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

Abstract

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...

Active colloidal chains with cilia- and flagella-like motion

“…These structures can in principle self-assemble, and in fact the 'reaction path' for at least one of them is known [19]. The (self-assemblable) chains are stable in a wide range of the parameter space and for which the building blocks (dimers and trimers) have been already experimentally realized [17]. These building , where a secondary instability takes place in the form of a low-frequency oscillation.…”

“…Although the equations are derived, and valid, for a three-dimensional system, we restrict the movement to two dimensions for two reasons: first, most of the experimental realizations of colloidal systems are not density matched, thus the particles tend to sediment and their final motion occurs in a plane [16,17]. Secondly, the analysis of the geometrical structures that are formed is simpler in two dimensions, without unnecessary complications from the increased number of degrees of freedom in three dimensions.…”

Pairing, waltzing and scattering of chemotactic active colloids

“…As active particles function under a sustained energy input within an ambient medium, their effective pair interactions are in general non-reciprocal and can systematically draw both linear and angular momentum from their surroundings. The roles of hydrodynamic flow [18][19][20][21][22][23][24], diffused solute [25][26][27][28][29], local phase change [30] and optical shadowing [31,32] as mediators of these interactions have been studied, as well as guidance by nearby boundaries [33][34][35]. Unravelling the dependence of phoretic and chemotactic effects on the surface profiles of catalyst concentration and solute-colloid interaction [8,[36][37][38] and symmetry-based classifications of pair interactions [39] have opened up the possibility of engineering active colloids with desired behaviour [40,41].…”

Generalized Stokes laws for active colloids and their applications

“…An enormously rich variety of phenomena have been observed in suspensions of such 'active colloids' and a systematic study of this new class of colloidal particles is now underway. It has also been realized that phoretic phenomena in gradients of externally applied fields [21], the swimming of micro-organisms [22,23], and the self-propulsion of drops [24,25], though distinct in the microscopic mechanisms that produce fluid flow, share many points of similarity with synthetic active colloids when viewed at the suspension scale. Their study has been revitalized [26][27][28] and subsumed into the field of active colloids.…”