Fingering Of Droplet Spreading Containing Insoluble Surfactants On A Precursor Film

When a droplet containing surfactants is deposited on the precursor film or solid substrate, the spreading process will be presented due to the surfactant concentration gradient. Due to the existence of initial disturbance, the thinning region of the drop spreading shows complex patterns, namely fingering phenomena. The fingers undergo branching, coalescence and tip splitting as they spreads in the direction of uncontaminated liquid. The spreading process of stability study has become the frontier in the field of fluid mechanics, colloid and surface science subject, and it is an interdisciplinary research hot spot.In this paper, for the flow of the droplet with insoluble surfactant, the liquid film thickness and concentration of surfactant evolution models are established using the lubrication theory, and the fingering phenomena appearing in the process of droplet spreading are examined with the numerical simulation, as well as the impacts on the fingering phenomena caused by the related parameters are investigate. The main contents of the paper are listed as follows:(1) For the spreading of the droplet with insoluble surfactant, the liquid film thickness and concentration of surfactant evolution models are established using the lubrication theory. Adding the initial disturbance to the initial conditions of the liquid film thickness equation is performed. The fingering phenomena in the process of droplet spreading are simulated with FREEFEM, and the influence of the change of the initial disturbance on fingering features are compared. The most dangerous disturbance wave number is found with the power spectrum diagram of the liquid film thickness from the simulation results.(2) The effects of different parameters, including the precursor film thickness, initial concentration of surfactant, capillary number C and the substrate, on the fingering characteristics are investigated with numerical simulation of FREEFEM procedure. The notable factors on the degree of the finger bifurcation are revealed by calculating the fractal dimension.