Pair Interaction between Two Catalytically Active Colloids

Abstract

Due to the intrinsically complex non‐equilibrium behavior of the constituents of active matter systems, a comprehensive understanding of their collective properties is a challenge that requires systematic bottom–up characterization of the individual components and their interactions. For self‐propelled particles, intrinsic complexity stems from the fact that the polar nature of the colloids necessitates that the interactions depend on positions and orientations of the particles, leading to a 2d − 1 dimensional configuration space for each particle, in d dimensions. Moreover, the interactions between such non‐equilibrium colloids are generically non‐reciprocal, which makes the characterization even more complex. Therefore, derivation of generic rules that enable us to predict the outcomes of individual encounters as well as the ensuing collective behavior will be an important step forward. While significant advances have been made on the theoretical front, such systematic experimental characterizations using simple artificial systems with measurable parameters are scarce. Here, two different contrasting types of colloidal microswimmers are studied, which move in opposite directions and show distinctly different interactions. To facilitate the extraction of parameters, an experimental platform is introduced in which these parameters are confined on a 1D track. Furthermore, a theoretical model for interparticle interactions near a substrate is developed, including both phoretic and hydrodynamic effects, which reproduces their behavior. For subsequent validation, the degrees of freedom are increased to 2D motion and resulting trajectories are predicted, finding remarkable agreement. These results may prove useful in characterizing the overall alignment behavior of interacting self‐propelling active swimmer and may find direct applications in guiding the design of active‐matter systems involving phoretic and hydrodynamic interactions.

Enhancing magnetically driven microswimmer velocity via low Reynolds number hydrodynamic interactions

“…In predatory flagellates equipped with hairs, the HI are the key to effective foraging [63]. For self-propelled chemical microswimmers, in addition to the evolution of their motion due to phoretic interactions, inter-HI are also found to affect their dynamics [64][65][66].…”

Pair Interactions of Self-Propelled SiO2–Pt Janus Colloids: Chemically Mediated Encounters

“…However, we believe that upon changing the fuel concentration, thickness of Pt coating, confinement of the optical cell, and particle size disparity, the relative strengths of the two interactions can be tuned resulting in a modified response. In addition, by altering the mode of swimming by either changing the active surface coverage , or altering the surface chemistry of the JPs, distinct yet connected response can be observed.…”

“…Additional experiments are needed to further understand the onset of dynamic assemblies because the pairwise investigation presented here and the one reported by Sharan et al reports an eventual scatter, but numerous experiments outlined in the Introduction section report assemblies and dynamical structures. 76 This might be due to the use of Au− Pt particles that operate predominantly via self-electrophoresis: electrochemical decomposition of H 2 O 2 generates electrostatic fields that act on top of the chemical and hydrodynamic fields, probably resulting in enhanced attraction between the JPs. 5 Indeed, pair-interaction studies on such rod-like particles have shown an array of robust pairwise assemblies.…”

“…For SiO 2 –Cu JPs, the study demonstrated far-field ( d cc ≫ 2 a ) repulsive interactions preventing contact. For SiO 2 –Pt JPs, they reported that the interactions are mostly steric in nature and no far-field hydrodynamic or chemical interactions were observed . They also reported (i) no significant speed fluctuations during the interaction, (ii) 3–4 s of contact where JPs slide over each other, and (iii) postcollision departure due to free in-plane thermal reorientations.…”

Open Questions of Chemically Powered Nano- and Micromotors

“…To clarify this vague statement requires careful experimental measurements and powerful simulations. On the experiment side, although numerous studies have demonstrated the attraction and repulsion of tracers to micromotors, , or the pairwise interaction between two or more micromotors, , measurements of the interaction potential around a moving/fixed micromotor are few. − These potentials can help distinguish between phoretic and hydrodynamic interactions, noting that the former is isotropic in space, while the latter is anisotropic at a far field. For example, two previous studies , have measured the attraction of tracers toward micromotors, and confirmed that the attractive force was isotropic in space and scaled with 1/r, which the authors argued to be consistent with phoretic interactions.…”