Influence of surfactants in forced dynamic dewetting

Abstract

In this work we show that the forced dynamic dewetting of surfactant solutions depends sensitively on the surfactant concentration. To measure this effect, a hydrophobic rotating cylinder was horizontally half immersed in aqueous surfactant solutions. Dynamic contact angles were measured optically by extrapolating the contour of the meniscus to the contact line. Anionic (sodium 1-decanesulfonate, S-1DeS), cationic (cetyl trimethylammonium bromide, CTAB) and nonionic surfactants (CE, CE and CE) with critical micelle concentrations (CMCs) spanning four orders of magnitude were used. The receding contact angle in water decreased with increasing velocity. This decrease was strongly enhanced when adding surfactant, even at surfactant concentrations of 10% of the critical micelle concentration. Plots of the receding contact angle-versus-velocity almost superimpose when being plotted at the same relative concentration (concentration/CMC). Thus the rescaled concentration is the dominating property for dynamic dewetting. The charge of the surfactants did not play a role, thus excluding electrostatic effects. The change in contact angle can be interpreted by local surface tension gradients, i.e. Marangoni stresses, close to the three-phase contact line. The decrease of dynamic contact angles with velocity follows two regimes. Despite the existence of Marangoni stresses close to the contact line, for a dewetting velocity above 1-10 mm s the hydrodynamic theory is able to describe the experimental results for all surfactant concentrations. At slower velocities an additional steep decrease of the contact angle with velocity was observed. Particle tracking velocimetry showed that the flow profiles do not differ with and without surfactant on a scales >100 μm.

Membrane emulsification: Formation of water in oil emulsions using a hydrophilic membrane

“…Therefore, advancing and receding contact angles were determined for different speeds (0-8 mm/s). These are relatively low speeds compared to the studies undertaken by G. K. Auernhammer and his colleagues [21][22][23][24], where dynamic receding contact angle measurements showed a decrease with increasing velocity as well as with increasing surfactant concentrations. It was postulated in [19][20][21][22] that Marangoni stresses were the main contribution to the contact angle shift.…”

“…K. Auernhammer and his co-workers are currently using a rotating drum setup for studying velocity-dependent wetting/dewetting processes of complex (surfactant) solutions between a solid-gas-liquid interface [21][22][23][24]. Below a similar setup is adopted, however, the vapour phase is replaced by an organic phase to allow the investigation of wetting properties between two liquids and a solid phase.…”

Versatile high-speed confocal microscopy using a single laser beam

“…In such wetting experiments, some of the interesting quantities are the position and dynamics of the contact line and the flow field inside the wetting liquid. [58][59][60][61][62] The setup described here allows measuring both quantities with the highest possible resolution simultaneously. The reflected light of the substrate allows a precise detection of the contact line, because the reflected intensity changes strongly at the contact line.…”

Forced dynamic dewetting of structured surfaces: Influence of surfactants

“…However, the dynamic wetting of multicomponent liquids, like surfactant solutions, is less understood. In recent years, an increasing amount of research has been done on this topic [3,[20][21][22][23][24][25][26][27]. Surfactant molecules absorb at the interface and change the interfacial tension [28].…”

“…We use four different motors equipped with suitable gearings to cover one decade in velocity per motor. For comparison measurements on a smooth surface were performed, where a drum with a spherical segment geometry (coated with polystyrene) as described in [24] was used.…”