Catalytic Janus Colloids: Controlling Trajectories of Chemical

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Catalytic Janus Colloids: Controlling Trajectories of Chemical Microswimmers Published as part of the Accounts of Chemical Research special issue “Fundamental Aspects of Self-Powered Nano- and Micromotors”. Stephen J. Ebbens* and David Alexander Gregory

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Department of Chemical and Biological Engineering, University of Sheffield, Mappin St, Sheffield S1 3JD, United Kingdom

CONSPECTUS: Catalytic Janus colloids produce rapid motion in fluids by decomposing dissolved fuel. There is great potential to exploit these “autonomous chemical swimmers” in applications currently performed by diffusion limited passive colloids. Key application areas for colloids include transporting active ingredients for drug delivery, gathering analytes for medical diagnostics, and self-assembling into regular structures used for photonic materials and lithographic templating. For drug delivery and medical diagnostics, controlling colloidal motion is key in order to target therapies, and transport analytes through lab-on-a-chip devices. Here, the autonomous motion of catalytic Janus colloids can remove the current requirements to induce and control colloid motion using external fields, thereby reducing the technological complexity required for medical therapies and diagnostics. For materials applications exploiting colloidal self-assembly, the additional interactions introduced by catalytic activity and rapid motion are predicted to allow access to new reconfigurable and responsive structures. In order to realize these goals, it is vital to develop methods to control both individual colloidal paths and collective behavior in motile catalytic colloidal systems. However, catalytic Janus colloids’ trajectories are randomized by Brownian effects, and so require new strategies in order to be harnessed for transport. This is achievable using a variety of different approaches. For example, self-assembly and control of catalyst geometry can introduce controlled amounts of rotary motion, or “spin” into chemical swimmer trajectories. Furthermore, rotary motion combined with gravity, produces well-defined orientated helical trajectories. In addition, when catalytic colloids interact with topographical features, such as edges and trenches, they are steered. This gives rise to a new approach for autonomous colloidal microfluidic transport that could be deployed in future labon-a-chip devices. Chemical gradients can also influence the motion of catalytic Janus colloids, for example, to cause collective accumulations at specific locations. However, at present, the predicted theoretical degree of control over this phenomenon has not been fully verified in experimental systems. Collective behavior control for chemical swimmers is also possible by exploiting the potential for the complex interactions in these systems to allow access to self-assembled, dynamic and reconfigurable ordered structures. Again, current experiments have not yet accessed the breadth of possible behavior. Consequently, continued efforts are required to understand and control these interaction mechanisms in real world systems. Ultimately, this will help realize the use of catalytic Janus colloids for tasks that require well-controlled motion and structural organization, enabling functions such as analyte capture and concentration, or targeted drug delivery.

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“swimming”.4 The potential advantages of active colloids over passive colloids are numerous. For example, in medical diagnosis, colloids are used to selectively bind analyte

INTRODUCTION Colloids currently play a key role in many applications including medical diagnosis,1 drug delivery,2 and photonics.3 This Account discusses the exciting possibilities enabled by replacing the colloids currently used in these applications, with active colloids that can produce enhanced motion, or © XXXX American Chemical Society

Received: May 30, 2018

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DOI: 10.1021/acs.accounts.8b00243 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Figure 1. (a) Schematic of a phoretic catalytic Janus colloid producing motion by asymmetrically decomposing hydrogen peroxide at the hemispherical platinum coated surface (shaded dark). (b) Experimental trajectory (2 μm diameter) for a phoretic catalytic Janus colloid (white line) determined from video microscopy observation. Reproduced with permission from ref 26. Copyright 2011 American Chemical Society. The orientation of the fluorescent colloid can be determined as the platinum coating masks fluorescent emission and appears dark. The effect of Brownian translations and rotations are visible as discussed in the text. Quantitative analysis shows that the angles θ1 (instantaneous orientation) and θ2 (subsequent propulsive step direction) are well correlated throughout the trajectory, proving that the direction of thrust corotates with the Janus colloids orientation. (c) Janus colloid propulsion velocity as a function of particle diameter at constant hydrogen peroxide concentration. Reproduced with permission from ref 27. Copyright 2012 APS. (d) Mean displacements predicted for Janus colloids as a function of time and colloid diameter, based on the variations in velocity shown in (c). The ridge feature in the 3D surface indicates there is a specific Janus colloid size that is predicted to have moved furthest from the origin after a given time period. The size of the expected “winner” of this virtual race between differently sized colloids increases as a function of time. This is because the smaller colloids rapid Brownian rotation produces a tortuous trajectory, despite their intrinsically higher propulsion velocities.

dissolved in the surrounding fluid. Catalytic active colloids consequently provide a viable route to reduce the technological complexity of current systems. For balance, it should be noted that drawbacks for catalytic propulsion include fuel depletion. Historically, the current experimental and theoretical interest in motile catalytic colloids stems from initial observations of motility for bimetallic nanorods,11 and the hypothesized and soon experimentally verified enhanced motion observed in catalytic Janus colloids12 (Janus refers to the hemispherical distribution of catalyst: the colloids are “two faced”, like the Roman god, Figure 1a. This asymmetry is often produced by line of sight metal evaporation). Since these findings in the mid-2000s, a large number of additional catalytic swimming “devices” have been studied.13 These examples can be categorized into two main groups: bubble swimmers, where motion is clearly linked to the momentum generated when catalytically formed gas bubbles detach from a catalytically active region14 (e.g., tubular swimmers15,16), and phoretic swimmers, where no bubble detachment is observed. Phoretic swimmers instead produce motion by self-generating local gradients,17 such as concentration or electric field. Often, this gradient driven mechanism requires the asymmetrical distribution of catalyst found in Janus colloids. Some common themes exist in the current work in this field. One strand has been to develop different motion producing

molecules that indicate the presence of disease. Increasing the motion of these colloids above their natural diffusion rate has been shown to speed up analyte capture.5 Also, developing labon-a-chip diagnosis devices often requires that detector colloids are moved between different reaction chambers, and then finally concentrated. While currently these tasks can be accomplished using external fields to manipulate passive colloids,6 active colloids may instead perform these functions with autonomy. In drug delivery systems colloids carry active ingredients and so directed motion can assist targeted therapies. In addition to acting as transporters, passive colloid self-assembly is also exploited to form regular structures such as photonic materials,3 and templates to pattern other materials.7 Active colloids have additional interactions that can augment current self-assembly applications, i.e., by forming new otherwise inaccessible structures, that are dynamically reconfigurable under changing conditions. Ultimately, combining both transport and assembly functions would allow active colloids to transport and concentrate analytes for diagnosis, or locate and organize around a therapeutic target. Catalytic colloids, the focus of this Account, are an attractive active colloid system to develop for these purposes. While alternative active colloid examples require the application of external fields such as light8,9 or ultrasound10 to produce motion, catalytic colloids are chemical swimmers, driven by molecular fuels B


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Figure 2. (a) Left: Range of trajectories observed when two catalytic Janus colloids self-assemble; Right: schematic of relative cap orientations. Reproduced with permission from ref 44. Copyright 2010 APS. (b) Upper left: schematic of glancing angle platinum deposition at angle θ onto a continuous colloidal monolayer. Lower left: schematic of the geometrically calculated glancing angle cap shapes and thickness variations for a colloidal monolayer member at three different angles (brighter regions indicates a greater cap thickness) Right: Mean ratio of propulsive angular velocity to translational velocity as a function of θ. Spun coat data indicates a control experiment where no shadowing effects were possible. Adapted with permission from ref 39. Copyright 2015 RSC. (c) Upper: results of Langevin simulation of a catalytic colloid with angular and translational velocity moving under the influence of gravity, showing the variety of possible gravitationally aligned trajectories. Lower: 3D experimental trajectories for catalytic Janus colloids with angular and translational velocity. Adapted with permission from ref 45. Copyright 2017 AIP.

experimentalists have focused on proof-of concept demonstrations, including drug delivery,19 lab-on-a-chip transport,20 and environmental remediation.21,22 Significant accompanying

catalytic, enzymatic, and chemical reactions, to improve solution compatibilities for Janus colloids in particular to allow deployment in biological medium.18 Additionally, many C


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the distance over which useful autonomous point A to point B transport using unconstrained Janus colloids is possible. In fact, some differences between this simple model of catalytic Janus colloids motion and experimental data exist. For example, the apparent Brownian rotation rate of Janus colloids has been found to increase with propulsion velocity, reducing the range of ballistic motion.12,30 The origin of this phenomenon has not yet been determined. In addition, it is practically difficult to produce a perfect hemispherical catalyst coating on a Janus swimmer, and deviations in cap symmetry in real systems introduce additional rotational propulsion. Finally, in experiments, gravity influences colloidal orientation, and interactions with boundaries and other nearby passive or active colloids occur, modifying colloidal trajectories, in ways that can be beneficially exploited as discussed below.

theoretical attention has focused on catalytically motile devices, ranging from fine scale simulations23 and analytical theory investigation of propulsion mechanisms,17 to simulation and analytical predictions of collective behavior in ensembles of devices.24 However, our theme here is the quest to make catalytic Janus colloids that exhibit well-defined trajectories, to maximize the potential to deploy active colloids for autonomous transport tasks. Additionally, this Account describes experiments that lay the foundations to control collections of catalytic Janus colloids, which can enable new applications exploiting dynamic structure formation combined with collective cargo transport and release. Consequently, the focus here is on phoretic catalytic Janus colloids, as bubble propulsive Janus colloids have less potential for control.25 While the range of catalytic reactions that can power Janus catalytic colloids is diverse, most of the studies described here still exploit chemical propulsion driven by platinum decomposing aqueous hydrogen peroxide. The prevalent use of this reaction reflects the rapid room temperature decomposition kinetics that produces rapid propulsion velocities. Finally, the focus here is on the micrometer size range: it may be more challenging to control motion at smaller scales.

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CIRCLES AND SPIRALS: CONTROLLING ANGULAR ROTATION RATE FOR CATALYTIC JANUS COLLOIDS As described above, Janus colloids have an intrinsic Brownian rotational rate. However, one way to exert control over trajectories is to also introduce rotational propulsion. The reasons for doing this are numerous: rapidly rotating colloids can agitate surrounding fluid to enhance microfluidic mixing, show increased selectivity when binding diagnostic molecules,31 and can also influence biological processes.32 Additionally, combining rotations with translations enables colloids to follow complex trajectories such as circles and helices, which could be used to control colloidal transport within a microfluidic device. Finally, the ability to transition between “running” and “tumbling” motion provides bacteria with the ability to navigate to specific targets,33 a capacity that could potentially be realized by analogy in synthetic systems.34 One approach to introduce driven rotations is to allow Janus colloids to self-assemble into larger agglomerates. Such agglomerates produce varying amounts of rotational and translational propulsion; determined by the random orientation of the catalytically active caps, Figure 2a.35 For example, trajectories for agglomerates containing two Janus colloids range from tight on the spot spinning to wide circling, Figure 2a. Schematically, Figure 2a depicts how perfectly aligned caps produce linear motion, while misaligned caps produce increasing amounts of rotation. A subsequent study showed that the relative orientation within propulsive Janus colloid clusters could be controlled.36 More recent in depth analysis for Janus dimers revealed that their time averaged position undergoes a chiral spiral diffusion phenomena.37 An alternative approach to produce higher rotational frequencies and controllable rotation rates entails deliberately breaking the symmetry of individual catalytic Janus colloid hemispheres. As mentioned above, commonly, Janus colloids are made by metal evaporation of the catalytic material (e.g., platinum metal) onto colloids dispersed onto a 2D substrate. The Janus structure originates from the directional “line of sight” nature of the metallisation method. To further control this process, if the substrate is completely covered with a continuous monolayer of colloids and the angle of evaporation is varied, then near-neighbor shadowing results in complex catalytic coverages, with broken symmetry, Figure 2b.38 This “glancing angle” method allows the production of batches of colloids with well-defined ratios of translational to angular velocity, Figure 2b.39 At shallow angles, this method produced higher rotational rates compared to self-assembly, with

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CATALYTIC JANUS COLLOID TRAJECTORIES Before discussing ways to increase control of Janus colloids, it is first instructive to describe their individual unconstrained trajectories. A key feature of phoretic Janus colloid propulsion is that the propulsive velocity vector corotates with the orientation of the asymmetrical colloid, Figure 1a. This was first predicted theoretically, and then confirmed by establishing a quantitative correlation between the orientation of the Janus colloid cap and the subsequent direction of motion, Figure 1b.26 From this finding, it is straightforward to predict trajectories, assuming propulsion does not modify the colloids’ Brownian translations and rotations. Intuitively, it is apparent that the Brownian diffusion will introduce translational “noise” to the trajectory, so a criteria for useful propulsion is that the magnitude of the catalytic velocity, v dominates Brownian translations. Simultaneously, Brownian rotation randomly reorientates the Janus colloid, and the propulsion vector. Consequently, while catalytic propulsion initially produces “ballistic” motion (i.e., displacements after time t given by vt), at longer time periods, Brownian rotation randomizes the displacement direction. Analysis shows that this leads to long time limit diffusive displacements, i.e., the effect of catalytic propulsion is to enhance the colloids’ diffusion rate. Because Brownian rotation rate scales as 1/r3, where r is the colloid radius, small particles are expected to rapidly lose their ballistic character and become diffusive; whereas larger particles should show greater persistence of directionality. Systematic statistical analysis of trajectories for differently sized Janus colloids confirms that experimental systems display these expected features.27 As a result, unconstrained Janus colloid trajectories can be simulated by simple stochastic equations,28 and provide a system that mimics the much-simulated active Brownian particle scenario.29 Experimental data also shows propulsion velocity significantly reduces at larger Janus colloid sizes (v scales as 1/r) due to intrinsic features of the propulsion mechanism, Figure 1c.27 This limits the utility of unconstrained Janus colloids for directed transport because a tradeoff between rapid motion and directional transport occurs as a function of particle size, Figure 1d. Practically, these factors cap D


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about additional axes, with the result that the Janus colloid trajectory is steered by the nearby boundaries, Figure 3a and

rotational frequencies of up to 2.5 Hz. Recently, the effect of varying catalytic patch shape on translational/rotational motion has also been theoretically investigated for self-phoretic colloids.40 These predictions show that additional trajectory control is possible by patterning both catalytic activity and surface mobility, including the ability to select the rotational axis. The behavior of colloids with adventitious propulsive angular rotation due to cap defects has also recently been experimentally and theoretically studied in 3D. When considering motion in 3D, gravitational alignment due to the asymmetrical mass distribution of the platinum cap becomes important. It has been found that the platinum coated hemisphere biases colloids’ orientation toward configurations where the cap points downward.41 This effect is increasingly prominent for larger colloids. Due to the propulsive direction pointing away from the catalyst, the colloid consequently moves upward in a gravitational field, a phenomenon termed gravitaxis. This is also observed for mass asymmetrical swimming microorganisms.42 Langevin simulations were able to classify the possible trajectory types for rotating and translating Janus colloids under the influence of gravity based on the axis of rotation, Figure 2c. This showed that under certain conditions gravitationally aligned helices result. Although the experiments could not control the rotational axis, similar aligned helical motion was observed, Figure 2c.43 By exerting additional control over experimental parameters as suggested by recent theory and simulations, it appears that in the future Janus colloids with specific relative rotation and translation propulsion, well-defined rotational axis and an adjustable degree of gravitational alignment can be made. As a result, gravitationally orientated helical trajectories with specific pitch, and a range of other circling or tight spinning trajectories will be accessible.

Figure 3. (a, b) Schematics of the axis about which Brownian rotations of platinum−polystyrene Janus colloids are predicted to be quenched when in proximity to geometries where multiple planes intersect (red axis are quenched, green allow free rotations. (c) Trajectory (yellow line) and superimposed time lapse images of a Janus colloid encountering the curved edge of a cuvette (inset, a). (d) Trajectory (yellow line) and superimposed time lapse images of a Janus colloid showing persistent linear motion within a close fitting trench. Adapted with permission from ref 47. Copyright 2015 Nature. (e) Trajectory (yellow line) for Janus colloids moving at the junction between the base of a colloidal crystal of larger colloids. Reproduced with permission from ref 49. Copyright 2016 RSC. (f) Example trajectories of Janus colloids being guided by circular posts. Reproduced with permission from ref 48. Copyright 2016 Nature.

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STRAIGHT LINE TRANSPORT: REDUCING BROWNIAN ROTATION RATE FOR JANUS COLLOIDS As described above, Brownian rotations are the main obstacle to using Janus colloids for autonomous long-distance transport. Resultantly, various methods have been investigated to overcome this limitation. One approach is to use a magnetic field to apply an aligning torque to prevent thermal rotations. This has been achieved by introducing magnetic components into the Janus colloid, and applying an external field. The resulting systems indeed demonstrate the ability to use catalytic propulsion for linear transport tasks, and also to steer colloids around corners.46 However, this strategy compromises the intrinsic autonomy of the catalytic propulsion mechanisms, and reintroduces the complexity of switchable external fields into any applications. These methods consequently fall short of the ideal goal: to be able to direct colloid motion in lab-on-a-chip fluidic networks without the use of external fields. Recently a range of experimental data has demonstrated that this goal can be achieved by using solid boundaries to steer Janus colloid motion. This idea follows from the finding that specific types of Janus colloids moving near to a horizontal planar solid surface do not exhibit Brownian rotation about the polar axis. Instead they retain an orientation that keeps their propulsion velocity aligned horizontally with the planar surface.26 In geometries where additional solid planes intersect, such as edges, trenches, or overhung constrictions, Brownian rotation is quenched

b.47−49 Specifically it has been found that Janus colloids can be steered by edges, trenches that are of similar width to the colloids, and at the overlap between larger spherical objects and a planar substrate, Figure 3c−f. This guidance effect is not limited to linear features: Janus colloids can also follow edges with curvature. Boundary steering consequently opens up the potential to topographically define pathways for Janus colloids, e.g., using silicon lithography, and enable controlled autonomous transport. Theoretical investigation of this alignment effect suggests the dominant contribution is hydrodynamic interactions between the Janus colloid and the nearby solid interfaces.47 Another related hypothesized substrate based approach to steering Janus colloids for transport applications is to use chemically patterned substrates.50 E


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Figure 4. (a) Analytical theory predicts a wide range of collective behavior for catalytic Janus colloids interacting through chemical gradients. Axis represent chemical and kinetic parameters, and different collective “phases” of behavior are mapped out. Adapted with permission from ref 51. Copyright 2014 APS. (b) Experimental observations of clustering behavior in Janus colloid systems. Reproduced with permission from ref 67. Copyright 2012 APS. (c) Experimentally measured hydrodynamic flow field around a moving catalytic Janus colloid.

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CHEMICAL GRADIENTS AND COLLECTIVE BEHAVIOR The above sections have discussed the behavior of isolated catalytic Janus colloids in homogeneous solutions. Here we turn our attention to environments where chemical gradients, e.g., variations in fuel concentration, exist, as well as considering interacting catalytic colloids. These two scenarios are linked because nearby catalytic colloids can interact through their self-generated chemical gradients. Predicting the behavior of catalytic colloids in fuel gradients has received considerable attention, due to the potential to direct motion toward specific locations, and control selfassembly.51−53 However, these theoretical suggestions have not yet been robustly tested in experimental systems. The first most obvious effect of a fuel gradient on a catalytic Janus colloid is to alter the propulsion velocity. Many mechanistic models for catalytic colloids predict that propulsion velocity will increase with the catalysts turnover rate,54 and indeed this has been confirmed experimentally.12 Typically, for catalytic motile colloids increasing fuel concentration up to a certain level increases reaction rate and measured propulsion velocity. Based on this, a Janus colloid moving in a fuel gradient is expected to speed up in higher fuel concentration regions and slow down in lower fuel concentration regions. This effect alone has been simulated for collections of Janus colloids, and found to result in collective accumulation at lower fuel concentration regions.34 However, concentration gradients also exert other influences on catalytic Janus colloids motion. One possibility is translational diffusiophoresis, whereby the gradient in concentration causes colloids to undergo drift toward higher or lower concentration regions depending on the surface mobility of the colloid. In addition, gradients can also cause Janus colloids to rotate, with a direction and magnitude that depends on their surface properties. These rotations will redirect the propulsion vector, and so produce significant deviations in trajectories. Against this complexity, experiments observed the motion of individual catalytic Janus colloids in a concentration gradient generated by a microfluidic flow device and reported a drift toward the higher fuel concentration regions55 (the opposite effect to that predicted based on velocity modulation).34 Likewise, some other related self-motile systems have shown collective drifts to high fuel concentration regions.56,57 However, despite the predictions that varying Janus colloid

surface properties will control chemical gradient response, to date no experimental studies have systematically demonstrated this link, or verified the prediction of reorientation in a chemical gradient. As described in the Introduction, in order to enable transport applications, and access materials with new structures, there has recently been a significant increase in attention devoted to the collective behavior of catalytic Janus colloids. In particular, analytical theory and simulations have shown the potential for catalytic colloids to organize into a wide range of configurations,24,58−63 including dynamic, oscillating structures and self-motile “molecules”.55,64,65 These configurations are predicted to be controlled by parameters including colloid properties and reaction kinetics, Figure 4a. An exciting prospect is the ability for colloids to switch between these collective behaviors in response to solution borne stimuli such as changes in fuel concentration. Experiments performed for catalytic Janus colloids have shown clustering phenomena which appear to support a limited subset of the theoretical possibilities, Figure 4b.66,67 However, despite this, there are many challenges remaining to fully exploit the potential to control multitudes of catalytic Janus colloids. In particular, the ability to control collective behavior relies on adjusting the parameters that determine interparticle interactions, such as chemical gradient meditated effects. As outlined above, the ability to control the response of catalytic Janus colloids to a chemical gradient has not yet been demonstrated experimentally. A recent study has shown the ability to exert control over the contactless interactions between catalytic Janus colloids, however this relied on introducing a magnetic dipole to the system.68 Additionally, theoretical and simulation predictions often struggle to incorporate all of the possible interaction mechanisms in catalytic Janus colloid systems, which include hydrodynamic and conventional colloidal interactions as well as chemical gradients. At present, there is also little experimental data available to verify the relative importance of these interactions. Recently, we have started to address some of this lack of experimental data, by performing experiments to visualize the hydrodynamic flow field around moving catalytic Janus colloids. Currently, hydrodynamic interactions, when included in theoretical treatments of collective behavior, are based on simple far-field approximations, where colloids are classified as pushers or pullers. However, no evidence for which of these F



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scenarios is most relevant to catalytic Janus colloids has been available. To achieve this goal, we adapted a methodology that had been used in the past to visualize the flow field around swimming micro-organisms. Tracer particles are used as indicators of the fluid flow, and their trajectories (smoothed to remove Brownian diffusion) are averaged and analyzed in order to arrive at streamlines and velocity fields. Figure 4c shows the resulting flow field, obtained for a moving Janus colloid. It is clear that the asymmetry of the Janus colloid is reflected in the flow field, with fluid being drawn toward the equator. Compared to the existing classification, this flow field can be approximated as a pusher, however it is clear that in reality the flow field has additional near field features, which can be parametrized and incorporated into future models. The flow field was also compared to theoretical predictions based on different propulsion mechanisms, and found to be consistent with the electrokinetic variant of phoretic propulsion.69 An additional recent contribution to the area of collective motion, has been a study of the onset of convective motion for symmetrical catalytic colloids. It was observed that as volume fraction increased, a circulating flow of colloids was initiated.70 A likely mechanism for this effect is convective flow driven by local solution density differences generated by the catalytic decomposition reaction. This feature is likely to be present in large collections of catalytic Janus colloids, and has the potential to interfere with experimental observations of collective behavior. A final requirement in order to deploy catalytic Janus colloids en masse for applications exploiting collective phenomena is that sufficient quantities of colloids can be manufactured. As described above, to date, Janus colloids exhibiting catalytic motility have been made via vapor phase metallisation, which limits scale up. This is a batch process limited to 2D coverages of colloids, and requires access to vacuum equipment. It is clearly attractive to be able to instead produce motile Janus colloids via a solution process. This has recently been shown to be possible by forming a wax Pickering emulsion with the core colloids, resulting in partial masking of the colloids surface. Subsequent solution phase deposition of platinum via salt reduction is then possible, producing a catalytically active Janus like structure. Testing these colloids shows comparable propulsive behavior to those made via 2D fabrication methods, and greatly increases the number of colloids that can be produced in a single batch, as well as removing the requirement for expensive instrumentation.71

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AUTHOR INFORMATION

Corresponding Author

*E-mail: s.ebbens@sheffield.ac.uk. ORCID

Stephen J. Ebbens: 0000-0002-4727-4426 David Alexander Gregory: 0000-0003-2489-5462 Notes

The authors declare no competing financial interest. Biographies Stephen J. Ebbens is a Senior Lecturer at the Department of Chemical and Biological Engineering, The University of Sheffield. His research group’s main interest is the investigation of catalytic reactivity induced motility in fluids. David A. Gregory is a postdoctoral researcher at the Department of Chemical and Biological Engineering, The University of Sheffield. His research interests include catalytic motility and reactive ink jet printing and their uses for bio applications.

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CONCLUSIONS This Account has surveyed developments in the field of catalytic Janus colloids, that have seen understanding for these systems grow from the initial reports of undirected enhanced diffusion, to the current case where a library of methods capable of exerting control over catalytic chemical swimmers exists. These findings lay the foundations for developing new approaches to microfluidic transport. It is clear that, to fulfill the potential to access the many collective behaviors possible in catalytic Janus colloidal systems, the important question of how the complex interparticle interactions in these systems can be controlled in experimental systems must be answered. Achieving this will allow access to new self-assembled, responsive structures with novel properties, and will further augment the existing potential to use motile catalytic Janus colloids for microfluidic transport. G


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Catalytic Janus Colloids: Controlling Trajectories of Chemical

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