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.

Nanoscopic interactions of colloidal particles can suppress millimetre drop splashing.

The splashing of liquid drops onto a solid surface is important for a wide range of applications, including combustion and spray coating. As the drop hits the solid surface, the liquid is ejected into a thin horizontal sheet expanding radially over the substrate. Above a critical impact velocity, the liquid sheet is forced to separate from the solid surface by the ambient air, and breaks up into smaller droplets. Despite many applications involving complex fluids, their effects on splashing remain mostly unexplored. Here we show that the splashing of a nanoparticle dispersion can be suppressed at higher impact velocities by the interactions of the nanoparticles with the solid surface. Although the dispersion drop first shows the classical transition from deposition to splashing when increasing the impact velocity, no splashing is observed above a second higher critical impact velocity. This result goes against the commonly accepted understanding of splashing, that a higher impact velocity should lead to even more pronounced splashing. Our findings open new possibilities to deposit large amount of complex liquids at high speeds.

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.

Rebound of self-lubricating compound drops.

Drop impact on solid surfaces is encountered in numerous natural and technological processes. Although the impact of single-phase drops has been widely explored, the impact of compound drops has received little attention. Here, we demonstrate a self-lubrication mechanism for water-in-oil compound drops impacting on a solid surface. Unexpectedly, the core water drop rebounds from the surface below a threshold impact velocity, irrespective of the substrate wettability. This is interpreted as the result of lubrication from the oil shell that prevents contact between the water core and the solid surface. We combine side and bottom view high-speed imaging to demonstrate the correlation between the water core rebound and the oil layer stability. A theoretical model is developed to explain the observed effect of compound drop geometry. This work sets the ground for precise complex drop deposition, with a strong impact on two- and three-dimensional printing technologies and liquid separation.

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.

Improved Resolution and Fidelity of Droplet-Based Bioprinting by Upward Ejection.

Bioprinting has emerged as a powerful biofabrication technology with widespread applications in biomedical fields because of its superiority in high-throughput, high-precision, 3D structure fabrication. For bioprinting, two of the most important parameters are the printing precision (i.e., droplets resolution) and structural fidelity (i.e., conformity of the printed objects to the design). The major factors that hinder resolution and fidelity are gravity and impact force between printed droplets and substrate. However, existing solutions to these two issues, including decreasing droplet volume and introducing sacrificial materials, cause other problems, such as complexity or poor biocompatibility. Here, we reported a variant 3D bioprinting technique, termed as upward bioprinting, in which the nozzle of bioprinter is overturned and the ejection direction is opposite to gravitational force. Employing this technique, we fabricated discrete droplets, continuous lines, and 3D multilayer constructs using alginate and gelatin methacrylate (GelMA). The characterizations show that the upward bioprinting could improve the resolution and also fidelity as compared with the conventional downward bioprinting. Meanwhile, this method enables cell printing without affecting the viability. In addition, this method can be easily implemented without upgrading any hardware. Such an upward bioprinting technique could be an alternative to scale down microtissues and to fabricate 3D complex bioconstructs. We envision that the upward bioprinting, as a general method, could be extended to other bioprinting processes or applied to 3D bioprinting in outer space.

Vortex-ring-induced large bubble entrainment during drop impact.

For a limited set of impact conditions, a drop impacting onto a pool can entrap an air bubble as large as its own size. The subsequent rise and rupture of this large bubble plays an important role in aerosol formation and gas transport at the air-sea interface. The large bubble is formed when the impact crater closes up near the pool surface and is known to occur only for drops that are prolate at impact. Herein we use experiments and numerical simulations to show that a concentrated vortex ring, produced in the neck between the drop and the pool, controls the crater deformations and pinchoff. However, it is not the strongest vortex rings that are responsible for the large bubbles, as they interact too strongly with the pool surface and self-destruct. Rather, it is somewhat weaker vortices that can deform the deeper craters, which manage to pinch off the large bubbles. These observations also explain why the strongest and most penetrating vortex rings emerging from drop impacts are not produced by oblate drops but by more prolate drop shapes, as had been observed in previous experiments.

von Kármán vortex street within an impacting drop.

The splashing of a drop impacting onto a liquid pool produces a range of different sized microdroplets. At high impact velocities, the most significant source of these droplets is a thin liquid jet emerging at the start of the impact from the neck that connects the drop to the pool. We use ultrahigh-speed video imaging in combination with high-resolution numerical simulations to show how this ejecta gives way to irregular splashing. At higher Reynolds numbers, its base becomes unstable, shedding vortex rings into the liquid from the free surface in an axisymmetric von Kármán vortex street, thus breaking the ejecta sheet as it forms.