Previous experimental data on mass transfer between particles and fluid in fixed and fluidized beds are reanalyzed and correlating equations are developed for the various situations.

The active transport of micron sized 4′-pentyl-4-biphenylcarbonitrile (5CB) liquid crystal droplets propelled through an aqueous solution of tetradecyltrimethylammonium bromide (TTAB) as surfactant and glycerol as a solute, is investigated. On addition of glycerol, it was observed that the motion of active 5CB droplets exhibited a transition from smooth to a jittery chaotic motion. The motion was further found to be independent of the droplet size and the nematic state of 5CB. Upon conducting analogous experiments with Polyacrylamide (PAAM) as the solute, it was confirmed that a mere increase in viscosity cannot explain the transition. We propose the physicochemical interactions of glycerol with TTAB and 5CB, as the main cause responsible for the observed jittery motion. Presence of glycerol significantly enhances the rate of solubilization of the 5CB droplets resulting in a quicker re-distribution of the adsorbed TTAB molecules on the interface causing the droplet to momentarily stop and then restart in an independent direction. This hypothesis is supported by the time evolution of droplets size and interfacial velocity measurements in the presence and absence of glycerol. Overall, our results provide fundamental insights into the scheme of complex interactions emerging due to the presence of a non-reactive solute such as glycerol.

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Rheotaxis is a well-known phenomenon among microbial organisms and artificial active colloids, wherein the swimmers respond to an imposed flow. We report the first experimental evidence of upstream rheotaxis by spherical active droplets. It is shown that the presence of a nearby wall and the resulting strong flow-gradient at the droplet level is at the root of this phenomenon. Experiments with optical cells of different heights reveal that rheotaxis is observed only for a finite range of shear rates, independent of the bulk flow rate. We conjecture that the flow induced distortion of an otherwise isotropic distribution of filled/empty micelles around the droplet propels it against the flow. We also show that nematic droplets exhibit elastic stress-induced oscillations during their rheotactic flight. A promising potential of manipulating the rheotactic behavior to trap as well as shuttle droplets between target locations is demonstrated, paving way to potentially significant advancement in bio-medical applications.

Understanding the motion of artificial active swimmers in complex surroundings, such as a dense bath of passive particulate matter, is essential for their successful utilization as cargo (drug) carriers and sensors or for medical imaging, under microscopic domains. In this study, we experimentally investigated the motion of active SiO2–Pt Janus particles (JPs) in a two-dimensional bath of smaller silica tracers dispersed with varying areal densities. Our observations indicate that when an active JP undergoes a collision with an isolated tracer, their interaction can have a significant impact on the swimmer’s motion. However, the overall impact of tracers on the active JPs’ motion (translation and rotation) depends on the frequency of collisions and also on the nature of the collision, which is marked by the time-duration for which the particles maintain contact during the collisions. Further, in the high-density tracer bath, our experiments reveal that the motion of the active JP results in a novel organizational behavior of the tracers on the trailing Pt (depletion of tracers) and the leading SiO2 (accumulation of tracers) side. In laboratory frame the emergence and the subsequent vanishing of the depletion zone are discussed in detail.