Chemical gradients provide an invisible but powerful way to transport microscopic objects. In this project, we explored how colloidal particles interact with solute gradients and discovered that such gradients can not only move individual particles but also organize collections of them into dynamic patterns. This mechanism, diffusiophoresis, arises because short-range interactions between a particle’s surface and surrounding solute molecules induce a slip velocity, driving motion without any external flow.

We first demonstrated that a single colloidal drop exposed to a solute gradient drifts with remarkable sensitivity, traveling distances far larger than its own size. Extending this to self-generated gradients, we showed that chemically active colloids can undergo “autophoresis,” propelling themselves by releasing or consuming solute at their surfaces. When many such particles are present, their overlapping chemical fields couple through the fluid, producing collective behaviors: clusters that move together, particles that “chase” one another, and dynamic assemblies that can merge, split, or reorient.

The novelty of this work is twofold. First, we built a general hydrodynamic framework for interacting active particles, allowing us to predict both translation and rotation of each particle directly from their surface chemistry. Second, we uncovered new emergent behaviors, such as spontaneous pairing and pattern switching, that demonstrate how simple chemical rules at the microscale can drive complex collective dynamics.

This research highlights chemical gradients as a versatile tool for engineering active matter. Diffusiophoresis links molecular-scale chemistry to mesoscale transport, suggesting strategies for targeted drug delivery, programmable self-assembly, and the design of synthetic microswimmers that harness ambient solute gradients.

Publications

  • 📄 F. Yang, S. Shin, and H. A. Stone. Diffusiophoresis of a charged drop. Journal of Fluid Mechanics (2018). HTML
  • 📄 F. Yang, B. Rallabandi, and H. A. Stone. Autophoresis of two adsorbing/desorbing particles in an electrolyte solution. Journal of Fluid Mechanics (2019). HTML
  • 📄 B. Rallabandi, F. Yang, and H. A. Stone. Motion of hydrodynamically interacting active particles. ArXiv (2019)

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