Moiré-Assisted Strain Transfer in Vertical van der Waals Heterostructures.

Strain provides a powerful method to study 2D monolayers and to tune their properties. The same approach also has great potential for van-der-Waals (vdW) heterostructures. However, we need to understand how strain can be applied to vertically stacked vdW structures, for which strain transfer from one layer to the next remains little explored. In our experiment, we fabricated vertical heterostructures consisting of transition metal dichalcogenides (TMDCs) monolayers that were deposited on a flexible substrate. These TMDC heterostructures allowed us to read out separately the strain in each monolayer by photoluminescence measurements. We find that, in TMDC heterostructures with large twist angles (>5°), strain transfer is limited. However, for aligned heterostructures with small twist angles (≤5°), near unity strain transfer efficiency is observed. We correlate this finding with the moiré domains formed in the aligned heterostructures by reconstruction.

Spreading Kinetics of Ultrathin Liquid Films Using Molecular Dynamics.

Ultrathin liquid films play a critical role in numerous engineering applications. Although crucial to the design and application of ultrathin liquid films, the physical mechanisms that govern spreading on the molecular scale are not well-understood, and disagreement among experiments, simulations, and theory remains. We use molecular dynamics simulations to quantify the speed at which the edge of a polymer droplet advances on a flat substrate as a function of various environmental and design parameters. We explain the physical mechanisms that drive and inhibit spreading, identify different spreading regimes, and clarify transitions between spreading regimes. We demonstrate that the edge of a droplet spreads according to a power law with two distinct regimes, which we attribute to competing physical mechanisms: a pressure difference in the liquid droplet and molecule entanglement. This research unifies many years of liquid spreading research and has implications for systems that involve designing complex ultrathin liquid films.

Anomalous diffusion kinetics of the precursor film that spreads from polymer droplets.

The radii R of the area wetted by precursor films spreading from small droplets of polymer liquids are found to vary as R ~ t(ν) with ν = 0.31-0.42, in contradiction to the 0.5 exponent indicated in previous studies. The experiments reported here are for three of the same polymer liquids used in the previous studies-methyl-terminated polydimethylsiloxane (PDMS), tetrakis(2-ethylhexoxy)silane, and squalane-and a linear chain perfluoropolyether polymer. When the droplets dissipate into pancake-shaped films, the spreading kinetics slows down to a variety of different diffusion processes for the different molecules, as they are no longer forced to move en masse across the surface by molecules being fed into the center of the film by the droplet.

Nanoscale origins of dynamic friction in an asymmetric contact geometry.

Friction, adhesion, and bounce dynamics are found to be asymmetric with respect to sliding direction for a tilted pad sliding at high speed. The differences originate from the nanoscale effects of the mechanical action against the lubricant layer forming an asymmetric meniscus around the pad and from a self-excited vibration induced by friction hysteresis coupled with adhesion hysteresis.