4 research outputs found
Charging of dielectric surfaces in contact with aqueous electrolyte -- the influence of CO
The charge state of dielectric surfaces in aqueous environments is of
fundamental and technological importance. We use super-heterodyne light
scattering in a custom-made cell to study the influence of dissolved CO on
the charging of three, chemically different surfaces. We compare an ideal,
CO-free reference state to ambient CO conditions. Systems are
conditioned under conductometric control at different low concentrations of
NaCl. As expected for constant charge densities, zeta-potentials drop upon
increasing the salt concentration in the reference state. Presence of CO
leads to an overall lowering of zeta-potentials. Moreover, for the inorganic
dielectric, the salt dependent drop is significantly weakened, and it is
inversed for the organic dielectrics. We suggest that at ambient conditions,
the charge state of dielectric surfaces is related to dielectric charge
regulation caused by the salt concentration dependent adsorption/desorption of
CO.Comment: Thoroughly revised version, extended data, improved interpretation,
Main text and supplementary materials, 23 Pages, 11 figures, submitted to
Angew. Chem. Int E
Spontaneous charging affects the motion of sliding drops
Water drops moving on surfaces are not only an everyday phenomenon seen on windows but also form an essential part of many industrial processes. Previous understanding is that drop motion is dictated by viscous dissipation and activated dynamics at the contact line. Here we demonstrate that these two effects cannot fully explain the complex paths of sliding or impacting drops. To accurately determine the forces experienced by moving drops, we imaged their trajectory when sliding down a tilted surface, and applied the relevant equations of motion. We found that drop motion on low-permittivity substrates is substantially influenced by electrostatic forces. Our findings confirm that electrostatics must be taken into consideration for the description of the motion of water, aqueous electrolytes and ethylene glycol on hydrophobic surfaces. Our results are relevant for improving the control of drop motion in many applications, including printing, microfluidics, water management and triboelectric nanogenerators
High Voltages in Sliding Water Drops
Water drops on insulating hydrophobic substrates can generate electric potentials of kilovolts upon sliding for a few centimeters. We show that the drop saturation voltage corresponds to an amplified value of the solid–liquid surface potential at the substrate. The amplification is given by the substrate geometry, the drop and substrate dielectric properties, and the Debye length within the liquid. Next to enabling an easy and low-cost way to measure surface- and zeta- potentials, the high drop voltages have implications for energy harvesting, droplet microfluidics, and electrostatic discharge protection
How charges separate when surfaces are dewetted
Charge separation at moving three-phase contact lines is observed in nature as well as technological processes. Despite the growing number of experimental investigations in recent years, the physical mechanism behind the charging remains obscure. Here we identify the origin of charge separation as the dewetting of the bound surface charge within the electric double layer by the receding contact line. This charge depends strongly on the local electric double layer structure close to the contact line, which is affected by the gas-liquid interface and the internal flow of the liquid. We summarize the charge separation mechanism in an analytical model that captures parametric dependencies in agreement with our experiments and numerical simulations. Charge separation increases with increasing contact angle and decreases with increasing dewetting velocity. Our findings reveal the universal mechanism of charge separation at receding contact lines, relevant to many dynamic wetting scenarios, and provide a theoretical foundation for both fundamental questions, like contact angle hysteresis, and practical applications