(654g) Convective Fluid Motions in Droplets Driven By Global and Local Chemical Gradients

Colloidal migration in chemical gradients are important to both scientific studies and industrial applications. Chemical gradients can generate interfacial flows between no-slip particle surfaces and bulk fluids, driving diffusiophoretic motion. Meanwhile, interfacial tension gradients may develop due to non-uniform distribution of chemicals, giving rise to Marangoni flows that yield solutocapillary motion. Here, we present studies on these two types of motions associated with droplets by creating global and local chemical environments in dispersed droplet systems.

First, we show diffusiophoretic transport of droplets in external electrolyte gradients. In addition, gradients of surface-active chemicals may direct droplet migration oppositely compared with solid particles, due to different boundary conditions at particle surfaces. Second, we demonstrate spontaneous flows inside sessile drops (immersed in oil) driven by local chemical gradients. In a ternary water-oil-butanol system, coupled interfacial transport of butanol between two adjacent droplets yield fluid flows. Patterns of these flows depend on the direction of interfacial mass transport, which can be understood in terms of induced Marangoni stresses at droplet interfaces, owing to the inhomogeneous transport. Such effects appear important in a variety of flows of both scientific and practical interest. Moreover, we find that convective flows even exist in single droplets when butanol is present. Internal flows of droplets are directed radially inward at the plane near the supporting substrate, regardless of interfacial mass transport direction.