Zipping-Depinning: Dissolution of Droplets on Micropatterned Concentric Rings.

The control of the surface wettability is of great interest for technological applications as well as for the fundamental understanding of surface phenomena. In this article, we describe the dissolution behavior of droplets wetting a micropatterned surface consisting of smooth concentric circular grooves. In the experiments, a droplet of alcohol (1-pentanol) is placed onto water-immersed micropatterns. When the drops dissolve, the dynamics of the receding contact line occurs in two different modes. In addition to the stick-jump mode with jumps from one ring to the next inner one, our study reveals a second dissolution mode, which we refer to as zipping-depinning. The velocity of the zipping-depinning fronts is governed by the dissolution rate. At the early stage of the droplet dissolution, our experimental results are in good agreement with the theoretical predictions by Debuisson et al. [ Appl. Phys. Lett. 2011 , 99 , 184101 ]. With an extended model, we can accurately describe the dissolution dynamics in both stick-jump and zipping-depinning modes.

Collective and convective effects compete in patterns of dissolving surface droplets.

The effects of neighboring droplets on the dissolution of a sessile droplet, i.e. collective effects, are investigated both experimentally and numerically. On the experimental side small approximately 20 nL mono-disperse surface droplets arranged in an ordered pattern were dissolved and their size evolution is studied optically. The droplet dissolution time was studied for various droplet patterns. On the numerical side, lattice-Boltzmann simulations were performed. Both simulations and experiments show that the dissolution time of a droplet placed in the center of a pattern can increase by as much as 60% as compared to a single, isolated droplet, due to the shielding effect of the neighboring droplets. However, the experiments also show that neighboring droplets enhance the buoyancy driven convective flow of the bulk, increasing the mass exchange and counteracting collective effects. We show that this enhanced convection can reduce the dissolution time of droplets at the edges of the pattern to values below that of a single, isolated droplet.

Stick-jump mode in surface droplet dissolution.

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, 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.

Mixed mode of dissolving immersed nanodroplets at a solid-water interface.

The dissolution dynamics of microscopic oil droplets (less than 1 µm in height, i.e. nanodroplets) on a hydrophobilized silicon surface in water was experimentally studied. The lateral diameter was monitored using confocal microscopy, whereas the contact angle was measured by (disruptive) droplet polymerisation of the droplet. In general, we observed the droplets to dissolve in a mixed mode, i.e., neither in the constant contact angle mode nor in the constant contact radius mode. This means that both the lateral diameter and the contact angle of the nanodroplets decrease during the dissolution process. On average, the dissolution rate is faster for droplets with larger initial size. Droplets with the same initial size can, however, possess different dissolution rates. We ascribe the non-universal dissolution rates to chemical and geometric surface heterogeneities (that lead to contact line pinning) and cooperative effects from the mass exchange among neighbouring droplets.

Water-induced blister formation in a thin film polymer.

A failure mechanism of thin film polymers immersed in water is presented: the formation of blisters. The growth of blisters is counterintuitive as the substrates were noncorroding and the polymer does not swell in water. We identify osmosis as the driving force behind the blister formation. The dynamics of the blister formation is studied experimentally as well as theoretically, and a quantitative model describing the blister growth is developed, which accurately describes the temporal evolution of the blisters.

Exposing nanobubble-like objects to a degassed environment.

The primary attribute of interest of surface nanobubbles is their unusual stability and a number of theories trying to explain this have been put forward. Interestingly, the dissolution of nanobubbles is a topic that did not receive a lot of attention yet. In this work we applied two different experimental procedures which should cause gaseous nanobubbles to completely dissolve. In our experiments we nucleated nanobubble-like objects by putting a drop of water on HOPG using a plastic syringe and a disposable needle. In method A, the nanobubble-like objects were exposed to a flow of degassed water (1.17 mg l(-1)) for 96 hours. In method B, the ambient pressure was lowered in order to degas the liquid and the nanobubble-like objects. Interestingly, the nanobubble-like objects remained stable after exposure to both methods. After thorough investigation of the procedures and materials used during our experiments, we found that the nanobubble-like objects were induced by the use of disposable needles in which PDMS contaminated the water. It is very important for the nanobubble community to be aware of the fact that, although features look and behave like nanobubbles, in some cases they might in fact be induced by contamination. The presence of contamination could also resolve some inconsistencies found in the nanobubble literature.

Particle tracking around surface nanobubbles.

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.

Nonintrusive optical visualization of surface nanobubbles.

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.