Arresting bubble coarsening: A two-bubble experiment to investigate grain growth in presence of surface elasticity

Many two-phase materials suffer from grain-growth due to the energy cost which is associated with the interface that separates both phases. While our understanding of the driving forces and the dynamics of grain growth in different materials is well advanced by now, current research efforts address the question of how this process may be slowed down, or, ideally, arrested. We use a model system of two bubbles to explore how the presence of a finite surface elasticity may interfere with the coarsening process and the final grain size distribution. Combining experiments and modelling in the analysis of the evolution of two bubbles, we show that clear relationships can be predicted between the surface tension, the surface elasticity and the initial/final size ratio of the bubbles. We rationalise these relationships by the introduction of a modified Gibbs criterion. Besides their general interest, the present results have direct implications for our understanding of foam stability

Probing the Mechanical Strength of an Armored Bubble and Its Implication to Particle-Stabilized Foams

Bubbles are dynamic objects that grow and rise or shrink and disappear, often on the scale of seconds. This conflicts with their uses in foams where they serve to modify the properties of the material in which they are embedded. Coating the bubble surface with solid particles has been demonstrated to strongly enhance the foam stability, although the mechanisms for such stabilization remain mysterious. In this paper, we reduce the problem of foam stability to the study of the behavior of a single spherical bubble coated with a monolayer of solid particles. The behavior of this armored bubble is monitored while the ambient pressure around it is varied, in order to simulate the dissolution stress resulting from the surrounding foam. We find that above a critical stress, localized dislocations appear on the armor and lead to a global loss of the mechanical stability. Once these dislocations appear, the armor is unable to prevent the dissolution of the gas into the surrounding liquid, which translates into a continued reduction of the bubble volume, even for a fixed overpressure. The observed route to the armor failure therefore begins from localized dislocations that lead to large-scale deformations of the shell until the bubble completely dissolves. The critical value of the ambient pressure that leads to the failure depends on the bubble radius, with a scaling of ΔP_{collapse}∝R^{-1}, but does not depend on the particle diameter. These results disagree with the generally used elastic models to describe particle-covered interfaces. Instead, the experimental measurements are accounted for by an original theoretical description that equilibrates the energy gained from the gas dissolution with the capillary energy cost of displacing the individual particles. The model recovers the short-wavelength instability, the scaling of the collapse pressure with bubble radius, and the insensitivity to particle diameter. Finally, we use this new microscopic understanding to predict the aging of particle-stabilized foams, by applying classical Ostwald ripening models. We find that the smallest armored bubbles should fail, as the dissolution stress on these bubbles increases more rapidly than the armor strength. Both the experimental and theoretical results can readily be generalized to more complex particle interactions and shell structures

Frugal Droplet Microfluidics Using Consumer Opto-Electronics

<div><p>The maker movement has shown how off-the-shelf devices can be combined to perform operations that, until recently, required expensive specialized equipment. Applying this philosophy to microfluidic devices can play a fundamental role in disseminating these technologies outside specialist labs and into industrial use. Here we show how nanoliter droplets can be manipulated using a commercial DVD writer, interfaced with an Arduino electronic controller. We couple the optical setup with a droplet generation and manipulation device based on the “confinement gradients” approach. This device uses regions of different depths to generate and transport the droplets, which further simplifies the operation and reduces the need for precise flow control. The use of robust consumer electronics, combined with open source hardware, leads to a great reduction in the price of the device, as well as its footprint, without reducing its performance compared with the laboratory setup.</p></div

Coupling the microfluidics and the DVD writer.

<p>(a) Schematic of the microfluidic device on top of the DVD writer. The DVD focus and tracking positions are controlled through an Arduino card, which also controls the on-off switching of the laser. (b-c) Images of the DVD writer, without and with the microfluidic device in place. (d) 3D profilometry of the microfluidic circuit. The colors indicate the local channel depth, as shown on the color bar. The left-most entrance transports the continuous oil phase. The entrance on the right side is used to inject the water phase. The sloped region produces monodisperse drops passively. finally, a default central rail and four side rails are visible on the right side of the image.</p

Drop size and velocity.

<p>(a) Value of the water inlet pressure that allows a constant droplet size, as a function of the imposed pressure on the oil inlet. (b) Velocity of the droplets, downstream of the sloped region, as a function of the imposed pressure on the oil inlet.</p

Multiple sorting positions.

<p>Sorting droplets onto different rails by controlling the position of the laser lens. Each panel shows a superposition of images for a single drop in the device. (a) If the laser is off, the drop remains on the default central rail. (b-d) Different laser positions force the drop to jump on different rails, which lead to different regions downstream.</p

Controlling the DVD laser and lens.

<p>The DVD laser was controlled using a constant current source. The power output of the circuit was determined manually by turning the variable resistor. The mobile lens position was controlled through the Arduino motor shield. Its position could be programmed through software. Extreme care must be taken while manipulating the laser, as these are relatively high power lasers that can cause severe eye damage.</p

Required power for derailing drops.

<p>Minimum electrical current required to derail droplets onto the first rail. Faster moving droplets require a more intense laser to switch rails. The largest current achieved on this laser was 0.28 A. Drops beyond a velocity of 6 mm/s could not be derailed at this maximum value.</p