Synchrotron X-ray nano-imaging reveals the dynamics of viscoelastic wetting ridges, directly visualizing the ridge-height “lowering” and the ridge-base “broadening” in real-time.

Elastocapillary waves appear on the surface of soft gels and by measuring the dispersion of these waves we are able to extract the surface tension.

A fundamental understanding of the flow of polymer solutions through the pore spaces of porous media is relevant and significant to enhanced oil recovery and groundwater remediation. We present in this work an experimental study of the fluid rheological effects on non-Newtonian flows in a simple laboratory model of the real-world pores—a rectangular sudden contraction–expansion microchannel. We test four different polymer solutions with varying rheological properties, including xanthan gum (XG), polyvinylpyrrolidone (PVP), polyethylene oxide (PEO), and polyacrylamide (PAA). We compare their flows against that of pure water at the Reynolds ( $Re$ ) and Weissenburg ( $Wi$ ) numbers that each span several orders of magnitude. We use particle streakline imaging to visualize the flow at the contraction–expansion region for a comprehensive investigation of both the sole and the combined effects of fluid shear thinning, elasticity and inertia. The observed flow regimes and vortex development in each of the tested fluids are summarized in the dimensionless $Wi-Re$ and ${\chi }_L-Re$ parameter spaces, respectively, where ${\chi }_L$ is the normalized vortex length. We find that fluid inertia draws symmetric vortices downstream at the expansion part of the microchannel. Fluid shear thinning causes symmetric vortices upstream at the contraction part. The effect of fluid elasticity is, however, complicated to analyze because of perhaps the strong impact of polymer chemistry such as rigidity and length. Interestingly, we find that the downstream vortices in the flow of Newtonian water, shear-thinning XG and elastic PVP solutions collapse into one curve in the ${\chi }_L-Re$ space.

Surface waves are excited by mechanical vibration of a cylindrical container having an air/water interface pinned at the rim, and the dynamics of pattern formation is analysed from both an experimental and theoretical perspective. The wave conforms to the geometry of the container and its spatial structure is described by the mode number pair () that is identified by long exposure time white light imaging. A laser light system is used to detect the surface wave frequency, which exhibits either a (i) harmonic response for low driving amplitude edge waves or (ii) sub-harmonic response for driving amplitude above the Faraday wave threshold. The first 50 resonant modes are discovered. Control of the meniscus geometry is used to great effect. Specifically, when flat, edge waves are suppressed and only Faraday waves are observed. For a concave meniscus, edge waves are observed and, at higher amplitudes, Faraday waves appear as well, leading to complicated mode mixing. Theoretical predictions for the natural frequency of surface oscillations for an inviscid liquid in a cylindrical container with a pinned contact line are made using the Rayleigh–Ritz procedure and are in excellent agreement with experimental results.

A soft cylindrical interface endowed with surface tension can be unstable to wavy undulations. The most unstable wavelength depends upon the viscoelastic properties of the material and is determined by a dynamic stability analysis.

Thin-film flow down a vertical fiber gives rise to a number of instabilities that define the bead-on-fiber morphology including Plateau-Rayleigh breakup, isolated bead formation, and convective instabilities. Experiments are performed that reveal an asymmetric instability which depends upon the liquid surface tension and fiber diameter and exhibits all the bead-on-fiber morphologies. The bead dynamics are described by the bead spacing and bead velocity, with the asymmetric morphology displaying more regular dynamics than the symmetric morphology. For the asymmetric morphology, the transition from the Plateau-Rayleigh to convective regime agrees well with predictions for a free viscous jet, indicating a minimal effect between the thin film and fiber. In addition, the dimensionless bead velocity is shown to scale with the capillary number for all experimental data. These observations for the asymmetric bead dynamics can be used as a design tool for heat and mass transfer applications.