Wetting Dynamics of a Lipid Monolayer

Direct measurement and control of the dynamic wetting properties of a lipid-coated water-air interface over a wide range of surface tension variations have many important applications. However, the wetting dynamics of the interface near its partial-to-complete wetting transition has not been fully understood. Here, we report a systematic study of the wetting dynamics of a lipid-coated water-air interface around a thin glass fiber of diameter 1-5 μm and length 100-300 μm. The glass fiber is glued onto the front end of a rectangular cantilever to form a "long-needle" atomic-force-microscope probe. Three surface modifications are applied to the glass fiber to change its wetting properties from hydrophilic to hydrophobic. A monolayer of phospholipid dipalmitoylphosphatidylcholine (DPPC) is deposited on the water-air interface in a homemade Langmuir-Blodgett trough, and the surface tension γ_L of the DPPC-coated water-air interface is varied in the range of 2.5 ≲ γ_L ≲ 72 mN/m. From the measured hysteresis loop of the capillary force for the three coated fiber surfaces with varying γ_L, we observe a sharp transition from partial to complete wetting when γ_L is reduced to a critical value (γ_L)_c. The obtained values of (γ_L)_c are 27 ± 1 mN/m for a DPPC-coated fiber surface and 23 ± 1 mN/m for an trichloro(1H,1H,2H,2H-perfluorooctyl) silane (FTS)-coated surface. Below (γ_L)_c, the contact angle θ₀ of the liquid interface is found to be zero for both hydrophobic fiber surfaces and the corresponding spreading parameter S becomes positive. For the FTS-coated fiber surface, the height of capillary rise exhibits a jump when γ_L is reduced to (γ_L)_c, which indicates that a rapidly advancing liquid film is formed on the fiber surface when the partial-to-complete wetting transition takes place. Our experiment thus establishes a quantitative method by which many other liquid interfaces coated with polymers, surfactants, and biomolecules (such as proteins and lipids) may be characterized dynamically.