From birth to health, surfactants play an essential role in our lives. Due to the importance, their environmental impacts are well understood. One of the aspects that has been extensively studied is their impact on bacteria, particularly on their motility. Here, we uncover an alternate chemotactic strategy triggered by surfactantsdiffusiophoresis. We show that even a trace amount of ionic surfactants, down to a single ppm level, can promote the bacterial diffusiophoresis by boosting the surface charge of the cells. Because diffusiophoresis is driven by the surface–solute interactions, surfactant-enhanced diffusiophoresis is observed regardless of the types of bacteria. Whether Gram-positive or -negative, flagellated or nonflagellated, the surfactants enable fast migration of freely suspended bacteria, suggesting a ubiquitous locomotion mechanism that has been largely overlooked. We also demonstrate the implication of surfactant-enhanced bacterial diffusiophoresis on the rapid formation of biofilms in flow networks, suggesting environmental and biomedical implications.

The transport of nanoparticles in biological hydrogels is often hindered by the strong confinement of the media, thus limiting important applications such as drug delivery and disinfection. Here, we investigate nanoparticle transport in collagen hydrogels driven by diffusiophoresis. Contrary to common expectations for boundary confinement effects where the confinement hinders diffusiophoresis, we observe a nonmonotonic behavior in which maximum diffusiophoretic mobility is observed at intermediate confinement. We find that such behavior is a consequence of the interplay between multiple size-dependent effects. Our results display the utility of diffusiophoresis for enhanced nanoparticle transport in physiologically relevant conditions under tight confinement, suggesting a potential strategy for drug delivery in compressed tissues.

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Electroosmosis on nonuniformly charged surfaces often gives rise to intriguing flow behaviors, which can be utilized in applications such as mixing processes and designing micromotors. Here, we demonstrate nonuniform electroosmosis induced by electrochemical reactions. Water electrolysis creates pH gradients near the electrodes that cause a spatiotemporal change in the wall zeta potential, leading to nonuniform electroosmosis. Such nonuniform EOFs induce multiple vortices, which promote the continuous accumulation of particles that subsequently form a colloidal band. The band develops vertically into a “wall” of particles that spans from the bottom to the top surface of the chamber. Such a flow‐driven colloidal band can be potentially used in colloidal self‐assembly and separation processes irrespective of the particle surface properties. For instance, we demonstrate these vortices can promote rapid segregation of soft colloids such as oil droplets and fat globules.

We present diffusiophoresis of ellipsoidal particles induced by ionic solute gradients. Contrary to the common expectation that diffusiophoresis is shape-independent, here we show experimentally that this assumption breaks down when the thin Debye layer approximation is relaxed. By tracking the translation and rotation of various ellipsoids, we find that the phoretic mobility of ellipsoids is sensitive to the eccentricity and the orientation of the ellipsoid relative to the imposed solute gradient, and can further lead to non-monotonic behavior under strong confinement. We show that such shape-and orientation-dependent diffusiophoresis of colloidal ellipsoids can be easily captured by modifying theories for spheres.