Koen G. Winkels

Initial spreading of low-viscosity drops on partially wetting surfaces

Liquid drops start spreading directly after coming into contact with a partially wetting substrate. Although this phenomenon involves a three-phase contact line, the spreading motion is very fast. We study the initial spreading dynamics of low-viscosity drops using two complementary methods: molecular dynamics simulations and high-speed imaging. We access previously unexplored length and time scales and provide a detailed picture on how the initial contact between the liquid drop and the solid is established. Both methods unambiguously point toward a spreading regime that is independent of wettability, with the contact radius growing as the square root of time.

Short time dynamics of viscous drop spreading

Liquid drops start spreading directly after coming into contact with a solid substrate. Although this phenomenon involves a three-phase contact line, the spreading motion can be very fast. We experimentally study the initial spreading dynamics, characterized by the radius of the wetted area, for viscous drops. Using high-speed imaging with synchronized bottom and side views gives access to 6 decades of time resolution. We show that short time spreading does not exhibit a pure power-law growth. Instead, we find a spreading velocity that decreases logarithmically in time, with a dynamics identical to that of coalescing viscous drops. Remarkably, the contact line dissipation and wetting effects turn out to be unimportant during the initial stages of drop spreading.

Bubble formation during the collision of a sessile drop with a meniscus

The impact of a sessile droplet with a moving meniscus, as encountered in processes such as dip-coating, generically leads to the entrapment of small air bubbles. Here we experimentally study this process of bubble formation by looking through the liquid using high-speed imaging. Our central finding is that the size of the entrapped bubble crucially depends on the location where coalescence between the drop and the moving meniscus is initiated: (i) at a finite height above the substrate, or (ii) exactly at the contact line. In the first case, we typically find bubble sizes of the order of a few μm, independent of the size and speed of the impacting drop. By contrast, the bubbles that are formed when coalescence starts at the contact line become increasingly large, as the size or the velocity of the impacting drop is increased. We show how these observations can be explained from a balance between the lubrication pressure in the air layer and the capillary pressure of the drop

Hopf Bifurcation in Acoustically Excited Faraday Ripples on a Bubble Wall

Observations of surface waves on the wall of a bubble which is subjected to an acoustical field demonstrate differences in the transient processes, as well as a marked variation in steady state amplitudes, that depend on the insonification conditions. To clarify these observations, the stability analysis of the system has been carried out and the presence of the Hopf bifurcation was established. The appearance of the limiting circle corresponds to the presence of periodic variations in the amplitudes of interacting breathing (volume) and distortion (surface) modes. Comparison between observation and modeling indicate qualitative similarities, for example in that the oscillations will be seen for sufficiently high pressure amplitudes and will be absent at the bottom of the threshold curve for the excitation of the ripples.