John Brady - US grants

This award is a collaboration between an experimental group at the University of Potsdam in Germany and a theoretical group at the California Institute of Technology to study the light-activated chemically induced flow of small particles in confined geometries. By applying simple optical stimuli, colloidal particles, both individually and in bulk, can be manipulated with unprecedented levels of control and precision. The research aims to establish the foundation for ‘virtual’ microfluidic devices that utilize soft boundaries generated from light-activated flow patterns whose strength and location are tunable and allow the manipulation and control of colloidal-scale objects. The research has potential technological applications in, for example, cell sorting, DNA manipulation, and the assembly of colloidal-based materials. The collaborative nature of the award will help broaden the horizon of research students as they work with others from different countries and backgrounds, which is increasingly important in this globally connected but fragile world.<br/><br/>This award builds on and extends the recently discovered phenomenon of light-driven diffusion-osmosis in which a chemical surfactant’s hydrophobicity is altered by illumination and generates an osmotic pressure gradient that drives fluid and particle motion. The study is aimed at a fundamental understanding of the diffusiophoretic flow; that is, how the motion depends on the basic physical properties of chemical concentration, ionic strength, light intensity, etc., as well as the hydrodynamic interactions among particles and with the confining substrate. Three different processes to manipulate ensembles of particles adjacent to a boundary are investigated: (i) global spatial patterns of light intensity that cause particles to accumulate in (vacate from) regions of low (high) solute concentration, allowing one to ‘paint’ with colloids; (ii) self-generated repulsion between porous (source) particles that crystalize and enhance motion of trapped passive colloids; and (iii) self-propelled Janus particles whose speed and duration can be dynamically controlled. Theory suggests that the light-induced flow profiles, despite being non-equilibrium phenomena, can be expressed in terms of an equilibrium-like chemical ‘solute potential,’ which suggests intriguing analogies to crystallization, phase separation, etc. The award is a close alignment of theory and experiment regarding a unique non-equilibrium system that touches upon many cutting-edge problems of phoretically-driven particle dynamics and hydrodynamics, such as segregation dynamics in mixtures of particles and the motion of active self-propelled particles in dynamically fluctuating confined geometries.<br/><br/>This project was awarded through the “Chemistry and Transport in Confined Spaces (NSF-DFG Confine)" opportunity, a collaborative solicitation that involves the National Science Foundation and Deutsche Forschungsgemeinschaft (DFG).<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.