Ions in an AC Electric Field: Strong Long-Range Repulsion between Oppositely Charged Surfaces

Transient fluctuation-induced forces in driven electrolytes after an electric field quench

“…These rich features point towards unexplored methods of force manipulation in practical applications, for example to control neutral colloidal particles that are immersed in an electrolyte solution. Moreover, they resemble some of the experimentally observed temporal patterns of force variation in surface measurements [29,31]; this implies that fluctuation effects which are generally discarded in mean-field models can indeed be relevant to understanding the force generation mechanisms in charged solutions out of equilibrium.…”

“…Since the typical Debye screening length is of the order of κ −1 ∼ 1 − 10 nm [1], such approximation is justified for studying the dynamics of the electrolyte beyond the screening scale. Accordingly, the corresponding FIFs that will be calculated below can in principle be realized in settings where the boundary separations are larger than the screening scale of the electrolyte which, for instance, may be the case for wet ion channels such as mechanosensitive channels [69], synthetic nanopores [70], and force measurement setups with large inter-plate separations [29,31]. Equation (13) shows that the charge density which persists beyond the relaxation time ∼ 1/(Dκ 2 ) is proportional to the gradient of the number density along the direction of the electric field.…”

“…Langevin formulations of the electrolyte dynamics provide a straightforward way of taking into account the stochasticity in the motion of the ions and can be used to obtain the required correlation functions, e.g., to compute the power spectrum of nanopores or the conductivity of strong electrolytes [21][22][23]. Inspired by some of the recent investigations of electrolytes and ionic liquids [24][25][26][27][28][29][30][31], we use a similar mathematical framework to study how fluctuations in a strong electrolyte in the presence of an external electric field may modify the force that is exerted on boundaries that confine the electrolyte. This direction would also be relevant for designing more efficient and environmental-friendly batteries and electrochemical capacitors [32][33][34] as electrolytes and ionic liquids are essential to the underlying conduction processes and energy storage.…”

Controlling adhesion using AC electric fields across fluid films

“…The force between the surfaces brought about by applying an AC field across fluid depends on (i) magnitude of the field, with the 'steady state' force imposed by the field, F ss , scaling as F ss ∼ V 2 0 (for both molecular and ion-containing fluid media) [18,36], (ii) dielectric and electrokinetic properties of the medium [18,36], and (iii) geometric factors D 0 , T m , and R. For dielectric fluids, such as molecular liquids, the force between the electrodes is simply the attraction between charged capacitor plates and has been established quantitatively in our experimental setup [18]. When the fluid contains ions -either as a pure ionic liquid as in the present work or as a diluted electrolyte -the magnitude of F ss is substantially larger than this simple capacitor force and evolves over slower timescales than the viscous drainage timescale, as reported recently [18,36]. The origin of this additional force induced by the action of the electric field on the mobile ions remains intriguing and is not yet fully resolved, although strong clues of the importance of ion asymmetry have been provided by Drummond and coworkers [36].…”

“…is reminiscent of the observed colloidal forces under AC fields in electrolyte reported by Bukosky, Ristenpart et al [37,38]; in their later calculations [39] it was pointed out that oscillating electric fields in electrolytes can create a long-range steady field. Separately, Stone, Holyst and Drummond have suggested the origin of the additional force lies in an excess osmotic pressure due to ions drawn into the region of high field from surrounding reservoirs [36]. Here, without attempting to resolve this mechanistic question, we simply make use of the property that F ss = K liq V 2 0 , with fitting parameter K liq dependent on the liquid and contact geometry.…”