Experimental observation of flow fields around active Janus spheres

Abstract

The phoretic mechanisms at stake in the propulsion of asymmetric colloids have been the subject of debates during the past years. In particular, the importance of electrokinetic effects on the motility of Pt-PS Janus sphere was recently discussed. Here, we probe the hydrodynamic flow field around a catalytically active colloid using particle tracking velocimetry both in the freely swimming state and when kept stationary with an external force. Our measurements provide information about the fluid velocity in the vicinity of the surface of the colloid, and confirm a mechanism for propulsion that was proposed recently. In addition to offering a unified understanding of the nonequilibrium interfacial transport processes at stake, our results open the way to a thorough description of the hydrodynamic interactions between such active particles and understanding their collective dynamics.

Floor- or Ceiling-Sliding for Chemically Active, Gyrotactic, Sedimenting Janus Particles

“…For example, a simple check is provided by locating the points (T, F ) corresponding to the experiments (the present ones and from, e.g., Refs. 9,11,25,28) within the state-diagrams and by checking the compatibility between the observed state(s) and the model-predicted state. This is illustrated in Fig.…”

“…This seems to be the situation in the experiments with Pt/PS and Pt/SiO 2 particles. 9,15,25 Turning to the parameters F and T , the mass densities of the core materials, of the catalyst, and of the solution, as well as the viscosity of the solution, are known. For the system of interest here, the particle parameters are ρ s ≈ 1050 kg/m 3 (PS) or 2196 kg/m 3 (amorphous SiO 2 ), and ρ a ≈ 21450 kg/m 3 (Pt catalyst), while for the aqueous solution ρ ≈ 1000 kg/m 3 and µ ≈ 9 × 10 −4 Pa⋅s (at 25 ○ C).…”

“…Accordingly, an additional study is required in order to understand if the models employed in the theoretical analysis are also compatible with the experimental observations of sliding states occurring at upper planar boundaries (ceiling). 11,12,25,28 Additional motivation for such a study arises from the question of how active particles, which are both heavy (i.e., they sediment to the floor in the absence of chemical "fuel") and bottomheavy (i.e., there is a gravitational torque due to the catalytic cap), may reach a state of sliding at the ceiling. One scenario, applicable for active particles moving with the active cap at the back, is the following.…”

Clustering-induced velocity-reversals of active colloids mixed with passive particles

“…Phoretic interactions may therefore be sufficient to describe the key aspects of the active-passive colloidal mixtures explored in 15 , although hydrodynamic interactions, which we neglect here, may generally be also important for some (other) active colloids and active-passive mixtures 46 (see also the interesting very recent flow field measurements in ref. 47 ).…”

Flow field around a confined active droplet

“…Experimental studies of the effect of confinement on phoretic swimmers concentrate on the kinetics of the particle trajectories and very little is known about the actual flow field. On some occasions, it has been measured in the median plane of the swimmer [16,40,51,52], and used for qualitative discussion. Yet, a precise and quantitative description of this flow field is critical, in particular to understand the role of hydrodynamic coupling between swimmers in setting their collective dynamics.…”