Dynamics at a Janus interface

Michael von Domaros, Dusan Bratko, Barbara Kirchner and Alenka Luzar, University of Bonn, Bonn, Germany

Electric field effects on water interfacial properties abound, ranging from electrochemical cells to nanofluidic devices to membrane ion channels. On the nanoscale, spontaneous orientational polarization of water couples with field alignment, resulting in an asymmetric wetting behavior of opposing surfaces – a field-induced analog of a chemically generated Janus interface. Using atomistic simulations, we uncover a new and significant field polarity (sign) dependence of polarization dynamics in the hydration layer. Applying electric fields across a nanoparticle, or a nanopore, can lead to ?close to two orders of magnitude difference in response times of water polarization at opposite surfaces. Typical time scales are within the O(0.1) to O(10) picosecond regime. Temporal response to the field change also reveals strong coupling between local polarization and interfacial density relaxations, leading to a nonexponential and in some cases, nonmonotonic response.

This work highlights the surprisingly strong asymmetry between reorientational dynamics at surfaces with incoming and outgoing fields, which is even more pronounced than the asymmetry in static properties of a field-induced Janus interface.