The dynamic wetting of simple liquids on planar, smooth, homogeneous, inert surfaces is relatively well understood. We study the dynamic wetting of complex liquids on surfaces. As a "complex liquids" we understand a liquid, which is structured at different length scale or shows internale relaxation. Examples include dispersion, emulsion, polymer solutions and melts, and surfactant solutions. Spontaneous and forced wetting and dewetting is analyzed.
Force wetting and dewetting of surfactant solutions: Forced wetting and dewetting of polymer surfaces in aqueous solutions containing cationic surfactant cetyltrimethylammonium bromide (CTAB) has been studied with a rotating cylinder half immersed in the solution. The receding contact angle decreases with faster withdrawing speeds. This decrease is enhanced when adding CTAB. The addition of salt to the CTAB solution further enhances the effect, but does not have a significant effect alone.We interpret this change in the dynamic contact angle with a surfactant-induced Marangoni effect.

• Fell, D., G. K. Auernhammer, E. Bonaccurso, C. Liu, M. Sokuler, and H.-J. Butt:
  Influence of Surfactant Concentration and Background Salt on Forced Dynamic Wetting and Dewetting.
  Langmuir 27, 2112-2117 (2011).
• Fell, D., N. Pawanrat, E. Bonaccurso, H. J. Butt, and G. K. Auernhammer:
  Influence of surfactant transport suppression on dynamic contact angle hysteresis.
  J Adhes. Sci. Technol., (2011), in press.

Fast dynamic wetting of polymer surfaces by miscible and immiscible liquids: Using a high speed camera, we studied the initial stages of spontaneous spreading of a solvent drop (toluene) on the surface of a soluble polymer (polystyrene). For drops of 1–4 μL volume, the increase in contact radius r follows a power law r ~ t^α, with the spreading exponent α=0.50 and for the first ≈8 ms. Thereafter, the three-phase contact line stayed pinned leading to a macroscopic static contact angle of Θ₀=12–15°. The insoluble liquids ethanol (α=0.47, Θ₀=0) and water (α=0.35, Θ₀=90°) showed a slower spreading. We attribute the fast spreading of toluene to the strong interaction with the polymer, like in reactive wetting. The finite macroscopic contact angle indicates the formation of a ridge by softening of polystyrene due to permeated toluene and the subsequent plastic deformation by the surface tension of the liquid. This interpretation is supported by experiments on polymers grafted from a silicon wafer. Toluene completely wets polymer brush surfaces. Transport of toluene through the vapor phase plays a significant role.

• Muralidhar, P., E. Bonaccurso, G. K. Auernhammer, and H.-J. Butt:
  Fast dynamic wetting of polymer surfaces by miscible and immiscible liquids.
  Colloid & Polymer Science 289, 1609-1615 (2011).

Spontaneous spreading of polymer melts: The spontaneous spreading of drops of polyisoprene melt terminated with groups on hydrophilic silicon surfaces has been studied experimentally. We sued methyl (PI-CH₃), hydroxyl (PI-OH), and carboxyl groups (PI-COOH) terminated poly-isoprene. Despite the fact that all three polymers have a similar surface tension (0.032 N/m) at the polymer-air interface, the equilibrium contact angles depend strongly on the end groups of the polymer. In the same way the spontaneous spreading of PI-OH and PI-COOH is slowed down as compared to PI-CH₃, most probably due to the strong interfacial binding of the hydroxyl or carboxyl end group to the surface.

• Liu, C., E. Bonaccurso, R. Sokuler, G. Auernhammer & H.-J. Butt:
  Dynamic wetting of polyisoprene melts with different end groups.
  Langmuir 26, 2544 - 2549 (2010).