Surfactant- and Aqueous-Foam-Driven Oil Extraction from Micropatterned Surfaces.

Liquid-infused surfaces are rough or patterned surfaces in which a lubricating fluid, such as oil, is infused, which exhibits various original properties (omniphobicity, biofouling, drag reduction). An outer flow in a confined geometry can entrain the oil trapped between the pattern of the surfaces by shearing the oil-water interface and cause the loss of the omniphobic properties of the interface. Starting from the theoretical analysis of Wexler et al. (Shear-driven failure of liquid-infused surfaces. Phys. Rev. Lett. 2015, 114, 168301), where a pure aqueous solution is the outer phase, we extend the predictions by introducing an extraction efficiency parameter α and by accounting for new dynamical effects induced by surfactants and aqueous foams. For surfactant solutions, decreasing the oil-water interfacial tension (γow) not only enhances oil extraction as expected but also modifies the dynamics of the receding oil-water interface through the variations of the receding contact angle (θ) with the capillary number (Ca), which is the ratio between the viscous and the capillary forces at the oil-water interface. For aqueous foams, the extraction dynamics are also influenced by the foam flow: oil is sheared by the thin film between the bubbles and the lubricating layer, which imposes a stronger interfacial shear compared to pure aqueous solutions. In both surfactant and foam cases, the experimental observations show the existence of nonuniform extraction dynamics related to the surfactant-induced instability of a two-fluid shear flow.

Shear-driven failure of liquid-infused surfaces.

Rough or patterned surfaces infused with a lubricating liquid display many of the same useful properties as conventional gas-cushioned superhydrophobic surfaces. However, liquid-infused surfaces exhibit a new failure mode: the infused liquid film may drain due to an external shear flow, causing the surface to lose its advantageous properties. We examine shear-driven drainage of liquid-infused surfaces with the goal of understanding and thereby mitigating this failure mode. On patterned surfaces exposed to a known shear stress, we find that a finite length of the surface remains wetted indefinitely, despite the fact that no physical barriers prevent drainage. We develop an analytical model to explain our experimental results, and find that the steady-state retention results from the ability of patterned surfaces to wick wetting liquids, and is thus analogous to capillary rise. We establish the geometric surface parameters governing fluid retention and show how these parameters can describe even random substrate patterns.

Robust liquid-infused surfaces through patterned wettability.

Liquid-infused surfaces display advantageous properties that are normally associated with conventional gas-cushioned superhydrophobic surfaces. However, the surfaces can lose their novel properties if the infused liquid drains from the surface. We explore how drainage due to gravity or due to an external flow can be prevented through the use of chemical patterning. A small area of the overall surface is chemically treated to be preferentially wetted by the external fluid rather than the infused liquid. These sacrificial regions disrupt the continuity of the infused liquid, thereby preventing the liquid from draining from the texture. If the regions are patterned with the correct periodicity, drainage can be prevented entirely. The chemical patterns are created using spray-coating or deep-UV exposure, two facile techniques that are scalable to generate large-scale failure-resistant surfaces.

Microfluidic in situ mechanical testing of photopolymerized gels.

Gels are a functional template for micro-particle fabrication and microbiology experiments. The control and knowledge of their mechanical properties is critical in a number of applications, but no simple in situ method exists to determine these properties. We propose a novel microfluidic based method that directly measures the mechanical properties of the gel upon its fabrication. We measure the deformation of a gel beam under a controlled flow forcing, which gives us a direct access to the Young's modulus of the material itself. We then use this method to determine the mechanical properties of poly(ethylene glycol) diacrylate (PEGDA) under various experimental conditions. The mechanical properties of the gel can be highly tuned, yielding two order of magnitude in the Young's modulus. The method can be easily implemented to allow for an in situ direct measurement and control of Young's moduli under various experimental conditions.

Capillary bridges between soft substrates.

A wetting droplet trapped in the thin gap between two elastic bodies will deflect the bodies towards one another. The deformation increases the total capillary adhesion force by increasing the contact radius and narrowing the gap height. For flat droplets, with a large ratio of radius to gap height, the Laplace pressure causes surface deformations that are orders of magnitude larger than those induced by a sessile droplet of the same radius. We present experiments, scalings, and closed-form solutions that describe the deformation. Using variational techniques, we also show that the problem exhibits a bifurcation, where the gap spontaneously closes due to an incremental increase in drop volume.

Microfluidic ultralow interfacial tensiometry with magnetic particles.

We describe a technique that measures ultralow interfacial tensions using paramagnetic spheres in a co-flow microfluidic device designed with a magnetic section. Our method involves tuning the distance between the co-flowing interface and the magnet's center, and observing the behavior of the spheres as they approach the liquid-liquid interface-the particles either pass through or are trapped by the interface. Using threshold values of the magnet-to-interface distance, we make estimates of the two-fluid interfacial tension. We demonstrate the effectiveness of this technique for measuring very low interfacial tensions, O(10(-6)-10(-5)) N m(-1), by testing solutions of different surfactant concentrations, and we show that our results are comparable with measurements made using a spinning drop tensiometer.