A drop impact study of worm-like viscoelastic surfactant solutions

In this paper, we investigate the impact dynamics of drops of complexed surfactant solutions. Dilute to semi-dilute solutions of rod-like to worm-like micelles of equimolar cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) in water (CTAB–NaSal (1:1)) have been dropped onto a smooth hydrophobic surface. The variations in impact behaviour of each solution are discussed in terms of the dynamic surface tension and viscoelastic properties. Increases in the concentration of CTAB–NaSal (1:1) in solution resulted in enhanced reduction in the maximum height achieved during recoil, even though the maximum diameter reached during spreading was equivalent for all solutions over the range of concentrations explored. The equivalence in the maximum diameter during spreading is due to the loss of fluid structure at high extension rates, whilst the resulting reduction in rebound tendency is due to a rapid restructuring of the fluid post impact. The drop impact of such complexed surfactant solutions provides significant insight into the contributions of a ‘dynamic’ fluid microstructure to the rebound behaviour of dilute ‘living polymer’ solutions.