(552g) Autonomous Ion Current and Bubble Oscillations in a Capillary Due to Localized Film Rupture Events During Electro-Dewetting

Onsager reciprocal relationship stipulates that, for linear response near equilibrium, ion and mass transport driven by constant external field gradients cannot exhibit sustained oscillations in time.  Yet, ion current through the ion channel of a heart cell does oscillate and so does some recent measurements of ion current through conic nanopores (see, for example, some data out of Siwy's group in JACS, 131, 5194(2009)). Such oscillations must be driven by non-equilibrium and nonlinear (possibly hysteretic) phenomena.  An earlier effort (Kobatake and Fujita, J Chem Phys, 40, 2219(1964)) employs non-equilibrium mixed electro-osmotic and pressure-driven flow in a capillary, generated by two opposing concentration and pressure gradients, to produce current and flow oscillations.  We show in this work that a more robust oscillation of both can be driven by an electric field, when bubble displacement by a mixed flow in a capillary is regulated by an irreversible film rupture event that results from a Maxwell interfacial instability during dewetting. We hence employ the extremely non-equilibrium, irreversible and hysteretic wetting and dewetting events to drive the observed oscillations. To sustain flow balance within and outside a nanoporous medium, an electric field through an ion-selective silica monolith within a capillary can sustain a large back pressure at the end of the monolith.  We use this back pressure to compress a trapped bubble at the end of a capillary opposite from the silica monolith.  At quasi-steady state, the meniscus of the bubble is stationary and the overall mixed flow rate is zero. However, the local liquid velocity is non-zero at this quasi-steady state--the system is far from thermodynamic equilibrium.  The electric field is focused into the surrounding film of the trapped bubble and the corresponding ion current can be short-circuited by the film rupture.  Beyond a critical voltage such that the quasi-steady state cannot be sustained, the electro-osmotic flow exceeds the pressure driven flow and expands the bubble volume by displacing its meniscus. This backward (relative to the liquid phase) meniscus motion resembles electro-dewetting except there is a liquid film surrounding the expanding bubble.  Using confocal imaging, we show the film to rupture abruptly during this forward motion to shut off the field and the electro-osmotic flow.  This reverses the meniscus motion and results in an opposite forward motion driven by the back pressure. The subsequent electrowetting event recoats the ruptured film to regenerate the electro-osmotic flow and restore the backward meniscus motion.  It is hence the hysteretic dependence of the film thickness, due to the irreversibility of the rupture event relative to the direction of meniscus motion, and the positive feedback offered by the compressed bubble that drives this curious oscillation phenomenon.  We find certain universal meniscus and current oscillation amplitude scaling that reflect the discrete rupture locations of the expanding bubble due to the Maxwell interfacial instability.  The discrete rupture location offers the lag, hysteretic gap and time delay that is necessary to produce finite amplitude oscillations. We hence offer precise experimental imaging of this rupture event and correlate it to measured current/meniscus oscillations, as well as a dewetting rupture theory that includes Maxwell pressure effects. The dewetting rupture event sometimes exhibit Oswald drop coarsening pattern dynamics, reminiscent of film ruptures by molecular forces at much smaller thickness (Oron et al, Rev of Mod Phys, 69, 931(1997)).