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When a droplet hits a surface fast enough, droplet splashing can occur: Smaller secondary droplets detach from the main droplet during impact. While droplet splashing on smooth surfaces is by now well understood, the surface roughness also affects at which impact velocity a droplet splashes. In this paper, the influence of the surface roughness on droplet splashing is investigated. By changing the root-mean-square roughness of the impacted surface, we show that the droplet splashing velocity is only affected when the droplet roughness is large enough to disrupt the spreading droplet lamella and change the droplet splashing mechanism from corona to prompt splashing. Finally, using Weber and Ohnesorge number scaling models, we also show that the measured splashing velocity for both water and ethanol on surfaces with different roughnesses and water-ethanol mixtures collapses onto a single curve, showing that the droplet splashing velocity on rough surfaces scales with the Ohnesorge number defined with the surface roughness length scale.

Droplets impacting on a surface are key to a wide range of applications such as spray deposition and inkjet printing. Yet, a full understanding of how they spread and retract is still lacking. Surfactants are often added to improve spreading and coverage of aqueous solutions, resulting in variations of the surface tension at timescales beyond the reach of conventional dynamic surface tension measurement methods. Here we study the impact dynamics of aqueous surfactant solutions on hydrophobic surfaces at millisecond timescales. We find that the spreading and retraction of droplets cannot be adequately described by the equilibrium surface tension. We infer the dynamic surface tension in the first milliseconds after impact from the spreading dynamics of droplets. "Slow"surfactants that take a lot of time to reach newly created droplet surface, only slightly decrease or even increase the surface tension, while "fast"surfactants, on the other hand, allow efficient wetting of aqueous solutions on hydrophobic surfaces. Our findings allow us to tailor surfactants for efficient drop deposition or spray application.

We investigate the influence of surfactants on capillary rise and corner flow in angular pores. We therefore study capillary rise for simple fluids and surfactant solutions, comparing square to cylindrical capillaries. We show that fingers start to form in the corners of the square capillaries when the capillary rise slows down before reaching the equilibrium height. The corner flow scales as t1/3 and its quantitative understanding necessitates that the surface wettability is taken into account in terms of the liquid’s advancing contact angle on the capillary walls inside the corner. Adding surfactants to water greatly influences the corner flow in square capillaries: depending on the nature of the surfactant, the corner flow can be either suppressed completely due to autophobic effects or enhanced due to the presence of Marangoni stresses caused by a surface tension gradient inside the liquid fingers.

The spreading of liquid droplets impacting a surface at high speed is well understood by now. However, when a droplet impacts a surface at relatively low impact velocities (

We investigate which property of non-Newtonian fluids determines the deceleration of a high-speed impacting object. Using high-speed camera footage, we measure the velocity decrease of a high-speed spherical object impacting a typical Newtonian fluid (water) as a reference and compare it with a shear thickening fluid (cornstarch) and a shear thinning viscoelastic fluid (a weakly cross-linked polymer gel). Three models describing the kinetic energy loss of the object are considered: fluid inertia, shear thickening and viscoelasticity. By fitting the three models to the experimental data, we conclude that the viscoelastic model works best for both the cornstarch and the polymer gel. Since the cornstarch is also viscoelastic, we conclude that the ability to stop objects of these complex fluids is given by their viscoelasticity rather than shear thickening or shear thinning.