Capillary pressure-based measurement of dynamic interfacial tension in a spontaneous microfluidic sensor

The size of droplets and bubbles, and the properties of emulsions and foams strongly depend on dynamic interfacial tension (γ_d) - a parameter that is often inaccessible due to the very short time scales for droplet and bubble formation, and the inaccessibility of (e.g., food) production lines. To solve this challenge, we developed a microfluidic tensiometer that can measure γ_d by monitoring the formation time of both droplets and bubbles. Our tensiometer is a pressure-driven microfluidic device that operates based on the principle of a pressure balance: the formation of a droplet (or a bubble) is initialized when the Laplace pressure of the interface is decreased below the externally applied pressure, and this decrease is caused by a reduction in γ_d that can be calculated from the applied pressure and the Young-Laplace equation. The decay of γ_d due to surfactant adsorption can be followed at the characteristic time scale, which is dependent on surfactant type and concentration. For 0.05-1% wt sodium dodecyl sulfate (SDS), we were able to measure γ_d at time scales down to 1 ms and 0.1 ms for droplet and bubble interfaces, respectively, at increasing applied pressures and SDS concentrations. Our tensiometer proves to be a simple, robust method that inherently allows access to nearly the full range of dynamic interfacial tension at relevant time scales.

Dynamics of an impacting emulsion droplet

Emulsions are widely used in agriculture where oil-based pesticides are sprayed as an emulsion. However, emulsion droplets can bounce off hydrophobic plant surfaces, leading to major health and environmental issues as pesticides pollute water sources and soils. Here, we report an unexpected transition from bouncing to sticking to bouncing as the droplet impact speed increases. We show that the physics are governed by an in situ, self-generated lubrication of the surface leading to a suction force from the nascent oil layer around the droplet. We demonstrate that this phenomenon can be controlled by a careful balance of three time scales: the contact time of the droplet, the impregnation time scale of the oil, and the oil ridge formation time scale. We lastly build a design map to precisely control the bouncing of droplets and the oil coverage of the target surface. These insights have broad applicability in agriculture, cooling sprays, combustion, and additive manufacturing.

Effects of dynamic adsorption on bubble formation and coalescence in partitioned-EDGE devices

HYPOTHESIS

Dynamic adsorption effects can play a crucial role in bubble formation and stabilization. We hypothesize that microfluidic tools provide direct insights to these effects, and that the final bubble size depends on the intersection of time scales for bubble formation versus adsorption of proteins.

EXPERIMENTS

We use a microfluidic device to study Laplace pressure-driven formation of bubbles that are stabilized by whey proteins. Bubble behavior is studied as a function of the pressure difference imposed across the pores (P_d^∗), and thus the bubble formation time (τ, ranging from μs to s), using highspeed recordings, quasi-static pressure arguments and a semi-empirical coalescence model.

FINDINGS

We observe two distinct bubble formation regimes, delimited by the pressure difference required to initiate bubble formation in pure water, P_d^∗= 1400 mbar. When P_d^∗<1400 mbar, protein adsorption is a requisite to lower the surface tension and initialize bubble formation. Individual bubbles (fixed d₀~ 25 μm) are formed slowly with τ≫1 ms. When P_d^∗ exceeds 1400 mbar, bubbles (fixed d₀~ 16 μm) experience no adsorption lag and thus are formed at steeply increasing frequency, with τ < 1 ms. Interaction between these bubbles causes finite coalescence to a diameter d_coal that increases for lower τ. A minimum time of 0.4 ms is needed to immediately stabilize individual bubbles. Our study provides a promising microfluidic tool to study bubble formation and coalescence dynamics simultaneously.

Mapping Bubble Formation and Coalescence in a Tubular Cross-Flow Membrane Foaming System

Membrane foaming is a promising alternative to conventional foaming methods to produce uniform bubbles. In this study, we provide a fundamental study of a cross-flow membrane foaming (CFMF) system to understand and control bubble formation for various process conditions and fluid properties. Observations with high spatial and temporal resolution allowed us to study bubble formation and bubble coalescence processes simultaneously. Bubble formation time and the snap-off bubble size (D0) were primarily controlled by the continuous phase flow rate (Qc); they decreased as Qc increased, from 1.64 to 0.13 ms and from 125 to 49 µm. Coalescence resulted in an increase in bubble size (Dcoal>D0), which can be strongly reduced by increasing either continuous phase viscosity or protein concentration-factors that only slightly influence D0. Particularly, in a 2.5 wt % whey protein system, coalescence could be suppressed with a coefficient of variation below 20%. The stabilizing effect is ascribed to the convective transport of proteins and the intersection of timescales (i.e., μs to ms) of bubble formation and protein adsorption. Our study provides insights into the membrane foaming process at relevant (micro-) length and time scales and paves the way for its further development and application.

Fungal artillery of zombie flies: infectious spore dispersal using a soft water cannon

Dead sporulating female fly cadavers infected by the house fly-pathogenic fungus Entomophthora muscae are attractive to healthy male flies, which by their physical inspection may mechanically trigger spore release and by their movement create whirlwind airflows that covers them in infectious conidia. The fungal artillery of E. muscae protrudes outward from the fly cadaver, and consists of a plethora of micrometric stalks that each uses a liquid-based turgor pressure build-up to eject a jet of protoplasm and the initially attached spore. The biophysical processes that regulate the release and range of spores, however, are unknown. To study the physics of ejection, we design a biomimetic 'soft cannon' that consists of a millimetric elastomeric barrel filled with fluid and plugged with a projectile. We precisely control the maximum pressure leading up to the ejection, and study the cannon efficiency as a function of its geometry and wall elasticity. In particular, we predict that ejection velocity decreases with spore size. The calculated flight trajectories under aerodynamic drag predict that the minimum spore size required to traverse a quiescent layer of a few millimetres around the fly cadaver is approximately 10 µm. This corroborates with the natural size of E. muscae conidia (approx. 27 µm) being large enough to traverse the boundary layer but small enough (less than 40 µm) to be lifted by air currents. Based on this understanding, we show how the fungal spores are able to reach a new host.

Application of Microfluidics in the Production and Analysis of Food Foams

Emulsifiers play a key role in the stabilization of foam bubbles. In food foams, biopolymers such as proteins are contributing to long-term stability through several effects such as increasing bulk viscosity and the formation of viscoelastic interfaces. Recent studies have identified promising new stabilizers for (food) foams and emulsions, for instance biological particles derived from water-soluble or water-insoluble proteins, (modified) starch as well as chitin. Microfluidic platforms could provide a valuable tool to study foam formation on the single-bubble level, yielding mechanistic insights into the formation and stabilization (as well as destabilization) of foams stabilized by these new stabilizers. Yet, the recent developments in microfluidic technology have mainly focused on emulsions rather than foams. Microfluidic devices have been up-scaled (to some extent) for large-scale emulsion production, and also designed as investigative tools to monitor interfaces at the (sub)millisecond time scale. In this review, we summarize the current state of the art in droplet microfluidics (and, where available, bubble microfluidics), and provide a perspective on the applications for (food) foams. Microfluidic investigations into foam formation and stability are expected to aid in optimization of stabilizer selection and production conditions for food foams, as well as provide a platform for (large-scale) production of monodisperse foams.

Photothermal trap utilizing solar illumination for ice mitigation

Ice buildup is an operational and safety hazard in wind turbines, power lines, and airplanes. Traditional deicing methods, including mechanical and chemical means, are energy-intensive or environmentally unfriendly. Superhydrophobic anti-icing surfaces, while promising, can become ineffective due to frost formation within textures. We report on a "photothermal trap"-a laminate applied to a base substrate-that can efficiently deice by converting solar illumination to heat at the ice-substrate interface. It relies on the complementing properties of three layers: a selective absorber for solar radiation, a thermal spreader for lateral dispersal of heat, and insulation to minimize transverse heat loss. Upon illumination, thermal confinement at the heat spreader leads to rapid increase of the surface temperature, thereby forming a thin lubricating melt layer that facilitates ice removal. Lateral heat spreading overcomes the unavoidable shadowing of certain areas from direct illumination. We provide a design map that captures the key physics guiding illumination-induced ice removal. We demonstrate the deicing performance of the photothermal trap at very low temperatures, and under frost and snow coverage, via laboratory-scale and outdoor experiments.

Buoyant droplets on functional fibers

In the absence of gravity, the wetting of droplets on fibers is characterized by the competition between an axisymmetric barrel morphology engulfing the fiber and a symmetry-broken clamshell morphology with the droplet sitting on the side of the fiber. In the generic case of nonzero buoyancy the cylindrical symmetry of the barrel morphology is broken, yet barrels and clamshells can still be distinguished based on their different interfacial topologies being multiply and simply connected, respectively. Next to contact angle and droplet size the capillary length appears as a third parameter controlling the droplet morphology. For droplets of variable size, contact angle and buoyancy are independently varied in experiments by use of electrowetting and density mismatch. This approach--together with the complementary numerical calculations--provides new insights into the gradual shifts of the stability limits in the presence of an external volume force. Overall, the parameter space for stable clamshells is found to expand with increasing gravitational forces, gradually shrinking the regimes of stable barrels and bistability. In addition, a new stability limit is introduced for the clamshell morphology related to a partial detachment of the wetting liquid from the fiber, appearing toward higher droplet volumes.

Dynamics of collapse of air films in drop impact

Liquid drops hitting solid surfaces deform substantially under the influence of the ambient air that needs to be squeezed out before the liquid actually touches the solid. Nanometer- and microsecond-resolved dual wavelength interferometry reveals a complex evolution of the interface between the drop and the gas layer underneath. For intermediate impact speeds (We∼1…10) the layer thickness can develop one or two local minima-reproduced in numerical calculations-that eventually lead to the nucleation of solid-liquid contact at a We-dependent radial position, from a film thickness >200  nm. Solid-liquid contact spreads at a speed involving capillarity, liquid viscosity and inertia.

Polymer microcapsules with a fiber-reinforced nanocomposite shell

Polymer microcapsules can be used as controlled release systems in drugs or in foods. Using layer-by-layer adsorption of common food proteins and polysaccharides, we produced a new type of microcapsule with tunable strength and permeability. The shell consists of alternating layers of pectin and whey protein fibrils, yielding a fiber-reinforced nanocomposite shell. The strength can be tightly controlled by varying the number of layers or the density and length of the fibrils in the protein layers. The mechanical stability of these microcapsules appears to be superior to that of currently available multilayer capsules. The method involves only standard unit operations and has the potential for scaling up to industrial production volumes.