Axel Huerre (Department of Chemical Engineering, Imperial College London) - Dynamics of complex interfaces: lubrication films and high-frequency deformation of colloidal monolayers

New experimental tools and theoretical concepts allow for the design of interfaces with a large degree of complexity exhibiting original dynamics and reviving the interest for the physics of interfaces. In this seminar, we show two examples of this complexity through droplet confinement and dynamics of colloidal monolayers.

In a first part, we are interested in the motion of droplets in a confined, micrometric geometry, by considering the lubrication film effect on droplet velocities. When capillary forces dominate, the lubrication film thickness evolves non-linearly with the capillary number due to the viscous dissipation between the meniscus and the wall. However, this film may become thin enough (tens of nanometres) that intermolecular forces come into play and affect classical scalings. We present our interferometric method which yield highly resolved topographies of the shape of the interface and allow us to bring new insights into droplet dynamics in microfluidics. We then discuss the characterization of two dynamical regimes as the capillary number (droplet velocity) increases:

(i) at low capillary numbers, the film thickness is constant and set by the disjoining pressure, (ii) above a critical capillary number, the interface behaviour is well described by a viscous scenario considering the different sources of viscous dissipation.

Finally, we briefly present a refined model to predict the droplet velocity.

In a second part, we focus on particle-coated bubbles subjected to ultrasonic driving. Exposing a particle-coated bubble to ultrasound waves enables us to achieve high-frequency compression-expansion of the monolayer in the range 10-100 kHz. Interestingly, we find that the periodic compression and expansion of the interface drives a qualitatively different dynamical evolution of the monolayer compared to what is commonly observed under shear deformation. In particular, we observe self-assembly of the particles into a network of strings.

We ascribe the emergence of this microstructure to transient interparticle interactions occurring during dynamic deformation. A simple force balance on a sphere attached to the interface by capillary forces, and undergoing oscillations normal to the interface, reveals that the inertia of the particle is important for a micron-sized colloid on the very short timescale of our experiments. The motion of the particle normal to the interface causes a dynamic deformation of the interface leading to transient capillary interactions between the particles. Particle-based simulations confirm that the emergence of strings can only be explained by the coupling of this transient deformation with the equilibrium deformation that is initially present due to nanoscale undulations of the contact line.

This work provides the first demonstration of unique dynamical phenomena upon extreme deformation of complex fluid interfaces, and lays the foundations for future studies of out-of-equilibrium behaviour of 2D soft matter.