IEEE Transactions on Pattern Analysis and Machine Intelligence

Annual review of condensed matter physics

Journal of Statistical Mechanics: theory and experiment

Tables of integrals

Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves

Bioinspired soft machines made of highly deformable materials are enabling a variety of innovative applications, yet their locomotion typically requires several actuators that are independently activated. We harnessed kirigami principles to significantly enhance the crawling capability of a soft actuator. We designed highly stretchable kirigami surfaces in which mechanical instabilities induce a transformation from flat sheets to 3D-textured surfaces akin to the scaled skin of snakes. First, we showed that this transformation was accompanied by a dramatic change in the frictional properties of the surfaces. Then, we demonstrated that, when wrapped around an extending soft actuator, the buckling-induced directional frictional properties of these surfaces enabled the system to efficiently crawl.

Kirigami skins make a simple soft actuator crawl

The dynamics of friction have been studied for hundreds of years, yet many aspects of these everyday processes are not understood. One such aspect is the onset of frictional motion (slip). First described more than 200 years ago as the transition from static to dynamic friction, the onset of slip is central to fields as diverse as physics1,2,3, tribology4,5, mechanics of earthquakes6,7,8,9,10,11 and fracture12,13,14. Here we show that the onset of frictional slip is governed by three different types of coherent crack-like fronts: these are observed by real-time visualization of the net contact area that forms the interface separating two blocks of like material. Two of these fronts, which propagate at subsonic and intersonic velocities, have been the subject of intensive recent interest12,13,14,15,16,17. We show that a third type of front, which propagates an order of magnitude more slowly, is the dominant mechanism for the rupture of the interface. No overall motion (sliding) of the blocks occurs until either of the slower two fronts traverses the entire interface.

Detachment fronts and the onset of dynamic friction

We present a visualization of the predicted instability in ionic conduction from a binary electrolyte into a charge selective solid. This instability develops when a voltage greater than critical is applied to a thin layer of copper sulfate flanked by a copper anode and a cation selective membrane. The current-voltage dependence exhibits a saturation at the limiting current. With a further increase of voltage, the current increases, marking the transition to the overlimiting conductance. This transition is mediated by the appearing vortical flow that increases with the applied voltage.

/pdf/direct-observation-of-a-nonequilibrium-electro-osmotic-2yass5u3of.pdf

Direct Observation of a Nonequilibrium Electro-Osmotic Instability

The behaviour of systems as diverse as earthquakes and hard drives is influenced by frictional motion and its strength. What at first glance appears to be a continuous sliding process between touching surfaces is in fact a product of a series of 'slip' and 'stick' events on the microscopic scale. The mechanism of evolution of frictional strength at this level, though, is still unclear. Ben-David et al. have studied the evolution of the local contact area between two sliding bodies (PMMA plastic blocks) and the motion of their interface, and find that it involves four distinct phases. Within microseconds, all the contact area reduction has occurred. This is followed by a rapid slip phase, then a sharp transition to much slower slippage culminating in a 'stick' phase when motion is arrested. After several hundred microseconds the contact area begins to increase again. These results provide a basis for a better understanding of this kind of motion in many technologically important contexts. From earthquakes to hard drives, frictional motion and its strength are involved in a wide range of phenomena. The strength of an interface that divides two sliding bodies is determined by both the real contact area and the contacts' shear strength. By continuous measurements of the concurrent local evolution of the real contact area and the corresponding interface motion from the first microseconds when contact detachment occurs, frictional strength is now characterized from short to long timescales. The evolution of frictional strength has great fundamental and practical importance. Applications range from earthquake dynamics1,2,3,4 to hard-drive read/write cycles5. Frictional strength is governed by the resistance to shear of the large ensemble of discrete contacts that forms the interface that separates two sliding bodies. An interface’s overall strength is determined by both the real contact area and the contacts’ shear strength6,7. Whereas the average motion of large, slowly sliding bodies is well-described by empirical friction laws3,8,9,10, interface strength is a dynamic entity that is inherently related to both fast processes such as detachment/re-attachment11,12,13,14 and the slow process of contact area rejuvenation6,7,13,15,16. Here we show how frictional strength evolves from extremely short to long timescales, by continuous measurements of the concurrent local evolution of the real contact area and the corresponding interface motion (slip) from the first microseconds when contact detachment occurs to large (100-second) timescales. We identify four distinct and inter-related phases of evolution. First, all of the local contact area reduction occurs within a few microseconds, on the passage of a crack-like front. This is followed by the onset of rapid slip over a characteristic time, the value of which suggests a fracture-induced reduction of contact strength before any slip occurs. This rapid slip phase culminates with a sharp transition to slip at velocities an order of magnitude slower. At slip arrest, ‘ageing’ immediately commences as contact area increases on a characteristic timescale determined by the system’s local memory of its effective contact time before slip arrest. We show how the singular logarithmic behaviour generally associated with ageing is cut off at short times16. These results provide a comprehensive picture of how frictional strength evolves from the short times and rapid slip velocities at the onset of motion to ageing at the long times following slip arrest.

Slip-stick and the evolution of frictional strength

The commonly accepted description of drops impacting on a surface typically ignores the essential role of the air that is trapped between the impacting drop and the surface. Here we describe a new imaging modality that is sensitive to the behavior right at the surface. We show that a very thin film of air, only a few tens of nanometers thick, remains trapped between the falling drop and the surface as the drop spreads. The thin film of air serves to lubricate the drop enabling the fluid to skate on the air film laterally outward at surprisingly high velocities, consistent with theoretical predictions. Eventually this thin film of air breaks down as the fluid wets the surface via a spinodal-like mechanism. Our results show that the dynamics of impacting drops are much more complex than previously thought, with a rich array of unexpected phenomena that require rethinking classic paradigms.

/pdf/skating-on-a-film-of-air-drops-impacting-on-a-surface-4d1pc9nihs.pdf

Shmuel M. Rubinstein

Papers

Kirigami skins make a simple soft actuator crawl

Detachment fronts and the onset of dynamic friction

Direct Observation of a Nonequilibrium Electro-Osmotic Instability

Slip-stick and the evolution of frictional strength

Skating on a film of air: drops impacting on a surface.