Effects of contamination-induced dynamic surface tension on sessile drop fragmentation via inertial detachment

Upon impact of a rain, irrigation, or dew droplet on a sufficiently compliant substrate, the substrate's motion can transfer its impulse to a contaminated sessile drop residing on it. For substrates of intermediate wetting, the fragmentation of such contaminated sessile drop occurs via inertial detachment (Gilet & Bourouiba, J. R. Soc. Interface, vol. 12, 2015, 20141092) and can be particularly efficient at producing contaminated ejected microdroplets with little dilution, via end-pinching. Understanding this process informs the quantification of pathogen surface removal and their subsequent dispersal. Inertial detachment fragmentation, from substrates of intermediate wetting, involves asymmetric axial stretching of part of the sessile drop, with rich contact-line dynamics (Shen et al. JFM vol 1002, A6, 2024). Surface-active pathogens could alter surface tension and the associated contact dynamics, on a timescale comparable to that of the inertial detachment fragmentation. In this work, using direct numerical simulations, we examine inertial fragmentation of pathogen-laden drops sessile on surfaces of average wetting, incorporating possible surface-active pathogen-fluid interactions. A hybrid level-set/front-tracking method is employed, coupled with bulk-interface surfactant transport and a generalised dynamic surface tension model. With this framework, we examine the role of surfactant elasticity, pathogen concentration, and sorption kinetics on the regimes of fragmentation, and emission of secondary contaminant-laden microdroplets.