Self-Propulsion of Chemically Active Droplets

Abstract

Microscopic active droplets are able to swim autonomously in viscous flows. This puzzling feature stems from solute exchanges with the surrounding fluid via surface reactions or their spontaneous solubilization and from the interfacial flows resulting from these solutes’ gradients. Contrary to asymmetric active colloids, these isotropic droplets swim spontaneously by exploiting the nonlinear coupling of solute transport with self-generated Marangoni flows; such coupling is also responsible for secondary transitions to more complex individual and collective dynamics. Thanks to their simple design and their sensitivity to physico-chemical signals, these droplets are fascinating to physicists, chemists, biologists, and fluid dynamicists alike in analyzing viscous self-propulsion and collective dynamics in active-matter systems, developing synthetic cellular models, or performing targeted biomedical or engineering applications. I review here the most recent and significant developments of this rapidly growing field, focusing on the mathematical and physical modeling of these intriguing droplets, together with their experimental design and characterization. Expected final online publication date for the Annual Review of Fluid Mechanics, Volume 55 is January 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Weakly nonlinear dynamics of a chemically active particle near the threshold for spontaneous motion. II. History-dependent motion

“…In the forced case, it has been incorrectly guessed in [10] that both the parallel and larger-magnitude anti-parallel states are stable, while the smaller-magnitude anti-parallel state is unstable (we have shown that the larger-magnitude anti-parallel state is transversely unstable). This claim has been referenced in a recent review on active drops [4] and in a recent axisymmetric numerical study [13]. We note that earlier weakly nonlinear analyses of forced active drops have concluded similarly to here that there is a stable parallel state and, in some cases, also two unstable anti-parallel states.…”

“…In contrast, the two anti-parallel steady states that exist above a critical Péclet value are both unstable, however the larger-magnitude anti-parallel state is stable under longitudinal perturbations. These findings agree with the heuristic 'relative-stability' predictions in [6,7] for a closely related active-drop model, but contradict the guess made in [10], later referenced in [4] and [13], that the larger-magnitude anti-parallel state is stable. Motivated by the instability of the anti-parallel states, we have studied the alignment dynamics of a spontaneously moving particle following the sudden application of a force.…”

“…The starting point for the theory is the canonical model proposed by Michelin et al [1] describing an autocatalytic colloid that self-propels despite having uniform surface properties, the underlying mechanism being a hydro-chemical instability that can occur when diffusion is sufficiently weak relative to advection. While in practice diffusion dominates in experiments involving autocatalytic colloids [2,3], the canonical active-particle model serves as a simplified yet physically consistent reference model for self-solubilizing chemically active drops whose spontaneous motion has been observed in many experiments (see the recent review [4] and references therein). As discussed in the first part of this series [5] (henceforth 'part I'), weakly nonlinear analyses of the canonical active-particle model and similar models of self-solubizing active drops have previously been limited to steady, axisymmetric solutions [6][7][8][9][10].…”

Dynamics of Active SiO2–Pt Janus Colloids in Dilute Poly(ethylene oxide) Solutions

“…Garnering inspiration from micro-organisms such as bacteria, algae, and spermatozoa, over the past couple of decades, researchers have devised artificial micro-swimmers . − By utilizing an in-built mechanism which breaks the symmetry of their interactions with the surrounding medium, these artificial machines can perform autonomous motion analogous to the locomotion of microorganisms. On choosing appropriate chemistry, they typically self-generate either a chemical or a physical gradient in their vicinity, which drives their motion.…”

New types of complex motion of a simple camphor boat

“…Many types of non-biological self-propelled objects have been reported so far, e.g. Janus particles, [6][7][8][9] swimming droplets, [10][11][12][13][14][15] Quincke rollers, 16,17 and so on.…”