Wetting States and Departure Diameters of Bubbles on Micro-/Nanostructured Surfaces.

Wetting states for droplets have been extensively investigated in the past. As the counter phase of the droplets, bubbles' wetting states have rarely been systematically explored. The wetting state of a bubble is closely related to its departure diameter, which plays significant roles in bubble-generated processes in boiling heat transfer and gas-evolving reactions. Based on the principle of minimum surface energy, we explicitly define three equilibrium wetting states (hemi-wicking state, Wenzel state, and Cassie-Baxter state) for bubbles on micro-/nanostructured surfaces in this paper. We analyze the three-phase contact line profiles for bubbles under these wetting states and propose theoretical models for predicting departure diameters of hemi-wicking-state bubble and Wenzel-state bubble on micro-/nanostructured surfaces. We identify competing effects of bubble departure in Wenzel state: the augmentation of contact line length due to the roughness, which would delay bubble departure, and the decrease of contact line length due to the reduced apparent contact angle, which would facilitate bubble departure. We demonstrate that hemi-wicking-state bubble exhibits a much smaller departure diameter on the textured surfaces. These findings are supported by numerical simulations by the three-dimensional (3D) multiple-relaxation-time lattice Boltzmann method. It is found that the length of the outermost contact lines instead of all contact lines determines the departure diameter of hemi-wicking-state bubble based on bubble detachment processes captured by our 3D numerical simulations. This work offers an avenue for the accurate prediction and control of bubble departure behaviors from micro-/nanostructured surfaces, and therefore can guide optimal designs of micro-/nanostructured surfaces in a variety of applications in boiling, desalination, and hydrogen production by electrolysis.

Temperature-induced coalescence of colliding binary droplets on superhydrophobic surface.

This report investigates the impact of droplet temperature on the head-on collision of binary droplets on a superhydrophobic surface. Understanding droplet collision is critical to many fundamental processes and industrial applications. There are many factors, including collision speed, collision angle, and droplet composition, that influence the outcome of the collision between binary droplets. This work provides the first experimental study of the influence of droplet temperature on the collision of binary droplets. As the droplet temperature increases, the possibility increases for the two droplets to coalesce after collision. The findings in this study can be extended to collision of droplets under other conditions where control of the droplet temperature is feasible. Such findings will also be beneficial to applications that involve droplet collision, such as in ink-jet printing, steam turbines, engine ignition, and spraying cooling.

[Numerical analysis on the influence factors of sample stacking in capillary electrophoresis].

Sample stacking in capillary electrophoresis can concentrate sample species through the electrical field strength gradient caused by the inhomogeneous buffer concentration field in capillary. The factors that affect the sample stacking process have been analyzed in detail by using a 1-D mathematic model. It was found from the simulation results that the electrical charge number and the electrical charge sign of sample particles can affect the electrophoretic velocity, which in turn has an important influence on the stacking process. The electrical potential can affect the migration captime of sample particles to reach detection window, and the initial length of sample plug has significant influence on the maximal sample concentration after stacking and the time to get the optimal stacking effect. The results obtained are helpful to the improvement of the sample stacking technique.