Non-equilibrium transport in inhomogeneous fluids

Binary mixture (well-mixed)
well-mixed state
Binary mixture (critical opalescence)
critical opalescence
Binary mixture (phase separation)
A binary mixture of critical composition above (top: mixing), just at (centre: critical opalescence), and below (bottom: phase separation) its consolute temperature. The images show 700,000 particles each.

Nanofluidic applications rely on mesoscopic amounts of liquid guided across lab-on-the-chip devices. The liquids are naturally confined by solid walls or by liquid–liquid or liquid–vapour interfaces and driven by external fields. Particularly, large gradients of pressure, density, concentration, or temperature in conjunction with a phase transition lead to interesting non-equilibrium behaviour, which so far has remained largely unexplored. For example, there is evidence that local heating of a mixture near its consolute point can induce stationary mass currents, which appear to be at the root of a novel type of microswimmer driven by light [3]. Unravelling the mechanism that converts the heat absorbed from bright-field illumination into mechanical work is the goal of an ongoing project.

[3]    G. Volpe, I. Buttinoni, D. Vogt, H.-J. Kümmerer, and C. Bechinger, Soft Matter 7, 8810 (2011).