Electrical surface charge patterns induced by droplets sliding over polymer and photoresist surfaces

Abstract

We have studied the generation of surface charge distributions on polymer surfaces due to the deposition, motion and evaporation of liquid droplets. Using liquids with different static dielectric constants, we found that the magnitude of the surface charge conforms well to the Boltzmann relation for the dissociation probability of ions in solution, indicating that the phenomenon is electrokinetic in nature. The origin of the sensitive dependence of the surface charge on the substrate thickness was identified to be an image charge effect. Experiments on polycarbonate and chemically-amplified photoresist yield surface charges of opposite polarity. By varying the photoresist composition, we identified the leaching of negative quencher ions as the reason for the charge inversion.

Electrowetting‐Assisted Generation of Ultrastable High Charge Densities in Composite Silicon Oxide–Fluoropolymer Electret Samples for Electric Nanogenerators

“…Somewhat intriguingly, the actual nature of the charge carriers—electronic or ionic—in our experiments is not clear. The same uncertainty applies to other corona‐ and triboelectric charging experiments [ 17,18,20,43–45 ] and also to the origin of the long debated negative surface charge of FP–water interfaces. [ 17,53–55 ] While previous EW experiments demonstrated a clear dependence of the dielectric breakdown of thin FP films on the nature of ions, [ 50 ] test extensions of the present measurements using purely electron‐conducting liquid metals (data not shown) displayed similar charging behavior as the water‐based experiments discussed above.…”

“…In fact, these states might be the ones that are populated spontaneously upon continued exposure of FP surfaces to water, [ 53 ] and by the slide/triboelectrification process. [ 2–4,12,19,43–45 ] The majority of charge carriers in our experiments, however, is only injected when the electric field exceeds the critical field E BD,P . The conclusion derived above that all injected charge carriers accumulate at the FP–oxide interface implies that the energy barriers within the polymer are much smaller than the “injection barrier” at the water–FP interface (i.e., Φ P ≪ Φ I ).…”

“…On the basis of separate tests with hundreds of impinging drops (Figure S1, Supporting Information), we tentatively attribute this small U 0 to slide/triboelectrification, as described earlier by others. [ 19,43–45 ] In the opposite limit, for high charging voltages, the appearance of bubbles indicated dielectric breakdown and electrolysis beyond a critical voltage U BD ( d ox , d P ). [ 46 ] (In fact, the thickest sample types turned out to be stable up to the highest charging voltage of U C = −815 V available in our setup.)…”

“…In view of the homogeneity and electrical neutrality of water, the energy within the liquid is constant, except for the immediate vicinity of the polymer surface, where the electric double layer and other short range molecular forces give rise to an interfacial energy barrier (Φ I , see local potential maximum at the water–FP interface). Within the polymer, the charge carriers (which could be electrons or ions or both) [ 17,18,43–45 ] experience a distribution of energetically more favorable locations (traps) separated by local energy barriers (Φ P ). Amorphous fluoropolymers such as Teflon AF are known to be porous on the molecular scale (this is essential, e.g., for their application as gas filtration membranes), implying the presence of sub‐nanometric voids separated by polymer material.…”

Escape dynamics of liquid droplets confined between soft interfaces: non-inertial coalescence cascades

“…Moreover, we have measured electric surface charge distribution induced by solid-on-solid contact and separation, but it was below the detection limit (10 12 ions per m 2 ). 62,63 Since the droplets move smoothly and continuously without stick-slip behavior and since they maintain a round and mirror-symmetric morphology, it is highly unlikely that surface irregularities and defects are responsible for the slow motion and the cascading behavior. Non-homogeneities tend to cause irregular droplet footprints and non-monotonic velocity profiles.…”