Clean data with dirty surfaces in electrokinetics

The field of electrokinetics, or electrically driven motion of particles and fluids, recently celebrated its 200th birthday [1, 2] and yet it remains a frontier of theoretical physics. Stringent tests of the standard model used by workers in this field are now possible in microfluidic devices using alternating-current [3] or induced-charge electro-osmotic (ICEO) flows [4]. Unlike classical electroosmosis, where the electric field acts on ions in the double layer screening a surface of fixed charge, ICEO flows involve electrically induced, rather than chemically derived, diffuse charge near polarizable (e.g., metal) surfaces, and this enables more direct tests of the theory. In most experiments, however, the model tends to underpredict observed velocities, sometimes by orders of magnitude [5]. Remarkably, the first quantitative agreement between theory and experiment, now reported in Physical Review Letters[6] by Andrew Pascall and Todd Squires at the University of California, Santa Barbara, involves the use of dirty metal surfaces, “controllably contaminated” by thin oxide films (Fig 1). Besides its fundamental significance, this work suggests that surface treatment may be the key to developing robust ICEO-based microfluidic devices.