Experimental study of a surfactant-driven fingering phenomenon in a Hele-Shaw cell

We present an experimental study of a new surfactant-driven fingering phenomenon in a Hele-Shaw cell. First, the threshold of instability is examined and compared with the linear theory. Second, the nonlinear evolution of fingering is quantified, and steady and unsteady patterns are distinguished. A wide range of dynamical behaviour is observed from drifting and merging fingers to cusp formation between fingers and subsequent ejection of air bubbles. All experiments are performed with a pure surfactant - sodium dodecyl sulphate - thus allowing us to obtain a well-defined bifurcation map for the specific kinetic and material properties of this surface-active substance. The measurements are conducted in a Hele-Shaw cell with smooth and roughened walls. A basic physical model is proposed to obtain further insight into the influence of the surfactant properties on the dip-coating process and, as a result, on the critical phenomena. The study allows us to resolve the discrepancies between previous experimental results and linear theory.