To Split or Not to Split: Dynamics of an Air Disk Formed under a Drop Impacting on a Pool.

When a drop falls and impacts on a liquid pool, it entraps an air disk below the drop, which then contracts into a central bubble. Here, we use high-speed imaging and high-resolution numerical simulations to characterize the air-disk contraction dynamics for different liquid properties. We show that the air disk can contract into a single central bubble, form a toroidal bubble, or split vertically into two smaller bubbles. We demonstrate that the transitions between the different regimes can be separated by an Ohnesorge number, Oh_{e}, based on the air-disk thickness. For the lowest Oh_{e}, we find a new regime, where vortex shedding from the rim of the contracting air disk breaks the vertical symmetry and prevents the bubble from splitting in two.

Drop impact on a mesh - Viscosity effect.

Using a mesh surface is a promising technique in oil-water separation applications. In this paper, we investigated the dynamic impact of a silicone oil drop with different viscosities on an oleophilic mesh experimentally, which will help to define the critical conditions of the oil-water separation process. Four impact regimes were observed by controlling the impact velocity: deposition, partial imbibition, pinch-off, and separation. Thresholds of deposition, partial imbibition, and separation regimes were estimated, by balancing the inertia, capillary, and viscous forces. During the deposition and partial imbibition phenomena, the maximum spreading ratio (ßmax) increases with the Weber number. In contrast, in the case of the separation phenomenon, no significant effect of the Weber number on ßmax has been observed. Based on energy balance, we predicted the maximum elongation length of the liquid under the mesh during the partial imbibition phenomenon; the predicted data agrees well with the experimental data.

Ultra-Porous Nanocellulose Foams: A Facile and Scalable Fabrication Approach.

Cellulose nanofibril foams are cellulose-based porous materials with outstanding mechanical properties, resulting from the high strength-to-weight ratio of nanofibrils. Here we report the development of an optimized fabrication process for highly porous cellulose foams, based on a well-controlled freeze-thawing-drying (FTD) process at ambient pressure. This process enables the fabrication of foams with ultra-high porosity, up to 99.4%, density of 10 mg/cm3, and liquid (such as oil) absorption capacity of 100 L/kg. The proposed approach is based on the ice-templating of nanocellulose suspension in water, followed by thawing in ethanol and drying at environmental pressures. As such, the proposed fabrication route overcomes one of the major bottle-necks of the classical freeze-drying approach, by eliminating the energy-demanding vacuum drying step required to avoid wet foam collapse upon drying. As a result, the process is simple, environmentally friendly, and easily scalable. Details of the foam development fabrication process and functionalization are thoroughly discussed, highlighting the main parameters affecting the process, e.g., the concentration of nanocellulose and additives used to control the ice nucleation. The foams are also characterized by mechanical tests and oil absorption measurements, which are used to assess the foam absorption capability as well as the foam porosity. Compound water-in-oil drop impact experiments are used to demonstrate the potential of immiscible liquid separation using cellulose foams.