Dynamics of spreading thixotropic droplets

- Capillarity driven spreading of thixotropic droplets is considered.
- A 1.5D model is derived and compared against various depth closures.
- Depth-averaging is shown to be the most robust for the parameter range considered.
- Change of viscosity function can lead to different behaviour such as self-pinning.

The effect of thixotropy on the two-dimensional spreading of a sessile drop is modelled using lubrication theory. Thixotropy is incorporated by the inclusion of a structure parameter, λ, measuring structure build-up governed by an evolution equation linked to the droplet micromechanics. A number of models are derived for λ coupled to the interface dynamics; these range from models that account for the cross-stream dependence of λ to simpler ones in which this dependence is prescribed through appropriate closures. Numerical solution of the governing equations show that thixotropy has a profound effect on the spreading characteristics; the long-time spreading dynamics, however, are shown to be independent of the initial structural state of the droplet. We also compare the predictions of the various models and determine the range of system parameters over which the simple models provide sufficiently good approximations of the full, two-dimensional spreading dynamics.