Ernst A. van Nierop

Reactive spreading and recoil of oil on water

Droplets of oil containing oleic acid were observed to spread, then recoil, on an aqueous solution of sodium hydroxide. Surfactant is produced at the interface during spreading, and for reagent concentrations of order O(1mM) spreading is observed to be much faster than in the absence of a chemical reaction [radius R(t)∝tα with 0.64<α<0.89]. After t∼10s, drops reach a maximum radius Rmax∼3–5 times the initial radius. Spreading is faster and Rmax is larger for higher concentrations of reagents. The drops are then observed to recoil (with apparent power-law behavior −0.34<α<−0.14), due to diffusion of surfactant away from the oil/water interface, with the rate of recoil being controlled by the NaOH concentration.

Bubble formation via multidrop impacts

Gas bubbles are often generated when droplets impact a liquid-air interface. For the impact of single droplets, a critical impact velocity must be exceeded for air to be entrained in the form of bubbles. Here we establish that bubbles can be generated at much lower velocities provided that two or more drops impact the liquid-air interface within a sufficiently short time interval. Using high-speed imaging, we show that bubbles are entrained when a drop lands in the impact crater of a previous drop. We quantify the critical crater depth formed upon impact and the necessary time interval between drop impacts for bubble entrainment to occur. For 1 mm diameter water drops falling at 1 m/s, the critical separation time is approximately 5 ms. This critical time is consistent with a scaling analysis of the time required for an impact crater to close by capillarity.

Thermocapillary-assisted pulling of thin films: Application to molten metals

We study the thermocapillary stabilization of a free liquid film as it is formed by being pulled out of a bath at constant speed. For sufficiently large stresses induced at the interface through a controlled temperature gradient, a continuous film of liquid can be processed. For negligible inertial effects, the film thickness only depends on the capillary length and on the strength of the surface tension variation. The theory suggests that very thin ribbons or foils of molten material can be drawn out of a melt over a wide range of thicknesses and at speeds relevant to manufacturing.

Thermocapillary-assisted pulling of contact-free liquid films

We study the formation of a free liquid film that is pulled out of a bath at constant speed and stabilized by the action of thermocapillary stresses prescribed at the free surfaces. The basic concept was introduced recently by Scheid et al. [“Thermocapillary-assisted pulling of thin films: Application to molten metals,” Appl. Phys. Lett. 97, 171906 (2010)]10.1063/1.3505523. The theory suggests that very thin ribbons of molten material can be drawn out of a melt by adequately tuning the temperature gradient along the dynamic meniscus that connects the static meniscus at the melting bath to the region of the drawn flat film. In the present paper, we extend our original analysis by investigating the roles of inertia and gravity on the film thickness, and show how the results depend on heat transfer/conduction properties. Furthermore, we analyze the one-dimensional transverse stability of the free film with respect to the long-wave thermocapillary instability.

Mechanical Inhibition of Foam Formation via a Rotating Nozzle

We recently discovered that bubble formation can be substantially prevented when an aqueous solution is sprayed into a bath of the same solution provided that any two consecutive drops impacting the same surface location do so with a time interval greater than the capillary relaxation time. Building on this observation, here we report a mechanical means of preventing foam formation during liquid addition: the nozzle delivering the liquid is rotated sufficiently rapidly so that no two successive drops impact the interface at the same location. Foam formation is reduced by as much as 95% without any chemical anti-foaming agents.