Viscoplastic drop impact on thin films

We quantitatively test a dimensionless group (Blackwell et al., Phys. Fluids, vol. 27, 2015, 043101) for predicting the onset of splash behaviour for drop impacts of yield-stress fluids on horizontal surfaces pre-coated with the same fluid. Two classes of complex fluid are considered, with different chemistry and material microstructure: 'gel' suspensions of attractive hard colloidal clay particles (Laponite RD), and 'glassy' suspensions of crowded soft polymeric particles (Carbopol 940). High-speed imaging identifies drop impact regimes as a function of material composition, drop diameter, velocity and coating thickness. The high-dimensional parameter space collapses into the single dimensionless group, which we find successfully separates the impact results into distinct regimes. Moreover, it gives a constant critical value for the regime boundary as a function of dimensionless coating thickness, and remarkably this critical value is only a factor of 2 larger for Carbopol when compared to Laponite at several different concentrations. The results demonstrate insight that depends on macroscopic dynamic conditions and rheological properties largely independent of chemistry and material microstructure within the range of conditions studied.