Ph.D Scholarship Programme "Heraclitus II"

Thin liquid films appear in a variety of natural and biological settings and are prominent in many industrial applications. They also represent one of the prototypes  of free surface flow, where theories of nonlinear dynamics and chaos are applicable. The addition of surfactants in film flow is encountered in a wide spectrum of applications, ranging from detergent production to modern therapeutic treatments.

The present study investigates experimentally the modifications in the dynamics of liquid film flow from the addition of Isopropanol (IP) and of Sodium Dodecyl Sulfate (SDS). Conductivity probes are used to record temporal variation of liquid film thickness at various locations along the flow. These data permit detection of the primary instability threshold, and documentation of the wave dynamics in the unstable regime.

Aqueous solutions of Isopropanol and Glycerol exhibit similar behavior, despite the fact that their surface tension is significantly different. The dependence of the critical Reynolds number on fluid properties is expressed by the Kapitza number. A delay in the onset of the primary instability (in comparison to the classical, two-dimensional prediction) is observed, which depends on channel width, and is most prominent at high Kapitza values. The non-linear evolution, which leads to the development of solitary waves with well developed front running ripples, is a common characteristic of all clean liquids.

Aqueous solutions of SDS exhibit a remarkably different behavior from that of clean liquids.  Inlet disturbances decay drastically and the critical Reynolds number is strongly dependent on surfactant concentration. Small amounts of SDS result in drastic delay of the primary instability, and maximum stabilization occurs at concentrations around 10% of the critical micelle concentration. Further addition of surfactant has an adverse effect, and at large concentrations the clear liquid limit is asymptotically approached. The dominant structures for the majority of inlet frequencies tested, even at high flow rates, are sinusoidal travelling waves of very small amplitude. 

The above observations for SDS may be interpreted by talking into account the elastic behavior of the free surface, due to Marangoni stresses caused by gradients in the surfactant concentration.  Increasing the amount of SDS dissolved in the liquid   enhances mass transport between the interface and the bulk, and therefore mitigates surface concentration gradients and the concomitant Marangoni stresses..