Erik Dietrich

The analogy between evaporating surface droplets in air to dissolving long-chain alcohol droplets in water is worked out. We show that next to the three known modi for surface droplet evaporation or dissolution (constant contact angle mode, constant contact radius mode, and stick-slide mode), a fourth mode exists for small droplets on supposedly smooth substrates, namely the stick-jump mode: intermittent contact line pinning causes the droplet to switch between sticking and jumping during the dissolution. We present experimental data and compare them to theory to predict the dissolution time in this stick-jump mode. We also explain why these jumps were easily observed for microscale droplets but not for larger droplets.

The exceptionally long lifetime of surface nanobubbles remains one of the biggest questions in the field. One of the proposed mechanisms for the stability is the \emph{dynamic equilibrium} model, which describes a constant flux of gas in and out of the bubble. Here, we describe results from particle tracking experiments to measure this flow. The results are analysed by measuring the Vorono\"i cell size distribution, the diffusion, and speed of the tracer particles. We show that there is no detectable difference in the movement of particles above nanobubble-laden surfaces, as compared to nanobubble-free surfaces.

The exceptionally long lifetime of surface nanobubbles remains one of the biggest questions in the field. One of the proposed mechanisms for producing the stability is the dynamic equilibrium model, which describes a constant flux of gas in and out of the bubble. Here, we describe results from particle tracking experiments carried out to measure this flow. The results are analysed by measuring the Voronoï cell size distribution, the diffusion, and the speed of the tracer particles. We show that there is no detectable difference in the movement of particles above nanobubble-laden surfaces as compared to ones above nanobubble-free surfaces.

Individual surface nanobubbles are visualized with nonintrusive optical interference-enhanced reflection microscopy, demonstrating that their formation is not a consequence of the hitherto used intrusive atomic force microscopy technique. We then use this new and fast technique to demonstrate that surface nanobubbles form in less than a few seconds after ethanol-water exchange, which is the standard procedure for their preparation, and examine how they react to temperature variations.

The existence of surface nanobubbles has been previously suggested using various experimental techniques, including attenuated total reflection spectroscopy, quartz crystal microbalance, neutron reflectometry, and x-ray reflectivity, but all of these techniques provide a sole number to quantify the existence of gas over (usually) hundreds of square microns. Thus `nanobubbles' are indistinguishable from a `uniform gassy layer' between surface and liquid. Atomic force microscopy, on the other hand, does show the existence of surface nanobubbles, but the highly intrusive nature of the technique means that a uniform gassy layer could break down into nanobubbles \textit{due to} the motion of the microscope's probe. Here we demonstrate \textit{optical} visualisation of surface nanobubbles, thus validating their individual existence non-intrusively.