We demonstrate that besides gaseous pockets also a gas supersaturated spot on a substrate can be a nucleus for cavitation. The supersaturation is achieved by either a formerly dissolved bubble or by heating locally the surface below the boiling temperature. The experiments are conducted in a thin film of water; one side of the water film is in contact with a gold coated substrate that is heated by a continuous laser through plasmonic heating. For nucleation of a bubble, the pressure at the heated spot is reduced by a transient rarefaction wave. The experimental findings suggest that the local gas supersaturation is responsible to nucleate cavitation and thus connects the phase transitions of cavitation and boiling. Additionally, the pressure waves in the liquid gap are studied numerically.

The merging of two soap bubbles is a fundamental fluid mechanical process in foam formation. In the present experimental study, the liquid films from two soap bubbles are brought together. Once the liquid layers initially separated by a gas sheet are bridged on a single spot, the rapid merging of the two liquid films proceeds. Thereby, the connecting rim is rapidly accelerated into the separating gas layer. We show that, due to the dimple formation, the velocity is not uniform and the high acceleration initially causes a Rayleigh–Taylor instability of the liquid rim. At later times, the rim heals and takes on a circular shape. However, for sufficiently high concentrations of the surfactant, the unstable rim pinches off microbubbles, resulting in a fractal dendritic structure after coalescence.

To address the problems of virgin plastic production from fossil resources and the growing amount of plastic waste, a rapid transition to a circular economy is being pursued. The separation of mixed plastics into pure fractions is of paramount importance for promoting recycling and preventing downcycling. In this study, experimental parameters were determined for the selective bulk dissolution of polyamide 6 (PA 6) filaments (1.75 mm diameter, 1 cm length) in CaCl₂-EtOH-H₂O mixtures (CEW) at 75 °C. These parameters included the energy supply mode, dissolution time, CEW composition and CEW:PA mass ratio. Compared with energy supply by microwaves, energy supply by ultrasound improved the yield of dissolved and recovered PA 6 after 5 h from 31% to 52%. In total, the yield of PA 6 after 3 h of bulk dissolution increased from 18% to 69% when the energy supply mode was changed from microwave to ultrasound and the H₂O:EtOH molar ratio of CEW was increased from 0.40 to 1.33 while maintaining an optimal CEW:PA mass ratio of 8.5. Additionally, master plot analysis suggested that dissolution under microwave energy supply followed a contracting cylinder model, whereas dissolution under ultrasonic energy supply aligned with a 2D diffusion or third-order kinetic model. Microscopic observations suggested that, in the case of ultrasonic energy supply, oscillating bubbles on the particle surface enhanced the dissolution rate of PA 6 filaments in CEW.

The spreading of soap bubbles after forming contact with a substrate is experimentally studied. We find for dry glass substrate that the rim of the spreading soap bubble follows the well known scaling law for inertia dominated spreading $$r \sim t^{1/2}$$r ∼ t 1 / 2 [Eggers, J., Lister, J., and Stone, H., J. Fluid Mech. 401, 293–310 (1999)]. Varying the viscosity of the soap solutions and the coating of the glass does not affect this spreading behavior qualitatively. Yet, on a wetted surface, the rim obtains a constant radial velocity. Here, the rim splits into two and this new rim trails the main rim. Interestingly, the central film enclosed by the two rims develops radially oriented wrinkles.