Nanoscale Electrowetting Effects Observed by Using Friction Force Microscopy

Abstract

We report the study of electrowetting (EW) effects under strong electric field on poly(methyl methacrylate) (PMMA) surface by using friction force microscopy (FFM). The friction force dependence on the electric field at nanometer scale can be closely related to electrowetting process based on the fact that at this scale frictional behavior is highly affected by capillary phenomena. By measuring the frictional signal between a conductive atomic force microscopy (AFM) tip and the PMMA surface, the ideal EW region (Young-Lippmann equation) and the EW saturation were identified. The change in the interfacial contact between the tip and the PMMA surface with the electric field strength is closely associated with the transition from the ideal EW region to the EW saturation. In addition, a reduction of the friction coefficient was observed when increasing the applied electric field in the ideal EW region.

On the Derjaguin Offset in Boundary-Lubricated Nanotribological Systems

“…Over the past few decades, it was proven many times, − both experimentally and numerically, that Derjaguin’s extension holds and can accurately describe the friction force during sliding when adhesion is involved. Recently, the law was used to study the friction between a glass particle and oligopeptides grafted on a glass plate, the stiction suppression properties of ionic liquids, or the nanoscale electrowetting effects of poly(methyl methacrylate) under strong electric fields . One group performed friction force microscopy on numerous types of monolayer systems and polymers on various substrates, − obtaining data consistent with Derjaguin’s friction law.…”

Evaluation of the Electrowetting Effect on the Interfacial Mechanics between Human Corneocytes and Nanoasperities

“…While previous AFM studies have investigated electrowetting on dielectrics, the results are not quantitative and do not isolate electrowetting effects from electroadhesion and capacitive effects between the AFM tip and sample. 34,35 There is thus a need to develop an AFM technique that can effectively isolate the electrowetting effect from the myriad of other occurring effects, in order to better understand how electrowetting impacts adhesion and friction in electroadhesive haptic devices and ultimately engineer systems with higher sensitivity and more robust operation.…”

“…At the micro- and nanometer scale, humidity in the air causes water to condense and form capillary bridges at contacting asperities within a solid–solid interface, which contributes to the overall adhesion and friction within the interface. − Many studies have previously shown that the friction at a skin–solid interface highly depends on the humidity by hydrating and softening the skin surface, increasing adhesion via capillary bridges, and/or lubricating the contact at very high humidity. , However, it is unclear what the relative effects of each mechanism are, and it is further unclear what effect, if any, electrowetting would have in such systems. While previous AFM studies have investigated electrowetting on dielectrics, the results are not quantitative and do not isolate electrowetting effects from electroadhesion and capacitive effects between the AFM tip and sample. , There is thus a need to develop an AFM technique that can effectively isolate the electrowetting effect from the myriad of other occurring effects, in order to better understand how electrowetting impacts adhesion and friction in electroadhesive haptic devices and ultimately engineer systems with higher sensitivity and more robust operation.…”

Studying the Adhesion Force and Glass Transition of Thin Polystyrene Films by Atomic Force Microscopy

“…At this stage, the rougher surface and the softer surface of normal thin PS films could induce a larger tip indentation, which causes increasing the real contact area between the AFM tip and surfaces. As has been reported, the adhesion force is proportional to the real contact area between surface asperities [ 22 , 29 ]. Therefore, the greater contact area results in the greater contact force.…”

“…The F ad measurements were conducted under the cooling down process from a temperature higher than the T g of bulk materials with a cooling down rate of 2 K/min. The relative humidity is controlled below 10% since the capillary menisci formed between the tip and the film surface could contribute to the measured forces [ 22 ].…”