Transport of cargo by catalytic Janus micro-motors

Recent experiments have demonstrated a fluctuation-induced lateral trapping of spherical colloidal particles immersed in a binary liquid mixture near its critical demixing point and exposed to chemically patterned substrates. Inspired by these experiments, we study this kind of effective interaction, known as the critical Casimir effect, for elongated colloids of cylindrical shape. This adds orientational degrees of freedom. When the colloidal particles are close to a chemically structured substrate, a critical Casimir torque acting on the colloids emerges. We calculate this torque on the basis of the Derjaguin approximation. The range of validity of the latter is assessed via mean-field theory. This assessment shows that the Derjaguin approximation is reliable in experimentally relevant regimes, so that we extend it to Janus particles endowed with opposing adsorption preferences. Our analysis indicates that critical Casimir interactions are capable of achieving well-defined, reversible alignments both of chemically homogeneous and of Janus cylinders.

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“…The latter type of particles has recently been used for realizing self-propulsion (see, e.g., Refs. [88] and [89]). …”

Section: Discussionmentioning

confidence: 98%

Alignment of cylindrical colloids near chemically patterned substrates induced by critical Casimir torques

et al. 2014

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“…The resulting directed motion of colloidal particles in combination with volume exclusion leads to fascinating dynamic behavior ranging from clustering [30] and the formation of "living crystals" [31] to schooling and swarming [32]. Our results demonstrate how one can implement strategies to control and engineer interactions and directed motion on the same footing [33], which is a step towards designing active particles that can perform dynamical tasks such as transport of cargo [34,35]. Concerning the size of the particles used here, we note that there is no conceptual barrier to using smaller particles, the main issue being the ion exchange rate and ion capacity of singe particles that determines the flow strength and the time over which the flow is being generated.…”

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confidence: 97%

Self-Assembly of Colloidal Molecules due to Self-Generated Flow

2017

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The emergence of structure through aggregation is a fascinating topic and of both fundamental and practical interest. Here we demonstrate that self-generated solvent flow can be used to generate longrange attractions on the colloidal scale, with sub-pico Newton forces extending into the millimeterrange. We observe a rich dynamic behavior with the formation and fusion of small clusters resembling molecules, the dynamics of which is governed by an effective conservative energy that decays as 1/r. Breaking the flow symmetry, these clusters can be made active.Colloidal particles acting as "big" artificial atoms have been instrumental in studying microscopic processes in condensed matter, from the kinetics of crystallization [1] to the vapor-liquid interface [2]. Due to their size, colloidal particles are observable directly in real space. Moreover, interactions are widely tunable, ranging from hard spheres to long-range repulsive, short-range attractive, and dipolar [3,4]. Consequently, colloidal particles can be assembled into a multitude of different structures: from clusters [5][6][7] and stable molecules [8,9] composed of a few particles to extended bulk structures like ionic binary crystals [10]. In addition, self-assembly into useful superstructures can be controlled by factors such as confinement [11] and particle shape [12], which make colloids a versatile and fascinating form of matter [13].What is still missing are truly long-range attractions of like-charged (or uncharged) identical colloidal particles. There is much interest in the basic statistical physics of systems with such interactions, which play a role in gravitational collapse, two-dimensional elasticity, chemotactic collapse, quantum fluids, and atomic clusters [14]. One proposed realization are colloidal particles trapped at an interface [15] that experience screened, long-range attractions due to capillary fluctuations of the interface [16]. The attractive interactions then correspond to Newtonian gravity in two dimensions. Complex patterns are also known to arise for bacteria due to longrange chemotactic interactions [17]. Critical long-range Casimir forces have been reported for colloidal particles in a binary solvent [18], which are tunable by temperature and surface chemistry. Finally, a recent theoretical proposal are catalytically active colloidal particles that interact through producing or consuming chemicals [19,20]. For simple diffusion the concentration profile of a chemical decays as inverse distance, implying long-range interactions that can be tuned through activity (how chemicals are produced or consumed) and mobility (how particles react to gradients).Here, we implement long-range attractions through hydrodynamic flows coupling suspended particles [21,22]. We report on experiments using spherical ion exchange resin particles sedimented to the negatively charged substrate. The particles have diameters of 15 µm, for which Brownian diffusion is practically negligible on the experimental time scale. They interact due to self-genera...

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“…This swimmer is a good candidate for forming the basis of future micro-carriers, a claim supported as much by its simplicity of design as by the number of experimental micro-swimmer systems that are based on linearly connected beads. 1,[3][4][5]7,12 A simpler variant of this design, the three-sphere swimmer introduced by Naja and Golestanian, 32 has already proved its immense utility by establishing many basic properties of micro-swimming in spite of sparseness of body elements. [33][34][35][36] Starting with a force-based description 37 and considering only ellipsoids of revolution for the beads in our swimmer, we here determine the optimal forcing parameters as well as the ellipsoidal aspect ratios that lead to the fastest and the most efficient swimming.…”

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confidence: 99%