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Evaporation of suspension droplets on superhydrophobic substrates reveals a dominant shape dependency of the resulting supraparticles on initial particle concentration.

Spreading and stationary droplets of a thermally responsive fluid on a heated surface are studied. The fluid undergoes a reversible gel formation at elevated temperature. The spatio-temporal pattern of gel formation within the droplet is examined using an experimental method based on spectral domain optical coherence tomography and time varying speckle patterns. Two stages of gel formation can be distinguished: first, a thin crust appears starting at the contact line. Second, a gel layer appears above the heated plate and then expands upward. We attribute the first stage of gel formation to solvent evaporation and heating through the air and the second to thermal conduction through the fluid from the base. Gel formation at the contact line is likely responsible for the arrest of spreading droplets, but was not detectable with our experimental protocol at the time of contact line arrest, suggesting that this arose over a microscopic length scale. Overall, substrate heating provides an effective way to control the final shape of droplets of thermo-responsive fluids.

Capillary ripples on thin viscous films are important features of coating and lubrication flows. Here we present experiments based on Digital Holographic Microscopy, measuring the morphology of capillary ripples ahead of a viscous drop spreading on a prewetted surface with a nanoscale resolution. Our experiments reveal that upon increasing the spreading velocity, the amplitude of the ripples first increases and subsequently decreases. Above a critical spreading velocity, the ripples even disappear completely and this transition is accompanied by a divergence of the ripple wavelength. These observations are explained quantitatively using linear wave analysis, beyond the usual lubrication approximation, illustrating that new phenomena arise when the capillary number becomes order unity.

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