Self-Propulsion of a Volatile Drop on the Surface of an Immiscible Liquid Bath.

We demonstrate the self-propulsion of a volatile drop on the surface of a bath of an immiscible liquid. Evaporative heat pumping is converted into directed motion through thermocapillary stresses, which arise from the coupling between surface-tension-driven flows and temperature advection. A propulsive force arises from convection-sustained temperature gradients along the drop interface, resulting in a warmer pool of liquid being advected by the hydrodynamic flow in the underlying bath toward the back of the drop. The dependence of the drop speed on the activity source, i.e., the evaporation flux, is derived with scaling arguments and captures the experimental data.

Exact Results for the Boundary Energy of One-Dimensional Bosons.

We study bosons in a one-dimensional hard-wall box potential. In the case of contact interaction, the system is exactly solvable by the Bethe ansatz, as first shown by Gaudin in 1971. Although contained in the exact solution, the boundary energy in the thermodynamic limit for this problem is only approximately calculated by Gaudin, who found the leading order result at weak repulsion. Here we derive an exact integral equation that enables one to calculate the boundary energy in the thermodynamic limit at an arbitrary interaction. We then solve such an equation and find the asymptotic results for the boundary energy at weak and strong interactions. The analytical results obtained from the Bethe ansatz are in agreement with the ones found by other complementary methods, including quantum Monte Carlo simulations. We study the universality of the boundary energy in the regime of a small gas parameter by making a comparison with the exact solution for the hard rod gas.

Method for high frequency tracking and sub-nm sample stabilization in single molecule fluorescence microscopy.

While fluorescence microscopes and atomic force microscopes are widely used to visualize, track, and manipulate single biomolecules, the resolution of these methods is limited by sample drift. To minimize drift, active feedback methods have recently been used to stabilize single molecule microscopes on the sub-nanometer scale. However, these methods require high intensity lasers which limits their application in single molecule fluorescence measurements. Furthermore, these feedback methods do not track user-defined regions of the sample, but rather monitor the relative displacement of an unknown point on a fiducial marker, which limits their use in biological force measurements. To overcome these limitations, we have developed a novel method to image, track and stabilize a sample using low laser intensities. We demonstrate the capabilities of our approach by tracking a user-chosen point on a fiducial marker at 8.6 kHz and stabilizing it with sub-nanometer resolution. We further showcase the application of our method in single molecule fluorescence microscopy by imaging and stabilizing individual fluorescently-tagged streptavidin proteins under biologically relevant conditions. We anticipate that our method can be easily used to improve the resolution of a wide range of single molecule fluorescence microscopy and integrated force-fluorescence applications.