Evaporation is a ubiquitous phenomenon in the natural environment and a dominant form of energy transfer in the Earth's climate. Engineered systems rarely, if ever, use evaporation as a source of energy, despite myriad examples of such adaptations in the biological world. Here, we report evaporation-driven engines that can power common tasks like locomotion and electricity generation. These engines start and run autonomously when placed at air–water interfaces. They generate rotary and piston-like linear motion using specially designed, biologically based artificial muscles responsive to moisture fluctuations. Using these engines, we demonstrate an electricity generator that rests on water while harvesting its evaporation to power a light source, and a miniature car (weighing 0.1 kg) that moves forward as the water in the car evaporates. Evaporation-driven engines may find applications in powering robotic systems, sensors, devices and machinery that function in the natural environment.

The behavior of water confined at the nanoscale plays a fundamental role in biological processes and technological applications, including protein folding, translocation of water across membranes, and filtration and desalination. Remarkably, nanoscale confinement drastically alters the properties of water. Using molecular dynamics simulations, we determine the phase diagram of water confined by graphene sheets in slab geometry, at T = 300 K and for a wide range of pressures. We find that, depending on the confining dimension D and density σ, water can exist in liquid and vapor phases, or crystallize into monolayer and bilayer square ices, as observed in experiments. Interestingly, depending on D and σ, the crystal-liquid transformation can be a first-order phase transition, or smooth, reminiscent of a supercritical liquid-gas transformation. We also focus on the limit of stability of the liquid relative to the vapor and obtain the cavitation pressure perpendicular to the graphene sheets. Perpendicular cavitation pressure varies non-monotonically with increasing D and exhibits a maximum at D ≈ 0.90 nm (equivalent to three water layers). The effect of nanoconfinement on the cavitation pressure can have an impact on water transport in technological and biological systems. Our study emphasizes the rich and apparently unpredictable behavior of nanoconfined water, which is complex even for graphene.

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The magnetic tweezer is a single molecule manipulation instrument ideally suited to measuring biophysical systems at a constant applied force. The development of a magnetic tweezers with a high-speed camera and GPU-accelerated particle tracking has allowed for the measurement of molecular events at the millisecond time scale. However, as the spatial resolution of the instrument is improved, previously neglected sources of noise start to become limiting, such as: mechanical stability of the sample stage, and coherent light artifacts such as speckle. Here, we isolate the various sources of noise in an attempt to determine the fundamental limit to magnetic tweezer resolution. We use a state-of-the-art high spatial and temporal resolution magnetic tweezer to measure the dynamics of model systems such as DNA hairpins.

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Chitosan is a polysaccharide consisting of N-acetyl-glucosamine and glucosamine units, prepared by the deacetylation of chitin. Glucosamine contains an ionizable primary amine, rendering chitosan water-soluble at low pH and insoluble at pH above~6.5. This pH dependent solubility can be exploited to make hydrogels used for coatings and sensors. We have used constant-pH molecular dynamics (CpHMD) to investigate the pH dependence of chitosan. Starting from a stable aggregate of neutral chitosan, we performed replica exchange simulations over a pH range 4.0-8.5. The aggregate remains stable at high pH and dissociates at low pH, as expected. Interestingly, the transition occurs cooperatively at around pH 6.5, in a remarkable agreement with experiment. The calculated bulk pKa was found to be similar to the transition pH, again in agreement with experiment. The role of electrostatic interactions and aggregation-induced desolvation in the protonation equilibria of the amine groups was also examined. This work provides atomic-level insight into the pH-dependent behavior of chitosan which may aid in the design and development of various chitosan-based materials.

DNA-binding agents are broadly used in many biotechnological or biomedical applications for detecting DNA in cells and gels or as drugs in cytotoxic oncologic cancer treatments. Their binding alters the structural and nanomechanical properties of DNA and affects the associated biological processes. Although interaction modes like intercalation and minor grove binding already have been identified, associated mechanic effects like DNA strand elongation as well as unwinding and softening of the dsDNA strand often remain obscure. We used single-molecule magnetic tweezers force experiments to quantitatively investigate the impact of four DNA-dyes (YOYO-1, DAPI, DRAQ5 and PicoGreen) as well as the anti-cancer drug mitoxantrone at room temperature in a concentration dependent manner. By extending and overwinding individual, torsionally constrained, nick-free dsDNA molecules, we determined the contour lengths, persistence lengths and molecular forces, which allow estimation of several thermodynamic and nanomechanical binding parameters. Whereas for YOYO-1 and DAPI the binding mechanisms can be assigned to bisintercalation and minor groove binding, respectively, DRAQ5 and Pico-Green exhibit both binding modes in a concentration dependent manner. Similarly, the chemotherapeutic drug mitoxantrone is found to interact with dsDNA depending on the concentration as a groove binder and an intercalator. Our experiments show that single-molecule nanomechanical experiments can be used for elucidating the binding mechanics of DNA binders and can contribute to the registration process in drug regulation.