Spreading of drops exhibiting complex rheology

Understanding the drop spreading behaviour of rheologically complex liquids, like dense granular suspensions or concentrated polymer solutions, is of crucial importance for applications such as inkjet printing, spraying paints or coating. In this presentation, we focus in the correlation between complex rheological behavior, like shear thickening or shear thinning, and the on the early dynamics of drop spreading on hard substrates.

As shear thickening systems, we study the spreading behaviour of suspensions on clean hydrophilic glass surfaces, with increasing particle mass fractions (Φ = 0.3 to 0.65) and varying particle diameter (d = 10 to 20 μm), chemistry and morphology. Depending on the particle mass fraction, being close or not to the jamming transition of the suspension, two outcomes were observed: the spreading curve either exhibited a spreading behaviour qualitatively similar to the one of the Newtonian carrier liquid, with two spreading regimes or a spreading behaviour diverging from the reference, with three spreading regimes and an increase in spreading rate in-between two. The different spreading behavior strongly correlates with an increasing shear thickening of the suspensions. The correlation between rheological behavior and spreading dynamics is general for all investigates particle types. Shear thinning polymer solution also exhibit strong correlation of the spreading dynamics with the rheological properties of the solutions, like the shear rate at the end of the Newtonian plateau or the exponent in an assumed power law model. In both examples the relation of the time scale of the experiment and the intrinsic relaxation time of the sample is relevant and will be discussed.

In both cases, we relate the observed correlations between rheological behaviour and spreading dynamics of drops to the dissipative mechanisms close to the moving contact line of the drop.