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Active Patchy Colloids with Shape-Tunable Dynamics Zuochen Wang, ZHISHENG WANG, Jiahui Li, Simon Tsz Hang Cheung, Changhao Tian, Shin-Hyun Kim, Gi-Ra Yi, Etienne Ducrot, and Yufeng Wang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b07785 • Publication Date (Web): 25 Aug 2019 Downloaded from pubs.acs.org on August 26, 2019

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Journal of the American Chemical Society

Active Patchy Colloids with Shape-Tunable Dynamics Zuochen Wang1, Zhisheng Wang1, Jiahui Li1, Simon Tsz Hang Cheung1, Changhao Tian1, ShinHyun Kim2, Gi-Ra Yi3, Etienne Ducrot4 and Yufeng Wang1,*

1Department

of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR,

China 2Department

of Chemical & Biomolecular Engineering, KAIST, Daejeon 34141, Republic of

Korea 3School

of Chemical Engineering, Sungkyunkwan University, Suwon, 440-746, Republic of

Korea 4Center

for Soft Matter Research, Department of Physics, New York University, New York, NY,

11206, USA *To whom correspondence should be addressed. E-mail: [email protected]

ABSTRACT Controlling the complex dynamics of active colloids—the autonomous locomotion of colloidal particles and their spontaneous assembly—is challenging yet crucial for creating functional, out-of-equilibrium colloidal systems potentially useful for nano- and micro-machines. Herein, by introducing the synthesis of active “patchy” colloids of various low-symmetry shapes, we demonstrate that the dynamics of such systems can be precisely tuned. The low-symmetry patchy colloids are made in bulk via a cluster-encapsulation-dewetting method. They carry essential information encoded in their shapes (particle geometry, number, size, and configurations of surface patches, etc.) that programs their locomotive and assembling behaviors. Under AC 1

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electric field, we show that the velocity of particle propulsion and the ability to brake and steer can be modulated by having two asymmetrical patches with various bending angles. The assembly of mono-patch particles leads to the formation of dynamic and reconfigurable structures such as spinners and “cooperative swimmers” depending on the particle’s aspect ratios. Particle with two patches of different sizes allows “directional bonding”, a concept popular in static assemblies but rare in dynamic ones. With the capability to make tunable and complex shapes, we anticipate the discovery of a diverse range of new dynamics and structures when other external stimuli (e.g., magnetic, optical, chemical, etc.) are employed and spark synergy with shapes.

INTRODUCTION Active colloids are nano/microparticles able to self-propel or “swim” within a fluid by consuming external energy, giving rise to rich out-of-equilibrium phenomena1, 2. Interactive active particles can for example lead to the collective formation of complex patterns whose structures and dynamics mimic the assembling behaviors often seen in biology, such as colonies of bacteria, flocks of birds, and schools of fishes2-7. These unique dynamics, i.e., the autonomous locomotion and dynamic self-assembly, have endowed active colloids with great promise in creating microscale machinery or robotics8-10, where control of motion and coordination of the moving parts are the essential elements. In nature, biological machinery is ubiquitous, responsible for sophisticated life functions. Likewise, artificial systems of the kind are expected to aid in performing microscopic tasks, such as cargo (e.g., drugs, micro-devices) transport, fluid mixing, sensing, and microfluidics6, 8, 11-13. The last decade has witnessed a rapid advancement of the field, with many active particle systems developed. These include early examples such as platinum-gold bimetallic nanorods,

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which propel by catalyzing the decomposition of hydrogen peroxide (H2O2)14, and dielectric spheres half-coated with metal, which reveal rich dynamics under electric/magnetic field8, 9, 15-17. More recently, “colloidal surfers” have been synthesized by partially embedding a hematite cube into a polymer matrix; under light irradiation and in the presence of H2O2, those particles become active and can form dynamic and “living” crystals4. In the examples mentioned, a common design criterion is that the particles consist of two parts, an active “engine” and an inert matrix, which are distinct in composition, polarizability, or surface chemistry, and are arranged in an asymmetrical fashion (known as Janus particles). The break in symmetry leads to unbalanced fluid environment that triggers the particle propulsion. While the mechanisms accounting for the activity of particles have been gradually understood, being either self-electrophoresis14,

18,

self-diffusiophoresis19,

20,

or bubble propulsion21,

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and

others, there lacks a general approach for precisely tuning their dynamics largely due to the complex and out-of-equilibrium nature. Consequently, it remains challenging to fully control the particle’s locomotive trajectories as well as the assembled structures. It has been suggested that altering the shape of particles can serve as a handle to tune the dynamics of active colloids as it would influence the fluid environment around a particle, through which the propulsions and interactions occur23-25. Indeed, the initial exploration of the shape effect has been fruitful. For example, when two Janus particles randomly aggregate to form a dimer, they together spin26, 27. L-shaped particles show circular motions if only one of the arms is powered28, 29.

Polymeric dimers assemble to form chiral cluster30. Gear-shaped objects, platinum

micromotors31, and supracolloidal spinners32 have been demonstrated, all capable of rotating with a preferred direction. More recently, simulations predict new propulsion trajectories by placing

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multiple active sites around a particle with a determined symmetry23; one of the proposed examples, a helical trajectory, has been realized33. Although promising, to further take the advantage of shape as a means to control the dynamics and obtain what simple Janus spheres/rods cannot achieve, there are two major difficulties. First, synthetic methods that can integrate activity with sophisticated particle shapes are limited24. Lithography has been used to make arbitrary shapes for active particles but are restricted to produce 2D objects31, 32, 34. Numerous types of particles such as polyhedrons, clusters, etc. have been fabricated earlier by other means35-38, yet they need to be further modified or integrated to become active24. Second, it is largely unexplored how small changes in particle shape can influence the complex dynamics, particularly in the context of dynamic assembly. In static systems, tiny differences in particle shape (e.g., roughness or truncation) can play an important role in the outcome of particle packing39-42. If the same principle holds for active particles, diverse dynamic behaviors may be possible. To tackle the mentioned challenges, we report on the synthesis of active colloids that carry tunable shape information, realized by the use of particles with “active” surface patches. Key to our scheme is the ability to precisely adjust the patch number, size, configuration as well as the overall particle geometry, in particular the possibility to make particles with multiple patches and reduced symmetries (i.e., low-symmetry patchy colloids). This allows us to identify essential shape parameters that orchestrate the dynamics of active particles. For example, under AC electric field, mono-patch particles with various aspect ratios reveal drastically different behaviors in assembling, leading to new behaviors of dynamics such as spinning, “cooperative swimming” and “colloidal reactions”. Also, two-patch particles with bent or asymmetrical configurations enable

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controlled particle locomotion such as steering and braking. In addition, they show selective and directional bonding in assembly according to the patch sizes. Relying on the robust and tunable synthesis developed, our strategy significantly stretches the boundary of the field of active colloids. On one hand, the particles can be more sophisticated in shape, with multiple parameters one can dial in effectively. On the other hand, our results suggest that the system’s activity is sensitive to particle shape. These together bring about a greatly enlarged toolbox from which one may choose the “dynamical units” (spinning, steering, bonding, etc.) by picking the proper shapes to customize the desired active materials.

RESULTS AND DISCUSSIONS Synthesis of low-symmetry patchy particles by surfactant-aided dewetting. To synthesize particles with tunable shapes, our idea is based on making patchy particles with one or multiple patches possessing reduced symmetries. The concept of patchy particles has attracted tremendous attention recently and has stimulated the development of many ingenious fabrication methods, including interface templating43,

44,

glancing-angle deposition45, cluster-

encapsulation35, colloidal fusion36, etc. These methods are normally designed to yield particles with highly symmetrical patches adopting linear, triangular, and tetrahedral configurations, etc. These particles may not be suitable for acting as active colloids, since symmetrical fluid environment will be generated; the particle’s propulsion is likely to be halted due to the balanced hydrodynamic forces. To circumvent this issue, we report on a facile surfactant-aided dewetting strategy, which, when coupled with the cluster-encapsulation method35, creates patchy particles with a wide spectrum of tunable shapes particularly ones bearing low-symmetries. The overall scheme,

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including three main steps, is illustrated in Figure 1a. First, colloidal clusters consisting of several spheres (one, two, or three) are partially encapsulated by an oil droplet. The oil droplet wets the cluster surface with a relatively small contact angle. The vertices of the clusters protrude out of the oil droplet forming patches arranged in a symmetrical manner (except for those with one patch). Then, by introducing a surfactant, which triggers a dewetting process that increases the contact angle and squeeze the cluster out of the oil droplet, asymmetrical shapes are obtained. Finally, the liquid shell is hardened, fixing the particle shapes. Experimentally, we first assemble colloidal clusters as previously reported46, using polymer spheres about 1 micrometer in diameter (see Supplementary Information, SI). Serving as seeds, the clusters are encapsulated in an oil droplet generated by nucleating 3(trimethoxysilyl)propyl methacrylate (TPM) on the surface of the cluster under basic conditions. The dewetting process takes place instantly after a surfactant, in this case Triton X-100 (TX), is introduced, which leads to asymmetrical particles shapes (Figure 1a). To fix the shapes, the TPM droplets are thermally polymerized using azobisisobutyronitrile (AIBN) as the initiator. The morphology of patchy particles can be precisely tuned. Scanning electron microscope (SEM) images in Figure 1b show various particle shapes obtained as we adjust the amount of TX added. For mono-patch particles, the patch size as well as the particle’s aspect ratio increase with the amount of TX (Figure 1b, top row). For two-patch particles, besides the increased patch size, bent patch configuration is created. This happens when TPM droplet migrates toward the side of the cluster upon dewetting (as will be discussed below). The bent patch angle can be modulated by adjusting the TX amount (Figure 1b, middle row). Three-patch particles follow a similar trend, which results in pyramidal particle shapes (Figure 1b, bottom row).

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Another handle we use to control the morphology of patchy particles is the size of the TPM oil droplets, i.e., the encapsulation volume. Figure 1c shows the SEM images of particles made at a fixed TX concentration while the amount of TPM is varied. More TPM results in bigger emulsion droplets and leads to larger particle bodies. For two- and three-patch particles, the changes in the body size also affect the bent angles of the patches. We simulate the evolution of patchy particle shapes using Surface Evolver47 (see SI). We first calculate the surface energy of the two-patch particles while changing the contact angle, θ, of TPM droplet wetting the cluster surface, assuming that the addition of TX could alter θ. We then compare the energy difference between particles with symmetrical patches and asymmetrical ones. As shown in Figure 1d, when θ < 35˚, symmetrical shapes are slightly more favorable. But for θ >35˚, asymmetrical patch configurations become much more stable, which is in good agreement with experiments (Figure 1b). Symmetrical configurations are energetically unfavorable for high θ as it would require a large deformation of the TPM droplet into a non-spherical shape, unstable due to surface tension. For a given contact angle (e.g., at θ = 60˚), an increase of the encapsulation volume V contributes in stabilizing the asymmetrical configurations (Figure 1e). In the case of three-patch particles, similar results are obtained as shown in Figure S1 and S2, well matching our experimental results in Figure 1b and 1c. We attribute the dewetting mechanism to the fact that the chosen surfactant (TX) can preferentially adsorb to the surface of the seed particles and cause a change in the contact angle. To support this point, we choose seed particles of different surface hydrophobicity (therefore different affinity to TX), and study how the contact angles response to surfactants. As can be seen from Figure 1f, when hydrophilic polymerized TPM particles (p-TPM) are used as seeds and encapsulated by TPM oil, the contact angle only slightly increased, from θ  0˚ (fully encapsulated)

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to θ = 35˚, as TX is increased up to 2.4 mM. While for seeds with hydrophobic surfaces, e.g., C18coated TPM (C18-TPM, see SI) and polystyrene seeds, the contact angles increase rapidly as TX is added, and can reach θ = 70˚ and θ = 99˚, respectively. Besides TX, we also test other common surfactants (Pluronic F108 and sodium dodecyl sulfate, SDS), which have distinct dewetting capability (Figure 1g). In this work, we mainly use chlorine-functionalized TPM (Cl-TPM) as seeds, which is hydrophobic enough for dewetting by TX (θ = 24˚ to θ = 69˚, Figure 1f), yet can be further functionalized to incorporate activity. We note that the dewetting process can be easily monitored by an optical microscope, facilitating the synthesis of the desired shapes (Figure S3). Finally, a layer of gold is plated to the particle patches, creating metallo-dielectric patchy particles that are active under AC electric field (Figure 2a). The gold plating is initiated by converting the chlorine groups on the patches to amine groups, which then enables the specific gold coating using a two-step, solution-based seeding and growth protocol48. To ensure activity49, conditions are optimized to obtain a high coverage of gold nanoparticles normally 10-30 nm in diameter. Figure 2b, 2c and Figure S4 show the optical and electron microscope images of the gold-coated patchy particles: only on the patches can we see the gold layer. We choose to use AC electric field to drive the particle dynamics for two reasons. First, it’s versatile as multiple field parameters (e.g., direction, strength, and frequencies) can be adjusted. Second, the behaviors of metallo-dielectric Janus spheres are well studied, showing a spectrum of individual and collective dynamic behaviors depending on the field parameters and polarizability of the metallic and dielectric part5, 32, 50. This serves as the basis for our investigation on particle shapes that deviate from Janus spheres through the number, size, and locations of the gold patches.

Shape-tunable locomotion.

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Having demonstrated the diversity of shapes we can create for gold-patched particles, we study their influence on particle’s locomotive dynamics. For this purpose, we drive the propulsion of various patchy particles by the induced-charge electrophoresis mechanism (ICEP)15 under AC electric field (square waves). In this case, the gold patches act as the active engines that power the particle propulsion. At frequencies chosen, electro-hydrodynamic flow (EHD) is weak and is not considered for simplicity27, 51. Using mono-patch particles, we first verify that particles made by our method are active. Indeed, at AC frequency f = 2 kHz, a condition suitable for ICEP, patchy particles propel showing persistent random walk (Figure 3a, Supplementary Video 1). While moving, the particles orient its dielectric part in the forward direction, normal to the electric field (AC field applied perpendicular to the plane of microscope slides, see SI). The velocity v is proportional to the strength of the field squared E2 (Figure 3b). Both observations suggest an ICEP propulsion mechanism. To look into the shape effect, we explore the velocities of propulsion as the shape of particle varies. In Figure 3b we plot the v versus E2 for mono-patch particles synthesized with identical seeds but different aspect ratios (insets). When the field is weak, all particle shapes move at relatively low speeds (4-6 µm/s). As the field strengthened, particles with small contact angle (less exposed patch surface) swim much faster (up to 43 µm/s) than those with intermediate contact angle, which are faster than those with a larger contact angle (26 µm/s). To rationalize the observed trend, we use COMSOL to numerically simulate the induced electric potential (indicated by the color map) and the corresponding electric field (indicated by the red arrows) around patchy particles, taking into account their shape characteristics (Figure 3c; see SI for method). Under the ICEP conditions, the low AC frequency allows sufficient time for

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the ions within the electric double layer (EDL) to screen the induced charges, resulting in an opposite electric field inside the EDL. This EDL screening would cause induced-charge electroosmosis (ICEO)52, 53 when the AC field changes signs. Because the ICEO flow (indicated by yellow arrow and color map in Figure 3d) is much stronger around the metallic patches (labeled with Au), their shape and symmetry govern the locomotive dynamics of particles. Patchy particles with smaller contact angle bear a patch close to a hemispherical geometry, thus generating the largest propulsion due to the asymmetrical ICEO flow. In contrast, patchy particles with a bigger contact angle display a larger patch area, closer to a spherical shape. The ICEO will thus be significantly balanced (indicated by blue arrows Figure 3d). One notable advantage of our synthetic strategy is the ability to produce particles with two or more patches. In the context of active colloids, the two patches can be viewed as two engines that simultaneously contribute to the particle’s overall motion. By precisely controlling the size and location of the two patches, we show that the locomotive trajectories of such particles can be regulated (Figure 4). When the two patches on the same particle are of an identical size and arranged in a linear, symmetrical fashion, the particle barely moves under the AC electric field, as compared to the enhanced motion of mono-patch particles (Figure 4a, i). The ICEP forces derived from the two patches cancel out (Supplementary Video 2). When the similarly sized patches are at a bent position, net ICEP force is produced enabling particle locomotion with its dielectric part facing forward (Figure 4a, i, Supplementary Video 2). Comparison is then made across two-patch particles with different bent angles, their shapes and propulsion velocities shown in Figure 4a, ii. As summarized in the inset, the velocity first goes up when the bent angle is increased, and then goes down as the bent angle is further increased. Here,

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both the bent angle and the patch size play a role. Increased bent angles lead to fast propulsion due to decreased cancelling effect from the two patches. However, large bent angle is associated with large patch sizes, which provides smaller propulsion forces, as discussed earlier (Figure 3d). The competition of the two factors determines the final result. We also synthesize particles with different patch sizes. To do so, we co-assemble 1.0-μm and 0.65-μm spheres to produce colloidal dumbbells composed of a big and a small particle (Figure S4). Encapsulation of this cluster results in particles with a big and a small patch. When the contact angle is kept small during the synthesis, the patches are located at the two ends of the particle (Figure 4b, i). The ICEP effects stemming from the two patches are of opposite directions and compete with each other due to opposite directions; they propel in a way that the big patch is pushing the small one. The propulsion velocity is reduced, as compared to mono-patch particles with the same patch size (Figure 4b, ii). In this case, the big patch could be pictured as the engine while the smaller one can be regarded as the brake. An interesting situation is when the big and small patches on a particle do not follow the linear arrangement. Such particles are synthesized by applying the dewetting strategy to the nonsymmetrical dumbbell (Figure 4c, Figure S5). Under AC electric field, such particles can steer, due to the uneven ICEP forces of the two patches that produces “eccentricity” when the particles are anisotropic in shape. A geometrical analysis shows that the curvature of the particle steering can cover a wide range by adopting different patch sizes and configurations (see Supplementary Discussion, SI). In the example cited, the velocity of the particles is mainly determined by the size of the big patch, while the small patch influence the particle steering by applying a small perturbation that can shape the locomotion trajectory. Depending the exact position of the small patch; particles are shown to make clockwise (CW) and counter-clockwise (CCW) circles when

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swimming (Figure 4c, right). The steer directions may switch between AC on/off cycles as particle flips by Brownian motion (see Supplementary Video 3). Lastly, we study the propulsions of asymmetrical three-patch particles (Figure 2c, bottom), which are dependent on the particle’s initial orientation before the AC field is on. It can either stand up showing no propulsion or propel toward its dielectric body. While propelling, it always places two of the patches close to the substrate (see Figure S6).

Shape-tunable assembly. We next explore the shape-directed assembly of active patchy particles. We focus on binary systems consisting of mono-patch particles along with plain polymeric, dielectric spheres. Such systems are chosen for the ease of study as the interactions of polymer spheres are isotropic (or simply dipolar); the shape influence due to the low symmetry of patchy particles could be better reflected. The shapes of mono-patch particles can be described by three parameters (Figure 5a, left): the long axis L, the diameter of the gold patch D1, and the diameter of the dielectric matrix D2, respectively. Figure 5a shows particles made with identical seeds but different contact angles. For particle 1, P1, D1=1.46 µm, D2=1.43 µm, and L=1.81 µm; for particle 2, P2, D1=1.46 µm, D2=1.17 µm, and L=2.00 µm. Because D1 is the same, we define the aspect ratio r=L/D2, which can suggest how particle shapes deviate from a traditional Janus spheres where r=1. For P1, r=1.27; it shows obvious surface anisotropy and small shape anisotropy. For P2, r=1.71; it shows large anisotropy in both surface and shape. The dielectric spheres we employ is 4µm in diameter, denoted as S.

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At a high AC frequency, f=10 kHz, P1 alone swims with the gold patch facing forward, following a self-dielectrophoresis (s-DEP) mechanism54 (Figure 5b). When P1 encounters the dielectric sphere S, it lifts the sphere out of the plane and insert below it. The assembly (S+P1) is capable of propulsion too, which can be thought of as an active patchy particle (P1) carrying a cargo (S). The assembly can quickly pick up another patchy particle (P1) forming a stable cluster in which two P1 are at the bottom whereas one S stacks on top (S+2P1). Once formed, the assembly starts to spin, having both clockwise (CW) and counter-clockwise (CCW) directions (Figure 5b, Supplementary Video 4). The formation and dynamics of the spinning clusters can be attributed to the following factors. First, the patchy particles are confined underneath the dielectric sphere due to out-of-plane dipolar attraction (Figure 5b). Second, the patchy particles adopt a head-to-tail configuration to minimize the in-plane dipolar repulsion by avoiding the strongly polarized gold patch being close to each other. Third, because the patchy particles keep the tendency to propel, yet they cannot escape from the cluster, they spin together. The spinning speed increases as the field strength increases (Figure 5b). Although similar clusters have been observed previously using Janus spheres55, we note that in our system, due to shape anisotropy (r=1.27), clusters possessing two patchy particles are abundant. In the case of Janus spheres (i.e., r=1), higher order clusters seem to be more favorable. As we further increase the shape anisotropy by using P2 (r=1.71), our observations are completely different. When the field is on at the same frequency, f=10 kHz, P2 “stands up” and align their long axis with the field direction. The orientation of particles in AC electric field is determined by torques exerted on particles, summing up contributions from ICEP, dielectrophoresis (DEP), and electric dipole alignment. Because ICEP and DEP are weak at this

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frequency32, 54, 56, the particle prefers to align its largest dipole moment with the field. For P2, due to the more exposed gold patch and the overall elongated particle shape (r=1.71), the largest dipole moment is parallel to the long axis of the particle. As a result of the “stand-up” configuration, P2 no longer propels. Moreover, they show strong in-plane dipolar repulsions with each other and with S, which inhibits their assembly (Figure 5c, Supplementary Video 5). The shape-dependent assembly dynamics of patchy particles are also investigated at a lower AC frequency. At f=2 kHz, the majority of mono-patch particles (P1 and P2) show ICEP propulsion. Yet, their co-assembly behaviors with the large spheres S are rather different. For P1, there are negligible interactions with the dielectric spheres. They may approach and touch the sphere while swimming but would immediately escape by sliding around; the locomotion direction of P1 is unchanged during the process (Figure 6a, Supplementary Video 6). Under identical condition, however, P2 can assemble with the big sphere forming multiple dynamic, complex, and reconfigurable structures. For example, the dielectric lobes of P2 can stick under the edge of S affording a heterogeneous dimer cluster (S+P2). Because P2 does not lose the activity to propel, it adopts a special configuration to swim along the rim of the big sphere (Figure 6b, Supplementary Video 7). This also causes the assembled dimer to steer. The radius of the steering is 3.2 µm, and the steering frequency is dependent on the field strength (Figure S7). The steering assembly can pick up more patchy particles forming various “colloidal molecules” having a central hub and various number of patchy particles as ligands (S+nP2) (Figure 7a). Those colloidal molecules are highly dynamic, showing “colloidal reactions” where ligands are added, reconfigured, deleted, and exchanged (Figure S8, Supplementary Video 8 and Video 9). For example, when the second P2 meets the dimer (S+P2), it first couples with the sphere, and then slides along the rim of the sphere to be close to the first patchy particles. Both patchy particles

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then change their configurations so that their long axis is normal to the sphere surface (Figure 7b). Following this, the two patchy particles push the big sphere, and all particles (S+2P2) swim cooperatively due to unbalanced force around the big sphere. Higher order clusters such as S+3P2 is then formed following similar dynamics (Figure 7c). Before the surrounding of S is saturated by P2, the assemblies swim due to the unsymmetrical overall shapes. (Supplementary Video 8). To account for the differences between P1 and P2 regarding their dynamic assembly, we consider how they interact with the big sphere (Figure S9). At AC frequencies studied, the ICEP and DEP effect are strong, so both the spheres and patchy particles prefer to stay near the conducting substrate15, 55, 56. This prevents the formation of out-of-plane clusters. To assemble clusters, the dipole-dipole interaction between S and P must be attractive and strong enough to overcome the EDL repulsion. Based on the particle geometry and material composition, we calculate the dipole-dipole interactions between patchy particles and the big sphere, with the assumptions that both particles are touching the substrate (in plane) and that the patchy particles approach the spheres with its dielectric lobe. The calculated results show that, for P1, the interaction between the dielectric lobe is 1.10 pN, and that between the gold patch and the spheres is 2.79 pN, being both repulsive. When EDL repulsion is taken into account, the overall interaction is repulsive. For P2, the dielectric lobe is small enough to fit under the side of the big sphere (Figure S9), resulting in attractive interaction (-2.96 pN). Meanwhile, the increased particle aspect ratio keeps the gold-patch further away from the sphere, leading to a decreased repulsion, 2.58 pN. The net interaction is attractive, which should be comparable to or even stronger than the repulsion induced by EDL, making it possible to form quasi out-of-plane assembly (Figure S9 and more discussions in SI).

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Finally, we demonstrate “directional bond” by using two-patch particles with one big and one small gold patch (Figure 7d, Supplementary Video S10). At f=1 kHz, such patchy particle propels under the ICEP mechanism with its small patch facing the front (Figure 4). When encountering a dielectric sphere S, the patchy particle forms a bond with the sphere using its smaller patch. The big patch never binds due to the unfavorable dipolar repulsion. Such bonding selection and directionality is crucial in static systems57, 58 for making complex structures but has never been shown in dynamic systems. Here, it is achieved by the designed particle shapes. After coupling, the assembly steers, as if the big patch push the spheres with an angle (Figure 7d).

CONCLUSION We have put forward a new strategy, based on cluster-encapsulation and surfactantdewetting, that allows for the synthesis of patchy particles with, in principle, countless shapes (geometries, patches, etc.). When introduced to the arena of active colloid (e.g., driven by electric field), these particles exhibit a rich variety of shape-dependent dynamics that regulate their locomotive and assembling behaviors. For such particles especially those adopting reduced symmetries, we show that not only the speed of their propulsion, but also the ability to brake and steer can be controlled. Moreover, complex structures and dynamical processes are discovered, including assemblies that spin, steer, and swim cooperatively as well as “colloidal molecules” that undergo “reactions” such as ligand exchange. While we use electric field to power the particles though polarization differences, other source of energy can be possibly employed, which may bring about new synergy with the shape of particles we introduced. For example, if a thin layer of magnetic materials is coated on the patch, the particles can be manipulated by magnetic field59. Given the large number of dynamic structures

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produced earlier using magnetic Janus particles (tubes, rings, etc.)16,

17, 60,

new sophisticated

structures are to be created for particles we make bearing complex shapes. On the other hand, the patches may be modified with titanium oxide61, or other active materials, so that particles can be activated by chemical fuels and light. Ultimately, by making colloidal cluster with different types of particle, it may be possible to produce particles with two or multiple patches responsible for different external stimuli. This will further increase the precision to control particle trajectory as well as complexity of assemblies. Finally, in accordance with the recent literatures that predict new dynamics and structures for active colloids through computer simulations, our strategy may be able to provide the desired particle types for experimental realization (we note that our particles can be purified as shown in Figure S10). For example, the big-small bent patchy particles capable of steering may be used to verify the hyperuniform material phase62. The two-patch particles can be used to assemble Kagome lattice or other structures enhanced by particle activity63. Higher order of particles with symmetrical or asymmetrical patches may be tailored to achieve complex trajectories23. In the meanwhile, the flexibility and versatility of our synthetic strategy could also spur much effort from the computational soft matter community in pursuit of the critical design rules for active colloidal systems.

ASSOCIATED CONTENT Supplementary Information Materials and methods; supplementary figures and discussion; supplementary videos and video legends. The Supporting Information is available free of charge on the ACS Publications website.

17



AUTHOR INFORMATION. Corresponding Authors *[email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT. Y.F.W. acknowledges support from start-up fund of The University of Hong Kong, and the Early Career Scheme (ECS) from the Research Grants Council (RGC) of Hong Kong (Project number: 27303817). This project is also partially supported by Croucher Innovation Award (Croucher Foundation, Hong Kong). G.R.Y. acknowledges support from the NRF (Korea) under award nos. 2017M3A7B8065528 and 2017R1A5A1070259. E.D. acknowledges support from the US National Science Foundation under Award Number DMR-1610788.

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FIGURE LEGENDS. Figure 1. Synthesis of low-symmetry patchy particles. (a) Schematic illustration of the synthesis of low-symmetry patchy particles by a cluster-encapsulation-dewetting strategy. (b) Scanning electron microscope (SEM) images of patchy particles with different shapes. Particles with one, two, and three patches and different contact angles are shown. (c) SEM images of patchy particles with different encapsulation volume. Surface Evolver simulation compares the surface energy of two-patch particles with symmetrical and asymmetrical patchy configurations, when contact angles (d) and encapsulation volume (e) are varied. The unit of energy is the surface energy of a seed sphere; the unit of the volume is the volume of a seed sphere. (f) Comparison of different seed particles which show different sensitivity toward surfactant-aided dewetting. (g) Comparison of different types of surfactants in the dewetting process. TX is a preferred surfactant that shows large extent of dewetting. The unit Scale bar: 1 μm.

Figure 2. Site-specific gold-coating of patchy particles. (a) Synthesis of patchy particle with gold patches. A two-patch particle is shown as an example. (b) Electron and (c) optical microscope images of patchy particles with gold plated on the surface of patches. Scale bar: 1 μm.

Figure 3. Shape-directed locomotion of mono-patch particles. (a) Image showing the propulsion of particles and their trajectories. (b) The propulsion velocity of mono-patch particles as a function of field strength. Images of particles with small, medium, and large patches are shown (insets). (c) Numerical simulation using COSMOL Multiphysics of the electric potential (color map), the electric field (red arrow), and (d) the induced-charge electroosmotic flow (yellow arrow and color map) around mono-patch particles of various patches sizes. Scale bar: 1 μm.

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Figure 4. Shape-directed locomotion of two-patch particles. (a) The locomotive behaviors of two-patch particles with similarly sized patches but various bent patch angles. (i) Illustration of particles with linear and bent patches and representative images showing the locomotive trajectories, as compared with mono-patch particles. (ii) Comparison between particles with varied bent angles of their propulsion velocities. (b) (i) The locomotion of big-small two-patch particles in a linear fashion. The small patch serves as “brake”. (ii) The v-E2 profiles of the corresponding particles. (c) The locomotion of bent, big-small two-patch particles; the particle steers under AC electric field. Steering trajectories of bent two-patch particle with big and small patches. Scale bar: 1 μm.

Figure 5. Dynamic assembly of colloidal spinners. (a) Images showing the building blocks used: patchy particle P1 (left), P2 (middle), and 4-µm dielectric sphere, S (right). (b, i) The formation of the colloidal spinner by P1 and S. (ii) Images show multiple colloidal spinners and the corresponding force analysis accounting for their formation mechanism. (iii) The spinning speed is linearly related to the electric field strength squared E2 under AC electric field (f=10 kHz). (c) P2 stands up when the AC field is on at identical condition and shows strong repulsion with S preventing possible assembly. Scale bar: 1 μm for (a), 4 μm for (b) and (c).

Figure 6. Dynamic assembly at low AC frequency (f=2 kHz). (a) Illustration (side view and top view) and optical microscope images showing that when P1 approaches S, it touches but escapes. (b) when P2 approaches S, they couple and form steering assembly. Scale bar: 4 μm.

25



Figure 7. Colloidal molecules and cooperative swimmer and “directional bonding”. (a) Optical microscope images of colloidal molecules formed (AB1 to AB7). (b) Optical microscope images and schematics showing the steering of AB1, and how it picks up additional patchy particle and reconfigures to AB2, which cooperatively swims. (c) Image showing the trajectories of the locomotion of an AB3 colloidal molecule. (d) Optical microscope images and schematics showing the “directional bond”, occurred when big-small two-patch particles are assembled with S. The smaller patch preferentially assembles with S. Scale bar: 4 μm.

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1.Encapsulation

b

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Active Patchy Colloids with Shape-Tunable Dynamics | Journal of the

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