Understanding the drop spreading behavior of rheologically complex liquids, such as dense granular suspensions or concentrated polymer solutions, is critical for applications such as inkjet printing, paint spraying or coating. In this presentation we will focus on the correlation between complex rheological behavior, such as shear thickening or shear thinning, and the dynamics of drop spreading and coalescence on hard substrates. For low viscosity fluids, such as water, it is well known that the spreading dynamics consists of two regimes; the inertial dominated regime and the viscous dominated regime. In the inertial regime, the wetted radius increases with the square root of time (r(t)~t^0.5). After reaching a cap shape, the viscous force becomes dominant and the spreading rate decreases (t(t)~t^0.1) [1].

We compare the spreading of viscoelastic (aqueous polymer solution) and Newtonian viscous (aqueous glycerine solution) drops on solid substrates of different wettability. For drops of the same zero shear viscosity, we find in the early stages of spreading that viscoelastic drops (i) spread faster and (ii) their contact radius has a different power law versus time than Newtonian drops. We argue that the effect of viscoelasticity is only observable for experimental timescales of the order of or greater than the internal relaxation time of the viscoelastic polymer solution. We attribute this behavior to shear thinning of the viscoelastic polymer solution. As the contact line is approached, the shear rate increases and the steady state viscosity of the viscoelastic drop is lower than that of the Newtonian drop. [2] The same behavior is observed for drop coalescence of polymer solutions on substrates.

In the last part of the talk, I will briefly present other drop-related publications of mine [3-5].

References:

[1] First steps in the spreading of a liquid droplet, Anne-Laure Biance et al. Phys. Rev. 2004.

[2] Spreading of a viscoelastic drop on a solid substrate, Peyman Rostami et al. JFM, 2024.

[3] Gas-Phase Induced Marangoni Flow Causes Unstable Drop Merging, Peyman Rostami et al. Langmuir 2020.

[4] Capillary filling in drop merging: Dynamics of the four-phase contact point, Peyman Rostami and Günter K. Auernhammer, Phys. Fluids 2022.

[5] Dynamic wetting properties of PDMS pseudo-brushes: Four-phase contact point dynamics case, Peyman Rostami et al. J. Chem. Phys. 2023.