier descriptions 20–22 under a single theme described as the reversible transition of a colloidal system between states with different function, as generated by any of a set of tools that modulate thermodynamic variables and colloidal properties of the system in time. The organization of this paper is the following. After defining reconfigurable colloidal assembly, we identify the tools that have been used to achieve reconfiguration as well as the specific kinds of reconfigurable transitions that have been produced. The union of these tools and functions yields a matrix classification of work in the field. Next we approach reconfigurable colloidal assembly from the point of view of the characteristic time scales of the dynamic response. We identify current limits, which depend on the tool used and function performed. In the final two sections, we identify application areas in which the tools and functions of reconfigurable colloidal assembly have been applied and discuss avenues for future work in the field.

Reconfigurable Colloidal Assembly Defined The following features characterize reconfigurable colloidal assembly: 1) It involves reversible, back and forth transitions between two different equilibrium states of a colloidal system by the manipulation in time of thermodynamic variables or colloidal properties. The thermodynamic variables include the typical ones such as temperature and (osmotic) pressure, but also generalized variables such as applied electric and magnetic fields. The colloidal properties include particle size and shape as well as the potential interactions between the colloids. The focus on reversible, back and forth, transitions distinguishes reconfiguration from the broader study of phase transitions and self-assembly. Colloids – unlike other materials such as small molecules, surfactants and polymers – often undergo phase transitions that are, in practice, not reversible, even though they occur at equilibrium. For


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example, adding salt to a colloidal crystal may change its structure from bcc to fcc 23 by modulating the Debye length of the screened Coulombic repulsion; however, switching back and forth between the two phases is stymied by the irreversibility of the salt addition. Because thermodynamic variables and particle properties cannot commonly be reversibly manipulated to change colloidal assembly structure and function, advances in synthesis (e.g. shape mutable colloids), interactions (e.g. colloids with temperature-dependent potentials due to binding of complementary DNA strands), and applied fields (e.g. tunable osmotic pressure as generated by electrophoretic deposition) have been applied to reconfigure the structure of colloidal assemblies. 2) It is defined operationally: reconfigurable colloidal assembly refers to a functional change in microstructure or property of the assembly. An example functional change is a phase transition of the colloids – such as a change from fluid to crystal or between two different crystal structures. A change in lattice spacing of a crystal, which would yield a change in iridescent structural color, is also a kind of reconfiguration. Finally, changing the shape or position of an object assembled from colloids is reconfiguration; such shape morphing operations are foundational to microscale force generation, locomotion, and capture. The emphasis on functional change means that reconfiguration does not include changes in colloidal microstructure that are small, such as, for example, as introduced by fluctuations around a particular state point. The functional change in the colloids can be either with respect to a macroscopic property – such as an elastic modulus – or to a microscopic variable – such as an order parameter of a crystal phase. 3) Reconfigurable colloidal assembly includes structural transitions that are in the thermodynamic limit, as well as those which are spatially localized and involving only small numbers of particles.


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Large numbers of colloids can undergo reconfiguration, as in the case of a bulk phase transition that occurs in the thermodynamic limit. In addition, the self-assembly of clusters, thin layers and even objects of colloidal particles is an aim of the field, because of the usefulness of these small-number ensembles for microscale function. Although such localized regions may contain too few colloids to be in the thermodynamic limit, reconfiguration between states of these ensembles may still be described by a change in order parameter. The terminology of reconfigurable colloidal assembly thus identifies a recent research direction in colloids that seeks to develop dynamic, reversible function in small collections of particles; these new functions complement the many already existing uses for colloids based on bulk self-assembly. The literature has applied a broad range of tools to accomplish colloidal reconfiguration. As illustrated in the columns of Figure 1, these tools manipulate the colloidal entropy, the interaction potential among the colloids, or the potential energy landscape that the colloids experience in the system. The tools have then been applied to perform a variety of functions. As shown in the rows of Figure 1, functions include generating structural transitions between phases, changing the configuration of particles within particular phases, and manipulating the size, shape and location of objects that are themselves self-assembled from colloids. Classifying these instances based on the tool used and the function performed organizes efforts in the field in way that can assist its future development.

Tools for Reconfiguration of Self-assembled Colloidal Structures The use of the term reconfigurable with reference to colloidal assembly is recent 20; however, interest in switching the structural characteristics of phases generated by colloidal self-assembly is much older 24. At its most basic level, configurational change in colloidal systems is a kinetic process that can occur through mechanisms such as spinodal decomposition or nucleation and growth as well as transport processes such as diffusion and


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convection 25. Such kinetics have been extensively studied in colloidal systems 1. As discussed previously, reconfigurable self-assembly of colloids is distinguished from the study of phase transitions and their kinetics by the back-and-forth reversibility that reconfigurability achieves. That is, in addition to stepping from one thermodynamic state point to another, the system can then step back again. To generate the back-and-forth step, the thermodynamic variable or colloidal property that drives the change must itself be free to vary in time. Thus, equilibrium phase transitions of colloids are not necessarily instances of reconfigurable colloidal assembly, because many transitions of this kind proceed only in one direction. The thermodynamic variables or colloidal properties that drive reconfiguration can be classed as tools that affect colloidal entropy, potential interactions, or phoretic motion; each of these tools is discussed in the sections following.

Entropy Controlled Reconfiguration To illustrate the distinction between colloidal reconfiguration and phase transitions, consider colloidal particles confined in a fixed volume, and initially prepared as a homogeneous liquid phase at equilibrium. If the state variables are fixed, there is no thermodynamic driving force to support kinetic processes; the system is already at its free energy minimum. However, at fixed temperature and number density, the system may still transform into a different phase – if the colloidal properties that control its thermodynamic state are manipulated. For instance, a change in the volume and/or shape of the particle could drive the system into a different phase. A volumetric change of the particle could be induced, for example, by swelling a polymer sphere through a change in the dielectric constant of the solvent in which the particle is dispersed 26. A change in particle shape – as accomplished from a cube to a sphere 27 – could also be induced. These changes in the colloidal building block size and shape change the number of configurations available to the system; the system’s phase boundaries shift in response to this


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entropy change. If the shift places the number density at which the system was prepared in a new region of the phase diagram, a phase transition will occur. If the change in particle shape or volume is reversible, the system can transform back and forth between these two states. The capability to make this back-and-forth transition is the critical feature of reconfigurable colloidal assembly; in this case, the reconfigurability arises through manipulation of the colloidal contribution to the system entropy. Colloidal volumetric change by reversible swelling/deswelling was an early demonstration of entropy-driven reconfiguration 24,28; this expansion/contraction yields control of the lattice spacing of close-packed colloidal crystals. In a step up in complexity, shape-shifting colloids 29,30 increase the practicality of reconfiguring between two different crystal phases 22. Shape-shifting can be performed by differential swelling of dissimilar materials incorporated into a single colloidal building block or by reconfiguration of bond distances between the (atom-like) constituents of colloidal molecules 31. The effect of shape-shifting on self-assembly has been demonstrated by simulation for cases such as the reconfiguration of spheres into pentagonal prisms and of rods into rhombi 32. In each case, a reversible change in building block size and shape yields changes in self-assembly through their effect on the entropic contribution to the free energy.

Interaction Potential Controlled Reconfiguration The potential interactions between colloids represent a tool for reconfiguration when its strength, magnitude, functionality can be reversibly controlled. While reconfiguration of colloid volume controls the system’s free energy through its effect on entropy, reconfiguration of potential interactions exerts control through its effect on the internal energy contribution to colloidal free energy. For example, charged colloids interact through a screened Coulombic potential that is parameterized by the Bjerrum length, the surface potential, and the Debye


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length 25. Phase boundaries between fluid and crystal phases depend sensitively on these three parameters 33. Each can be controlled by manipulation of environmental and solvent conditions. The Bjerrum length depends on the solvent dielectric constant, the surface potential is often a function of solution pH, and the Debye length depends on the electrolyte concentration in solution. In reconfigurable colloidal assembly these shifts in potential interactions must be both controllable and reversible. Solvent conditions such as dielectric constant, pH, and electrolyte concentration are reversed with difficulty, typically by means of a slow step such as solvent exchange or membrane permeation 34. It is more common to generate single-instance step changes in such properties; however, such step changes are not reconfigurable. Instead, reconfiguration of colloidal interaction potentials by reversible manipulation of environmental or solvent conditions has been accomplished by, e.g. manipulation of the depletion interaction. The depletion interaction is generated by small non-adsorbing particles or polymers; their addition affects the system free volume to produce an effective attraction between larger colloids 35,36. The strength and range of the attraction is a function of the number density of the depletant and its size relative to the colloid. Although the depletion interaction arises due to the effect of colloidal configurations on the depletant entropy, its role in colloidal self-assembly is typically incorporated through the effect of the depletant size and number density on an effective potential of mean force between colloids 37. That is, the physical properties of the depletant control the interaction potential of the colloids. Because certain polymers, such as poly-N-isopropylacrylamide (poly-NIPAM), have strongly temperature dependent size, these thermoreversible polymers can be used to reconfigure the depletion interaction potential between colloids in ways that impact their selfassembly 38–41. These polymers are often the same as those used to generate particle volumetric changes in entropy-driven reconfiguration, as discussed in the previous section. In


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this case, however, it is the interaction potential between colloids that is affected by their size change, rather than the colloid itself. Surfactant micelles can also function as depletants 42; their formation can be controlled by temperature. Temperature can also affect the potential interactions between colloids through other mechanisms – for example, by changing the conformation of grafted layers that stabilize colloids 43,44, by controlling the adsorption of thin, small-molecule layers that mediate capillary bridging between particles 45, and by affecting preferential wetting in critical mixtures 46. Bonds between colloidal pairs can be produced by the hybridization of complementary DNA strands that have been grafted to each colloidal surface 6. These programmable, highly specific bonds can also produce reconfigurable potential interactions between colloids in a number of ways. First, the temperature-dependent, on-off nature of DNA hybridization can be applied to transition through the fluid-crystal boundary by means of thermal cycling 47. If the DNA strand sequence, surface density, as well as grafted molecule length and flexibility are designed to prevent irreversible aggregation, then crystals – including fcc, bcc, CuAu, AlB2 and diamond have been self-assembled in nano or micro-sized colloidal systems and with one or two particle basis 48–50. The DNA bond can be programmed to melt at temperatures ~ 30°C and greater, depending on solvent composition 51, because of the temperature dependence of DNA hybridization. Second, the colloid pair bond produced by surface-grafted, complementary DNA can be reconfigured by the addition of small DNA strands to the solution. If the DNA sequences of the three strands are designed as a system, DNA strand displacement reactions can create colloidal bonds of variable length. DNA colloids have been reconfigured between two crystal structures by this method. For example, reconfiguration between CsCl crystal structures with different colloidal substitution was accomplished when the DNA sequences were programmed to recognize the different colloidal species 47. In addition to exploiting DNA displacement


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reactions, the DNA hairpin state 52–54, mixed layers of complementary DNA 55, DNA intercalation 56

, and solvent dielectric constant 51 have all been used to control the potential interactions

between colloids. The rich phase behavior and defect physics of liquid crystal solvents has been used to reconfigure potential interactions among the colloids. Liquid crystal solvents align in the vicinity of colloids to accommodate an anchoring condition at the colloidal surface. Homeotropic anchoring at a colloidal surface in a nematic liquid crystal leads to an equatorial defect loop (i.e. a Saturn ring defect) around the particle 57 or a point defect at the particle pole (i.e. a hedgehog defect) 58. The quadrupolar and dipolar symmetry of the liquid crystal orientation, respectively, generates an anisotropic interaction potential that has been exploited to produce chains, clusters, sheets, and networked gels 59–61. Defect entanglement states are controlled by the relative positions of multiple particles 60. Because of the temperature 62 and chemical 63 sensitivity of the liquid crystal defect states around the colloid, these anisotropic potential interactions are tunable. The temperature sensitivity of the reconfigurable assemblies leads to switchable functional properties, such as elasticity and rheology 59. Therefore, temperature can affect colloidal assembly by mediating the strength and range of potential interactions, in addition to its role in the temperature/entropy conjugate pair. This role of temperature can have multiple physical origins, as shown in Figure 2. Temperature change mediates the strength and range of interaction potentials through its effect on colloidal properties such as: surface-grafted polymer and ligand conformation (for depletion and thermoreversible interactions, Figure 2a); DNA hybridization (for DNA strands grafted to colloidal surfaces, Figure 2b); or liquid crystal defect states (for the induction of dipolar and quadrapolar interactions, Figure 2c). The other common way to control potential interactions for reconfigurable colloidal assembly is by application of fields; a recent review is available in 21. Electric and magnetic


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fields induce dipolar interactions between polarizable and magnetically susceptible particles, respectively. These interactions are anisotropic; they depend on the orientation of the colloid pair relative to the direction of the applied field. For uniform fields and particles with isotropic polarizability, the induced force and torque align the pair in the direction of the applied field. In concentrated systems, these associations lead to chaining if the interactions are strong 64. Weaker interactions can reconfigure the unit cell of colloidal crystals 65,66. If the polarizability of the particles is anisotropic – either because of their non-spherical shape or their variable particle composition – then the orientation of the colloidal pair relative to the field direction is offset. These interactions yield staggered chain 67, chain-link 5, sheet 68, and tubular 69 structures as well as crystal-crystal phase transitions 70. Such field-induced interactions drive reconfiguration if their energy significantly modifies the strength or range of the field-free interaction potential. For colloidal self-assembly, the magnitude of the latter is ~ kBT, the energy scale of the Brownian motion that drives the system to explore its ensemble of configurations. For electric fields applied to dielectric spheres, the dipolar interactions generate new phases such as body centered tetragonal for dipolar interactions strengths of about 5-10 times thermal energy 66. Likewise, single ellipsoids orient in the field directions when the dipolar interaction strength exceeds thermal energy; phases such as sheets and tubes occur when the interaction strength is about ten times thermal energy 69. The conditions needed to generate dipolar interactions of this strength depend on the dielectric properties of the particle and medium, the solution electrolyte concentration, and the frequency and voltage of the applied electric field. In non-polar solvents, polymeric spheres ~ 1 µm in radius reconfigure to form strings and crystals at electric fields ~ 0.1 V/µm and ~ 0.5 V/µm, respectively (~ 1 MHz) 71. The differential polarizability of metallodielectric Janus particles leads to chaining and assembly at the lower field strengths of ~ 0.025 V/µm and 0.04 V/µm, respectively 72. Polarizability also depends on particle shape. Aqueous polymeric


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ellipsoids align and assemble at ~ 0.025 V/µm and 0.16 V/µm, respectively (0.16 MHz) 69; the required electric field strength to align and assemble is reduced to ~ 0.01 V/µm and ~ 0.02 V/µm at 10 kHz 73. Colloidal scale ferromagnetism can also be applied to generate dipolar interactions. The ferromagnetism can be introduced by depositing magnetic materials on colloidal surfaces 74,75 or by suspending paramagnetic or diamagnetic particles in ferrofluids 76,77. Even mild externally applied magnetic fields generate dipolar interactions that are stronger than thermal energy and therefore capable of generating chains, clusters, and crystals 78. For reconfiguration, the challenge instead is to maintain reversibility when the external field is released; the magnetic interactions can be so strong as to bring particle pairs into irreversible contact. Strategies such as field rotation 79,80, surface repulsion 81, and agitation 74 have been used to switch structures or restore equilibrium after field removal. Magnetic and electric fields have been applied orthogonally to generate two modalities for control and self-assembly 82.

Phoretically-Controlled Reconfiguration The third method to generate reconfigurability is to exploit field-induced colloidal motion in a bounded system. This motion can be generated by means of transport mechanisms such as sedimentation, electrophoresis, or diffusiophoresis. Such transport mechanisms produce motion of individual colloids by exerting forces either directly on the colloid (e.g. sedimentation, radiation pressure, dielectrophoresis) 25 or on a thin interfacial layer that surrounds it (e.g. electrophoresis, diffusiophoresis) 83,84. In a closed system, this colloidal motion generates spatial gradients in number density as the colloids move toward and concentrate at a system boundary. The force due to the applied field is locally balanced by a gradient in osmotic pressure; at equilibrium a spatially varying concentration is established which satisfies:


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ଵ સΠሺ‫ܚ‬ሻ ఘሺ࢘ሻ

= ࡲሺ࢘ሻ

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eqn (1)

Here, F(r) is the force applied to a colloid at position vector r. ρ( ) and Π( ) are the number density and osmotic pressure at that position. Π( ) is a function of colloid number density and potential interactions. The spatial dependence of these variables has been made explicit to emphasize that the conditions for local equilibrium vary with position vector r. Physically, the total force ρ(r)F(r) exerted by the applied field per unit volume is balanced by a gradient in chemical potential (i.e. osmotic pressure). The osmotic pressure is a function of number density through the equation of state. Eqn. (1) can be solved at each spatial position r; the number of colloids in the system is a parameter of this solution. If the field strength is tuned such that local equilibrium is maintained and a sufficiently large local osmotic pressure is generated, then phase transitions can be induced due to the local densification. Different self-assembly states can then be achieved if the applied field is varied in time 85. This phoretically-controlled (or transport-controlled) mechanism for reconfiguration is distinct from both entropy and interaction potential controlled reconfiguration. That is, the mechanism can be applied to switch regions of the system between different self-assembly states, even for fixed colloidal shape and potential interactions. If the applied field is uniform the colloid number density necessary to satisfy local equilibrium varies in one-dimension, along an axis parallel to the applied field. Sedimentation 86,87

, centrifugation 88, radiation pressure 89,90, and electrophoresis 91 act in this way. On the

other hand, if the applied field is non-uniform, then the spatial variation in colloidal density will be more complex. Dielectrophoresis 92–95 and light are two ways to generate such forces for colloidal transport. In the former cases, field gradients have been used to generate large volumes (~ 5 mm3) of crystalline phase 93. In the latter case, the field-induced potential energy localizes particles at equilibrium in particular regions either due to optical trapping 89,96, light-


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induced electrophoresis 97, or thermally-induced phoretic motion 98. In these cases, the force in eqn (1) is specified by the spatially varying potential energy generated by the electric field or light. Reconfiguration by dynamic change of system boundaries – that is, by manipulating the degree of confinement – is also of this class, because of the change’s effect on the osmotic pressure 34,99. The dispersing medium itself can furthermore generate the field that drives assembly. For example, liquid crystal defects create potential energy landscapes that can drive spatiallyresolved self-assembly in their vicinity; annealing out these liquid crystal defects – as through a temperature change – can thus lead to reconfiguration 100,101. Because of the change in potential energy that results from manipulating defect structure, this is an example of phoretically-induced reconfiguration. Furthermore, this kind of reconfiguration is distinct from the effects that liquid crystal alignment and anchoring conditions have on colloidal potential interactions, as introduced in the previous section and Figure 2c. This contrast is further illustrated in Figure 3a,b. Liquid crystal solvents can be used for reconfigurable colloidal assembly through their effect on potential interactions (as in, when colloids attract to each other through dipolar or quadrupolar interactions through combinations of defect states, as shown in Figure 3a for linked defect states in that loops around particles in a cluster 60) or through their ability of attract colloids to particular parts of the system (as in, when colloids are attracted to point or line defects in liquid crystals, as shown in Figure 3b for a largescale quadrupolar interaction of a micropost that attracts and orders colloids around it 102). Recently, these two kinds of reconfiguration by liquid crystals have been combined. For example, defect sites attract and concentrate particles – a phoretic mechanism; these proximate colloids then interact through hedgehog and Saturn ring defects – an interaction potential mechanism – to produce chains at the defect site 101.


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Applied fields can similarly generate two distinct kinds of reconfiguration, as shown in Figure 3 c,d. In the first mechanism – interaction potential controlled reconfiguration – strong fields of spatially uniform potential energy induce anisotropic interactions (e.g. dipolar forces) between colloidal pairs (c.f. Figure 3c, here showing a tubular structure assembled from interacting ellipsoids 103). AC fields are a convenient way to provide such strong fields of spatially uniform potential energy. Reconfiguration results because of a field-induced change in the system’s internal energy, as mediated through changes in potential interactions. In the second mechanism – phoretically-controlled reconfiguration – weak fields (~ kbT) that produce a spatially varying potential energy field generate a spatially varying osmotic pressure; the osmotic pressure is self-consistently determined by the field strength, equation of state, number density, and system boundaries (c.f Figure 3d, here shown a phase transitions generated after application of a dielectrophoretic field 104). That is, in phoretic reconfiguration the field acts on individual colloids independent of any modulation of the interaction potential between colloids. (As for liquid crystals, although the phoretic and potential interaction mechanisms for reconfiguration are distinct, they can be usefully combined. For example, strong AC fields can generate both dipolar and dielectrophoretic interactions 72,94,105. The former is interaction potential reconfiguration; the latter is phoretic reconfiguration.) Mechanisms of phoretic motion, such as diffusiophoresis, have recently been generated autonomously. An example is the self-diffusiophoretic motion of platinum coated Janus particles 106,107. The platinum half of the Janus particle catalyzes the decomposition of a fuel, such as hydrogen peroxide, which drives active motion. The collisional dynamics of these particles at finite concentration can lead to segregation 108. When such active motion is triggered by stimuli, such as light 109, reconfiguration between different segregation states can be achieved, in ways that are analogous to reconfiguration by other field-induced phoretic mechanisms. Although this kind of reconfiguration is clearly similar to the subject of this review,


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it differs from reconfigurable colloidal assembly because local equilibrium, as described by thermodynamic variables and generalized potentials, is absent. The previous discussion points out the multiple usages of the term reconfigurable colloidal assembly in the literature, and the variety of tools that are available. The dictionary definition of reconfigurability is a system’s capacity to rearrange either its settings or its elements. Likewise, in colloidal self-assembly, two levels of reconfigurability are available. The first level of reconfiguration involves the back-and-forth adjustment of settings (i.e., “knobs”) – these settings being the independent parameters that define the statistical mechanics of the system. As just discussed, the settings that can be changed involve the size and shape of building blocks, the strength and range of potential interactions, and the spatial variability of a system potential energy as specified by an applied field. Rearranging these settings manipulates the system’s free energy. As these tools of reconfiguration are applied, the elements of the system – the colloids themselves – can then rearrange to minimize the free energy. Reconfiguration in this way can shift the system across a phase boundary – such as in an order/disorder or a crystal/crystal transition. It can also shift the lattice spacing of a unit cell whose symmetry is otherwise conserved. Finally, it can rearrange the shape or position of an object self-assembled from colloids. Figure 1 introduced a matrix representation of these two determinants of reconfigurable colloidal assembly. Changing shape, interaction potential, or phoretic strength represents a tool for reconfigurable colloidal assembly. Any of these tools can yield any of three different functional classes of colloidal reconfiguration. These classes are a change in phase, a change in lattice spacing, or a change in the shape and position of an object that has been selfassembled from the colloids. This manner of describing this research area illustrates the diversity of ways in which reconfiguration has been generated, as well as gaps in what has so


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far been accomplished. In the next section, we describe examples of the three functions of reconfigurable colloidal assembly.

Functions of Reconfigurable Colloidal Assembly In the previous section, we identified the different categories of tools that can be applied to generate structural transitions in reconfigurable colloidal assembly. The tools can be broadly classified as entropy (e.g. shape shifting colloids), interaction potential (e.g. field-induced dipolar interactions), or phoretically (e.g. electrodeposition) controlled. To complete the classification, we describe the range of structural transitions that the reconfiguration tools have generated (c.f. Figure 4). The first function is reconfiguration across a thermodynamic state boundary, examples of which are shown in Figure 4 a,b. Colloidal systems display fluid, cluster, gel, and crystal phases, and reconfiguration between any of these phases is possible. For example, figure 4a shows a field-induced order disorder transition in colloidal spherocylinders 70. Figure 4b shows how magnetic particles can be toggled between free particles and chain structures 74. The second function is reconfiguration of the particle positions with a particular phase. An example is the reconfiguration of the lattice spacing of a crystal phase. Another example would be the reconfiguration of the hydrodynamic radius of clusters by varying the interparticle spacing of particles in the cluster 53. A final example would be reconfiguration of the structural color of colloidal crystals in droplets through manipulation of the colloidal separation 34 (Figure 4 c,d). The third function is the reconfiguration of the spatial boundaries of a colloidal phase, so as to generate self-assembled objects whose shape, size, and position can be reversibly modulated. For example, as should in Figure 4 e-j, micropatterned electrodes act as sources or sinks for clusters of colloids (panels e-h) 95, or colloids are organized into specific shapes by lightassisted electrophoresis (panels i-j) 97). I next discuss these three functions of reconfiguration.


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Thermodynamic State Reconfiguration Many of the tools identified in the previous section have been used to produce reconfiguration between fluid and crystal phases. From the perspective of applications, this structural transition is a common target because of the large difference between the functional properties of disordered fluids and ordered crystals. Iridescence is an example of a functional property that has been switched on and off as a result of reconfiguration between fluid and crystal phases 110. Each of the reconfiguration tools – based on entropy, interaction potential, and phoretic mechanisms – has been applied to generate this kind of thermodynamic transition. For example, temperature has been used to reversibly swell and shrink the volume of colloids, causing them to reconfigure between disordered liquid and ordered crystal phases 111. The phase diagram of colloids interacting through dipolar forces is rich 66. Reconfiguration among fluid, close-packed, body centered orthrorombic and body centered tetragonal phases has been reported for charged spheres 71,112 when high frequency AC electric fields are applied to induce dipolar interactions. Ellipsoids polarized by means of high frequency AC electric fields have furthermore been actuated between fluid and ordered microtubule phases 69 (c.f. Figure 3c). Temperature-mediated binding of complementary DNA yields reconfiguration of disordered colloids intro crystals phases with the symmetry of ionic crystals, including CsCl and CuAu 113. Examples of phoretically induced fluid to crystal structural transitions include dielectrophoretic concentration 92,114–116, electrophoretic deposition – as induced either by applied electric fields 85 or light 97, and radiation pressure. These fields displace colloids in the field direction, thereby generating large, near wall regions of high colloid concentration in which ordered phases are in local equilibrium. Other methods, such as optical trapping 89 and lightactivated AC electroosmotic trapping 117, have been used to generate microscopic wells with


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individual colloids attracted to each well. Reconfiguration occurs as the trap pattern is switched on and off. Colloids have been reconfigured between the fluid phase – characterized by individual colloids with random, disordered structure – and cluster phases. Cluster phases organize colloids into aggregates or crystallites of particular size, shape, and internal structure 118. The tools of ligand reconfiguration, DNA hybridization 53, and DNA toehold-mediated displacement reactions have been applied for fluid – cluster reconfiguration. Spheres with attractive Janus faces or patches are also well suited to this kind of reconfiguration because the prescribed region of the patch limits the number of bonds in which the colloid can participate 119–121. This geometry leads to self-limiting assembly. The reversibility between the self-limited, cluster state and a fluid of individual colloids has been generated through magnetic fields 77,122 as well as by changes in solvent properties 46. If the interaction potential between colloids is anisotropic, then low-dimensional structures such as chains can form. Such chaining leads to the self-assembly of fibers, networks, and gels. Reversible actuation between these phases – which can potentially support stresses – and other, ergodic phases 59,74 – which cannot – generate materials with reconfigurable mechanical properties. A mechanism for chaining is to introduce specific kinds of Janus functionality onto particles that are sufficiently anisometric 123, such as ellipsoids 5 and rods 124. Confinement 79, nanocapillary interactions 45, and the combination of shape and attractions 125,126 can also limit the dimensionality of colloidal structures. Alternatively, induced dipolar and quadrapolar interactions can generate strings, chains, and networks 59,74,127,128. Chains themselves can be reconfigured into other phases, including crystals 63. Reconfiguration between two different crystal phases was first demonstrated in the context of colloidal martensitic transitions through the application of AC electric fields 129. The transition was between close-packed and body-centered tetragonal unit cells. AC electric fields


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have been used to actuate between plastic and liquid crystal phases of colloidal rods 70, as well as between two different crystal phases of Janus ellipsoids 5. Temperature dependent depletion interactions have been used to induce solid-solid phase transitions in monolayers of colloidal superballs 41. Colloids coated with complementary DNA strands have been used to reconfigure between fcc and bcc phases by means of a fluid phase intermediate 52. Displacement reactions with DNA-grafted linkers have produced reconfiguration between two crystal phases of the same symmetry, but different colloidal substitution 47. Phases with different substitution have also been formed as magnetic field strength is varied 78. Evaporation of solvent in a solution of ligand-functionalized nanocubes has been used to reversible switch between fcc and simple cubic structures 27; this reconfiguration is an example of an entropy-induced transition in the sense that the conformational change of the ligand modulates the shape and volume of the nanocube, thereby affecting the colloidal contribution to the system entropy 22. Simulation suggestions further opportunities for shape-shifting colloids to reconfigure between multiple crystal phases 32,130.

Lattice Spacing Reconfiguration Reconfigurable self-assembly can function to tune the lattice parameters of a colloidal crystal or the separation distance between particles in a cluster. While state reconfiguration involves a transition between equilibrium phases, lattice parameter reconfiguration manipulates the separation distances of particles in the unit cell, while still conserving the unit cell symmetry. Functional properties of colloids depend sensitively on these separation distances. For example, the wavelength of Bragg iridescence in biomimetic structural color is a function of both the unit cell symmetry and its lattice parameters 10. The Bragg peak shifts with the lattice parameter 131. The lattice parameters and interparticle spacing of ordered colloidal structures


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have been reconfigured by means of DNA displacement reactions 54, electric and magnetic fields 5,80,132,133, temperature 24, osmotic pressure 34, and analyte concentration 28. Interaction potential tools for reconfiguration – including DNA displacement reactions 54,56

and solvent effects on ligand conformation and depletion interactions 42,51,134 – have also

been used to switch bond separation distances in clusters and columnar chains. Bond distance reconfiguration is central to the proposed use of colloidal clusters for information storage 135. The spectral response of a self-assembled cluster can be reconfigured by manipulating the plasmon coupling between particles in the cluster through interaction potential controlled changes in particle separation distance 53,134.

Self-assembled Object Reconfiguration To this point, we have focused on how reconfiguration can be used to control the microscopic features of self-assembled colloidal crystals – properties such as the symmetry of the unit cell and its lattice parameters. However, another important feature of these crystals is their size, shape, orientation, and domain structure. Phoretic mechanisms, which apply forces to colloids that vary in space and time, can be used to reconfigure the shape, orientation, position, and patterning of colloidal crystals. For example, illuminating an ITO-coated electrode with UV or blue light in an electrochemical cell yields sensitive control of ionic currents at the point of illumination. These ionic currents are sufficiently large to drive colloidal crystallization by electrophoretic deposition 97 or electrohydrodynamic flow 136. The position and shape of the illuminated region controls the spatial extent of the crystal. Electrothermal hydrodynamic forces – as induced by IR illumination – have similarly been used to create and reposition ~ 50 µm colloidal crystal regions 137. Electrode patterning can be used to localize colloids either on or off the pattern, depending on the properties of the electric field and the colloids 95. Synchronized optical traps have been used to rotate small colloidal crystals 96. Colloidal structures formed


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through interactions with liquid crystal defects can reconfigure in response to translation 138, rotation 139, or deformation 140 of the defect field. Liquid crystal defects created by substrate patterning can respond to electric 140 and light 138,139 fields. The reconfigured colloidal structures are one or two-dimensional arrays of particles decorated on the defects.

Dynamical Response of Colloidal Reconfiguration An important characteristic of a reconfigurable colloidal system is its dynamic response. What is the delay between triggering reconfiguration and the adoption of the new functional state? The answer to this question constrains applications of colloidal reconfiguration. Moreover, it reveals fundamental information about the physical processes that are foundational to both the tools and functions of reconfiguration. Here we use the matrix classification of Figure 1 to illustrate how the highly variable response times for colloidal reconfiguration can be rationalized based on adding the time constants for the use of the tool and the performance of the function. Figure 5 illustrates the components of the dynamic response; it distinguishes between the time to trigger the reconfiguration tool and the time for the tool to act on the colloidal system. The latter time constant can have two contributions – the time constant for the structure to switch states and the time constant for the colloids to traverse and accumulate in particular parts of the system. The time to activate the reconfiguration tool – τtool – varies considerably among the entropy, interaction potential, and phoretic mechanisms. Broadly, these three mechanisms are triggered either by changes in environmental conditions or electromagnetic field strength. The former mechanism is much slower than the latter. The trigger for entropy mechanisms is typically thermal or mass diffusion induced by changes in environmental conditions. For example, a temperature change that propagates across the system through thermal diffusion (i.e. conduction) will swell colloids comprised of thermoreversible polymers. Both environmental


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and field triggers are represented in the interaction potential and phoretic reconfiguration tools. For example, DNA hybridization is triggered by thermal diffusion, electric and/or magnetic fields induce dipolar interactions, and electrophoretic deposition requires ion transport to set up the steady electric field that drives colloids to the electrode. Once the tool has been activated by propagation of the temperature, solute, or electromagnetic wave, the functional change in colloidal structure occurs according to its characteristic dynamics. Depending on the three functions – phase change, lattice spacing shift, or assembled object displacement – structural reconfiguration can require time to: (i) generate a phase change or lattice position change that propagates though the system; or (ii) accumulate colloids at various positions in the system so that the thermodynamic state transition can be generated. The first time scale, τswitch, is determined by the kinetics of nucleation and growth, spinodal decomposition, melting or the dynamics of normal growth, polymorphic, or martensitic transitions 1. The balance of fields and drag that determines transport properties sets the second time scale, τacc. The total reconfiguration time is therefore τreconfig = τtool + τswitch + τacc. As we will see, the reconfiguration time constant compares favorably with the requirements of a variety of applications for reconfigurable colloidal assembly. Likewise, the constraints of a particular application on τreconfig inform the selection of particular tools and functions compatible with the specified application dynamics. For example, consider a display or fabric of interest for reconfiguration with a thickness ~ 0.1 mm. The value of τtool varies considerably by the tool selected. For liquids, thermal and small-molecule diffusivities are approximately 10-1 mm2/s and 10-3 mm2/s, respectively. Therefore, temperature-induced reconfiguration requires τtool ~ 10-1 s to propagate across the device. Reconfiguration triggered by a small-molecule solute requires larger times (τtool ~ 101


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s). On the other hand, at these dimensions, electromagnetic fields – such as those used to trigger interaction potential and phoretic mechanisms – propagate nearly instantaneously. If the functional reconfiguration involves a change in phase, additional time is required for the dynamics of phase change. For example, in one-dimensional, thermally activated growth 141

– as might occur in boundary nucleation 85,142 – this time requires:

߬௦௪௜௧௖௛ ~

௛ೞೢ೔೟೎೓ ௔ ቆ1 ஽ೞ ሺథሻ

୼ஜ ಳ்

− ݁‫ ݌ݔ‬ቀ− ௞ ቁቇ

ିଵ

eqn (2)

Here hswitch is the characteristic dimension of the region whose phase will be reconfigured. Ds is the (volume-fraction dependent) short-time self-diffusivity of the colloid, a is the colloid radius, and kBT is the thermal energy. ∆µ is the chemical potential difference between the two phases. For chemical potential differences that are large relative to thermal energy, the thermally activated growth rate saturates at a velocity, Ugrowth ~ Ds(φ)/a 142,143. Therefore, to induce a phase change throughout a 100 µm device, τswitch ~ 20 s for 0.1 µm colloids. This time increases to ~ 2400 s for colloids of radius 1.0 µm. The characteristic growth velocity sets a lower bound on τswitch that can be applied both to different phase transitions (e.g. fluid-fluid, fluidcrystal, crystal-crystal) as well as to lattice-spacing reconfiguration. Approaches to estimate τswitch of course will vary with both the kinetic mechanism of the phase transition and the structure produced. For example, heterogeneous nucleation and growth can lead to Avrami kinetics and diffusionless, martensitic transitions between crystal phases can occur very rapidly 112. Reconfiguration among fluid, cluster, and chain states may also occur rapidly because of the small number of participating colloids 5,82,101,144. Accumulation time, τacc, is required when the function of reconfiguration (such as a phase change) first requires a change in the number density profile of colloids across the device. For example, phoretically-driven reconfiguration might transform a system from a


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volume fraction that is uniformly low – with a fluid structure – into one in which all the particles have been concentrated into a local region of high density – with a crystal structure. Mass conservation requires that the region of high density pervade only a small portion of the device volume; all colloids need to be swept into this volume before phase reconfiguration is complete. The time to accomplish this densification is the accumulation time. An expression for this time, from a simple model of one-dimensional convection is 25: ߬௔௖௖ =

௛ೌ೎೎ థ ቀ ೌ೎೎ ௎బ ௄ሺథ೔ ሻ థ೔

− 1ቁ

eqn (3)

 τacc is the time to fill a height hacc to volume fraction φacc given an initial volume fraction φi. U0 is the convective velocity of a single colloid; K(φ) is a factor accounting for the retardation of dynamics due to crowding. Its value depends both on the colloid number density and the convective mechanism. Convection in this case is by one of the many mechanisms for microscopic colloidal transport – i.e. sedimentation, electrophoresis, dielectrophoresis, radiation pressure, etc. Although U0 varies considerably by mechanism and solvent, characteristic values range between ~ 0.5 to ~ 5.0 µm/s. Consider an application in which the colloid density is phoretically manipulated from a state of uniformly low density (φ ~ 0.05) to one in which a high density of colloids (φacc ~ 0.5) is localized in a small region of the device. For a device with smallest dimension ~ 0.1 mm, such accumulation times would fall between ~ 101 and 102 s. The accumulation time varies linearly with the smallest dimension of the device. The three characteristic time constants – τtool, τswitch, and τacc – sum to give the total reconfiguration time, τreconfig, as illustrated in Figure 5. Note that one or more of the three time constants might not contribute significantly for some combinations of tool and function. For example, using dielectrophoresis to reconfigure a dense suspension of colloids between a liquid and crystal phase yields τreconfig ~ τswitch, because neither τtool nor τacc are significant in this case.


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The parameter space illustrated in Figure 5 constraints the choice of reconfiguration tool if a specific reconfiguration function must be achieved in a particular time. Broadly, for 0.1 µm colloids, reconfiguration is never faster than about 60 s. However, many combinations of reconfiguration tools and function require times as long as 6000 s, especially for colloids of large radii (~ 1.0 µm). Literature measures 34,55,85,97,110,112,143,145,146 of the dynamic response of reconfiguration broadly confirm these approximate limits (c.f. Figure 5.) The expressions for τswitch and τacc point to parameters of the colloidal system or device that can be manipulated to design for a particular target reconfiguration time.

Applications of Reconfigurable Colloidal Assembly to Functional Materials The tools and functions of colloidal reconfiguration have been applied to manipulate the properties of materials produced by self-assembly. Properties that have been targeted for reconfiguration include iridescence and structural color, plasmonic response, conductivity, as well as rheological and mechanical properties, including the capacity for self-healing. Future concepts that have been explored for colloidal reconfiguration include information storage and microrobotics. Because the size of colloids matches the visible wavelengths of light, colloidal crystals can produce structural color. Because this color is generated by physical dielectric properties, rather than chemical absorption, it is both iridescent and resistant to fading through mechanisms such as photobleaching. Through its effect on the Bragg condition for optical diffraction, latticeparameter reconfiguration has been used to tune the characteristic wavelength of the structural color response 34,81,147, (as. e.g. generated when a macroscopic elongational strain affinely deforms the microstructure, cf. Figure 6a 131). Fluid-crystal phase reconfiguration has furthermore been used as an on-off switch for structural color 110.


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If the colloidal building blocks are smaller, then reconfigurable self-assembly can be used to produce optical metamaterials through control of plasmonic response. Localized surface plasmon resonance in metallic nanocolloids produces strong absorption at visible wavelengths; dielectrophoresis has been used to reversibly concentrate and orient such gold nanorods near an electrode 148. The concentration and orientation produced by dielectrophoresis generates a polarization dependent optical response in which a central electrode’s shadow – around which the rods are concentrated – changes in response to an electric field applied along a particular axis (as shown in Figure 6b, right). Conductive colloids can produce reconfigurable wires and interconnects potentially useful for applications involving mobile electronic components, such as neuromorphic engineering, rapid prototyping of circuit networks, or self-repairing electronics. Dielectrophoresis has been applied to self-assemble silicon nanowires into bundles that are then switched between two interconnect states 149 (c.f. Figure 6c, in which the electrodes are yellow; the connection among them switches according to the position of a self-assembled nanowire). Reconfiguration between fluid and gel phases produces materials with tunable elasticity. Weak elastic networks are an important constituent of complex fluids used in consumer products, agricultural formulations, and pharmaceuticals. In these products, gel networks maintain homogeneity of microscale particles and droplets, thereby ensuring dosage uniformity. Sensitive, temperature-dependent elasticity has been produced by reconfiguration of potential interactions between colloids dispersed in liquid crystal solvents. This reversible transition is shown in Figure 6d 59, for the particular case of colloids dispersed in a liquid crystal solvent. The colloids form a gel phase dependent on the temperature-dependent phase of the liquid crystal.


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Self-healing can be an outcome of colloidal reconfiguration. For example, magnetic nanoparticles coated with thin fatty acid layers bind above a critical temperature due to nanocapillary interactions of the fluidized layer 45. Magnetic fields drive assembly of the particles into chains. The nanocapillary interactions preserve the network structure even if the field is removed. If damaged, the networks can be reconstituted by renewed application of a magnetic field (c.f. Figure 6e – showing pre- and post- states of the chains upon rupture and healing). Reconfiguration between two phases can be used for actuation and force transduction. For example, Janus ellipsoids self-assemble into fibers with a four-particle unit cell. The unit cell elongates upon the application of an AC electric field. Fibers comprised of a sequence of these unit cells therefore deform when the AC field is switched on. The fibers snap back into their equilibrium conformation when the field is switched off. This on/off process is shown in Figure 6f, in which the four image frames show field off, on, on, and off states, respectively. The unit cells of the unstretched and stretched phases are also shown, at right. The reconfiguration arises because the potential energy barrier of the ellipsoids to sliding is relatively low. Simulation estimates that forces of ~ 5 pN magnitude can actuate this sliding transition, which yields a ~ 35% change in fiber length 5. Colloids with reconfigurable spacing, clustering, and assembled object shape have been proposed for rewritable information storage 90,135, antennas 150

, and microrobotics 18,151.

Outlook This review has highlighted the broad range of tools that have been discovered for reconfiguration of self-assembled colloidal structures. A single tool, used in different ways, can often produce different functions (as shown in Figure 3). Alternatively, individuals seeking a specific function frequently have the flexibility to select from among different tools (as shown in Figure 2).


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A scientific gap that remains in the development and use of colloidal reconfiguration is at the intersection of entropy-based tools and assembled object reconfiguration (as shown in Figure 1c). Entropy tools, which affect self-assembly by direct manipulation of particle shape and size, have strong advantages for reconfiguration 22. These advantages are: (i) access to particular equilibrium phases that are hard to produce because of kinetic limits. In this case the kinetically limited phase can be reached through an intermediate phase produced from a simple shape. Particle shape change can then yield an accessible pathway to the target phase. (ii) reconfiguration kinetics itself can be accelerated because entropy tools operate locally; the kinetics of crystal-crystal phase transitions can therefore avoid the slow, collective dynamics that retard other phase transition pathways. Albeit these advantages, experimental demonstrations of entropy-driven reconfiguration due to particle shape change 27 are not as widespread as the possibilities suggested by computer simulation 31,32. In a similar vein, the use of these tools to change the shape and position of a selfassembled colloidal object – a gateway to achieving more complex functions – is in an early stage of development. Examples of complex functions to which reconfiguration can contribute include those recently realized on large scales by soft robotics, such as: engulfing a target; grabbing and translating a target; and drawing targets into proximity with each other. On the microscale, these functions are biomimetic to phagocytosis, intercellular transport of vesicular cargo, and bacterial quorum sensing. All these aims can potentially be advanced by exploiting the hierarchy of scales present in objects self-assembled from colloids whose shape and position can be reversibly controlled. By applying the tools of reconfiguration, fine variations in object shape and position can be realized either autonomously, through local energy transformation, or at a distance, through application of fields. The phoretic tools that currently dominate the available methods for object transformation represent a kind of dynamic self-


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assembly. The intersection of dynamic self-assembly and reconfiguration combines energy input with reversibility, thereby yielding the potential for cyclic function, durability, and self-repair. The tools represented by reconfigurable colloidal assembly are limited if the pathway between the target structures is kinetically inaccessible. Earlier we discussed how entropy tools may open new pathways between structures by passing through intermediate phases. In addition, real-time monitoring of colloidal configurations could allow the identification of fluctuations favorable to a particular pathway; control algorithms could then be applied to direct and amplify these fluctuations to accelerate the otherwise slow dynamics of reconfiguration 116. Finally, the tools of reconfigurable colloidal assembly are diverse and versatile; they are ready for integration into devices and materials with switchable and tunable function. Figure 5 shows that the time scales for reconfiguration are compatible with many such applications. Engineering science questions to next address in pursuit of this aim include: (i) the quality of the mechanical, optical, or other functional response achieved; (ii) the dependence of that quality on the reconfiguration tool and structure; and (iii) the degree to which functional quality is conserved as device dimensions are scaled up into the ranges of greatest applications interest. Addressing these questions will assist in tool selection for specific applications. For example, consider the design of an architectural coating that changes color on demand. Would this kind of reconfiguration be better accomplished by adopting a phoretic method – e.g. by performing electrophoretic deposition – or by deploying a change in the interaction potential – e.g. by inducing dipolar interactions? The choice depends on the reconfiguration times and crystal quality achieved by the two methods at the scale of interest; these are questions that can be answered by future research that combines theory, simulation, and experiment.


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Acknowledgments Work on this feature article about colloidal self-assembly for reconfigurable biomimetic structural color and mechanics was supported by the US Department of Energy, Basic Energy Sciences, under grant DE-SC0013562 and the US Army Research Office under Grant Award No. W911NF-10-1-0518, respectively. Opinions and conclusions expressed in this article are the author’s; they do not necessarily reflect the views of the funding agencies. I thank Ron Larson, Carlos Silvera Batista, Sepideh Razavi, and Ashis Mukhopadhyay for discussion of the manuscript.


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Figures and Figure Captions

Figure 1. Classification of reconfigurable colloidal assembly by tool and function. (a) ligandinduced particle shape change generates reconfiguration from simple cubic (SC) to face centered cubic (FCC) via rhombohedral structures 22; (b) volumetric swelling of polymer colloids shifts the crystal lattice spacing 152; (c) no entry yet for the bottom left matrix element; (d) temperature-induced annealing of surface grafted complementary and displacing DNA strands yields reversible melting of a CsCl colloidal crystal 47; (e) microparticle spacing at an interface depends on the effects of surfactant concentration on liquid crystal ordering 63; (f) the length of Janus ellipsoid chains is modulated by AC electric fields 5; (g) homogeneous ellipsoids undergo a order-disorder transition by light-assisted electrophoretic deposition 153; (h) electrophoretic


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deposition and compression changes the lattice spacing of colloidal crystals 133; (i) tunable optical trapping rotates a self-assembled colloidal object 96.. Scale bars for e and g are 5 µm, f is 20 µm, h is 100 µm and I is 3 µm. Images reproduced with permission from the references as indicated. Reproduced with permission from AAAS, RSC, AIP Publishing, APS, NAS, Wiley, and Springer Nature from indicated references.


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Figure 2. The multiple roles of temperature on colloidal interaction potential reconfiguration. The strength and range of attractions between colloids can be manipulated with temperature through its effect on: (a) The configuration of polymeric or oligomeric species that are either dissolved in solution or grafted to the colloidal surface; (b) Hybridization of surface-grafted complementary DNA determines the binding state of the DNA pairs. (c) Solvent phase transitions, such as in liquid crystal solvents.


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Figure 3. The same control variable can be designed to generate either interaction potential or phoretic reconfiguration of colloidal systems. For example, liquid crystals can have a dual role. (a), Defect states – such as Saturn rings 60. – yield a temperature dependent potential of mean force between colloids that produces self-assembly (scale bar is 5 µm); (b) The system geometry may produce large-scale defect states that drive phoretic motion and self-assembly, such as the quadrupole generated around the ~ 100 µm micropost, as shown.102. Also, AC fields also have a dual role. (c) When of sufficient strength, they generate dipolar interactions between colloids 69. (d), Spatial gradients in AC electric field amplitude generate dielectrophoretic motion that concentrates colloids for self-assembly 115. Images reproduced with permission from the references as indicated. Reproduced with permission from AAAS, NAS, AIP Publishing and Nature Springer from indicated references.


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Figure 4. Different reconfiguration functions realized by application of the three types of tools. Phase reconfiguration: (a) Laser diffraction shows fluid/glass to a crystal phase transition of colloidal rods by AC electric fields 70; (b) fluid to chain reconfiguration by magnetic field swithcing 74. Lattice spacing reconfiguration: (c) spacing of satellite nanoparticles in a cluster, as controlled by DNA conformation 53; (d) Osmotic pressure changes the lattice constant – and thereby the structural color – of colloidal crystals confined in vesicle droplets 34; Shape and location reconfiguration (e-h): colloids applied to patterned electrodes reconfigure their location on the grid in response to the frequency of the applied electric field 95; liquid-induced phoretic reconfiguration produces colloidal crystals of mutable shape (i,j) 97. Images reproduced with permission from the references as indicated. Scale bars in b, i, j are 20 µm, e, g are 50 µm and f, h are 200 µm. Reproduced with permission from AAAS, ACS, RSC and Springer Nature from indicated references.


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Figure 5. Dynamics of Colloidal Reconfiguration. The total time to reconfigure a colloidal system, τreconfig (top, purple surface) is the sum of τtool, τacc, and τswitch (see text for definitions). τtool grows as the square of the characteristic dimension of the device and is independent of the particle size (bottom green surface). τacc – which arises in phoretic mechanisms – grows linearly with the device dimension and is independent of particle size. The sum of τtool and τacc is the middle green surface. τswitch grows linearly with the smallest device dimension and as the square of the colloidal size. How each characteristic time scales with particle size and device dimension is apparent from the cross section of each plane.


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Figure 6. Applications of reconfigurable colloidal assembly. (a) Iridescence and structural color, demonstrated through lattice spacing reconfiguration 131; (b) Phoretically-driven reconfiguration shifts the polarization dependent optical response of gold nanorods 148; (c) Nanowire bundles – self-assembled and manipulated by phoretic reconfiguration – switch between different connection architectures 149; (d) Sol-gel transitions generated by liquid crystal phases switch the mechanical state of colloidal materials 59; (e) Magnetic-field induced reconfiguration of strand networks applied to self-healing materials 45; (f) One-dimensional assemblies of reconfigurable length actuate forces on objects and boundaries 5. Scale bars in c, d and f are 15 µm, 1 cm, and 3 µm, respectively. Images reproduced with permission from the references as indicated. Reproduced with permission from AAAS, ACS, IOP, AIP Publishing and Springer Nature from indicated references.


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Application of Optically Enhanced, Direct Current Electric Fields. Soft Matter 2017, 6, 557–562.

Author Biography. Mike Solomon is Professor of Chemical Engineering at the University of Michigan. He received his Ph.D. at the University of California at Berkeley in 1996. After a post-doctoral appointment at the University of Melbourne, Australia, he joined the faculty at the University of Michigan. His research interests are in colloidal assembly, gel rheology, and the biomechanics of bacterial biofilms.


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Tools and Functions of Reconfigurable Colloidal Assembly - American

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