Adam R. Tetreault

A description of how droplets moving across a surface drive the depletion of infused liquid-repelling lubricant paves the way for broader adoption of liquid-infused repellant surfaces in a wide range of industries.

There are currently three main classes of liquid-repellent surfaces: micro- or nanostructured superhydrophobic surfaces, flat surfaces grafted with "liquidlike" polymer brushes, and lubricated surfaces. Despite recent progress, the mechanistic explanation for the differences in droplet behavior on such surfaces is still under debate. Here, we measure the dissipative force acting on a droplet moving on representatives of these surfaces at different velocities U=0.01-1 mm/s using a cantilever force sensor with submicronewton accuracy and correlate it to the contact line dynamics observed using optical interferometry at high spatial (micron) and temporal (<0.1 s) resolutions. We find that the dissipative force-due to very different physical mechanisms at the contact line-is independent of velocity on superhydrophobic surfaces but depends nonlinearly on velocity for flat and lubricated surfaces. The techniques and insights presented here will inform future work on liquid-repellent surfaces and enable their rational design.

The Langmuir-Blodgett technique is one of the most controlled methods to deposit monomolecular layers of floating or surface active materials but has lacked the ability to coat truly large-area substrates. In this work, by manipulating single-layer dispersions of graphene oxide (GO) and thermally exfoliated GO into water-immiscible spreading solvents, unlike traditional Langmuir-Blodgett deposition which requires densification achieved by compressing barriers, we demonstrate the ability to control the 2D aggregation and densification behavior of these floating materials using a barrier-free method. This is done by controlling the edge-to-edge interactions through modified subphase conditions and by utilizing the distance-dependent spreading pressure of the deposition solvent. These phenomena allow substrates to be coated by continuous deposition and substrate withdrawal-enabling roll-to-roll deposition and patterning of large-area substrates such as flexible polyethylene terephthalate. The aggregation and solvent-driven densification phenomena are examined by in situ Brewster angle video microscopy and by measuring the local spreading pressure induced by the spreading solvent acting on the floating materials using a Langmuir-Adam balance. As an example, the performance of films deposited in this way is assessed as passivation layers for Ag nanowire-based transparent conductors.

The production of exfoliated MoS2 via lithium intercalation has been widely used to prepare 1T polytype dominated MoS2 (1T-MoS2) monolayers. These metallic single layers hold promise as high-performance electrodes for various electrochemical applications as well as for direct conversion to the semiconducting 2H polytype (2H-MoS2), a material of significant interest for next-generation electronics and optoelectronics. The performance in these applications is largely determined by defects introduced during processing. In this work, we systematically investigate the degradation rate and products resulting from the aging of aqueous MoS2 dispersions, obtained by chemical exfoliation under a variety of common processing conditions. Depending on the size and number of defects initially present in the material, the resulting MoS2 is found to have a surprisingly short half-life of only 2 to 6 days under alkaline conditions and exposure to both light and air. By aging samples under various environments and analyzing by UV–vis, Raman, Fourier-transform infrared, and X-ray photoelectron spectroscopies, we demonstrate that light-induced generation of superoxide by MoS2 is largely responsible for its oxidation (both 1T and 2H polytypes are affected equally). This process is accelerated under alkaline conditions because of the solubility of MoO3, which we suggest would otherwise passivate defects and edge sites. The soluble molybdates are found to be a strong indicator of oxidation, which can easily be followed by measuring the absorption at 209 nm using UV–vis spectroscopy. We hope that this work will help guide researchers in minimizing or controlling degradation, which will accelerate the successful use of MoS2 in applications.