Static and Dynamic Optical Analysis of Micro Wrinkle Formation on a Liquid Surface

Abstract

A spatially periodic voltage was used to create a dielectrophoresis induced periodic micro wrinkle deformation on the surface of a liquid film. Optical Coherence Tomography provided the equilibrium wrinkle profile at submicron accuracy. The dynamic wrinkle amplitude was derived from optical diffraction analysis during sub-millisecond wrinkle formation and decay, after abruptly increasing or reducing the voltage, respectively. The decay time constant closely followed the film thickness dependence expected for surface tension driven viscous levelling. Modelling of the system using numerical solution of the Stokes flow equations with electrostatic forcing predicted that wrinkle formation was faster than decay, in accord with observations.

Dynamics of drying colloidal suspensions, measured by optical coherence tomography

“…Optical coherence tomography has recently been showing its potential for the study of soft materials. 26–32,34–36 While most applications, to date, have involved tracking particle motion or the shapes of surfaces or interfaces, we have shown that OCT can be a sensitive probe of the evolving composition of soft materials. Here, we compare our results with those obtained by other methods, chiefly SAXS (see introduction for a list of other methods recently applied to drying films), in order to demonstrate the strengths and limitations of OCT methods for the study of drying films and coatings.…”

“…Optical coherence tomography has recently been showing its potential for the study of soft materials. [26][27][28][29][30][31][32][34][35][36] While most 7, for initial particle concentrations of j 0 = 0.06 (dashed lines) and j 0 = 0.19 (solid lines). At the lower j 0 a long induction period of 78 min is observed, before any solid packed film appears, even though the evaporation rate (given by dx l /dt) is similar in both cases.…”

“…Optical coherence tomography was first developed for medical applications 25 but has been emerging as a powerful imaging method for applications in soft matter physics [26][27][28][29][30][31] and the heritage sciences. [32][33][34][35][36] Using OCT, dynamic measurements of surfaces or interfaces can be made, for example to monitor the roughness of a drying varnish layer, 33 or to measure the evaporation rates of droplets, 30 the spreading rates of extruded fluids, 31 and the formation and decay of wrinkles during dielectrophoresis.…”

“…[32][33][34][35][36] Using OCT, dynamic measurements of surfaces or interfaces can be made, for example to monitor the roughness of a drying varnish layer, 33 or to measure the evaporation rates of droplets, 30 the spreading rates of extruded fluids, 31 and the formation and decay of wrinkles during dielectrophoresis. 29 Internal features like shear bands can also be visualised. 28 Alternatively, it can be used to track reflective tracer particles, to measure fluid flows in liquids.…”

“…As such, OCT can detect changes in the refractive index of a drying droplet of a polymer solution, 26 for example, as well as characterising dynamic wrinkling on the surface of liquid films. 29 Within a colloidal suspension any optical distance will depend on the concentration of colloidal particles, φ , and on the difference between the refractive indices of the particles and their solvent. With proper calibration, we will show that OCT can thus provide a means of dynamically measuring properties of suspensions, like particle concentrations.…”