An important aim of microfluidics is to miniaturise reaction vessels and connecting conduits and to understand the fluid flow on these ``lab-on-a-chip" dimensions. However the influence of the walls or boundaries, while negligible in bulk, becomes dominant in small vessels and channels. Near solid walls, the hydrodynamic drag or friction on a micron-sized colloidal particle is doubled and becomes anisotropic, with implications on transport and energy requirements of the micro flow. But these boundaries don't have to be solid surfaces, they can also be soft surfaces such as vesicle membranes, having deformation energies on the order of thermal energy. The softness of such boundary walls, i.e. their response to flow and thermal fluctuations, plays a significant role in fluid microflow that remains virtually unexplored. As an example, consider the biological cell, the most ubiquidous of microfluidic devices: it has soft walls whose flexibility changes during processes such as budding or phagocytosis. In this project, we have devised an optical tweezers-based method to measure particle mobility near interfaces and we apply these to a number of problems.
In 2009, members of the group reported the first measurements of distance-dependent, anisotropic colloidal friction near fluid-fluid interfaces. Our technique uses optical tweezers to localise a transparent colloidal particle near the bounding interface of water. Our precise measurements of the particle's fluctuations in the optical trap provide us with a quantitative measure of the particle's friction and detail the local hydrodynamic boundary condition of the surface or interface. Near the liquid-vapor interface, our experiments show for the first time that friction decreases below that in the bulk, corresponding to predictions of a ‘‘perfect-slip’’ surface. More recently we measured the mobility of a particle in water near a high surface tension interface while changing the viscosity of an immiscible fluid on the opposite side of the interface, demonstrating that you can “tune-out” the distance-dependence of the particle mobility. These experimental results near high surface tension interfaces are the first of their kind to demonstrate hydrodynamic predictions of the 1980s. We are currently extending our measurements to soft and permeable model interfaces where predictions are scant and precise measurements are needed.