Drop spreading and gelation of thermoresponsive polymers

Spreading and solidification of liquid droplets are elementary processes of relevance for additive manufacturing. Here we investigate the effect of heat transfer on spreading of a thermoresponsive solution (Pluronic F127) that undergoes a sol-gel transition above a critical temperature $T_m$. By controlling the concentration of Pluronic F127 we systematically vary $T_m$, while also imposing a broad range of temperatures of the solid and the liquid. We subsequently monitor the spreading dynamics over several orders of magnitude in time and determine when solidification stops the spreading. It is found that the main parameter is the difference between the substrate temperature and $T_m$, pointing to a local mechanism for arrest near the contact line. Unexpectedly, the spreading is also found to stop below the gelation temparature, which we attribute to a local enhancement in polymer concentration due to evaporation near the contact line.Comment: 9 pages, 10 figure

Solidification of liquid metal drops during impact

Hot liquid metal drops impacting onto a cold substrate solidify during their subsequent spreading. Here we experimentally study the influence of solidification on the outcome of an impact event. Liquid tin drops are impacted onto sapphire substrates of varying temperature. The impact is visualised both from the side and from below, which provides a unique view on the solidification process. During spreading an intriguing pattern of radial ligaments rapidly solidifies from the centre of the drop. This pattern determines the late-time morphology of the splat. A quantitative analysis of the drop spreading and ligament formation is supported by scaling arguments. Finally, a phase diagram for drop bouncing, deposition and splashing as a function of substrate temperature and impact velocity is provided

Folding Films, Inverse Coarsening, and the Bubble-Bursting Cascade

International audienceBubble rupture is one of the principle mechanisms in which foams are assumed to coarsen, creating a smaller population of larger bubbles over time. Here, however, we demonstrate that for a large range of situations bubbles that rupture on an interface 'inverse coarsen', leading to a larger population of smaller bubbles. We present high speed images and numerical simulations to demonstrate that when a bubble bursts the retracting film can fold and entrap air. The folding leads to a torus of entrapped air which breaks up into a ring of daughter bubbles. These results have broad implications for any process involving bubbles on an interface, including air-water transfer and interfacial foam coarsening