Wetting Dynamics of a Lipid Monolayer

Abstract

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 "longneedle" 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,2Hperfluorooctyl) silane (FTS)-coated surface. Below (γ L ) c , the contact angle θ 0 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.

FM-AFM with a Hanging Fiber Probe for the Study of Liquid–Liquid Interfaces

“…12 During the past decade, they have been applied to the imaging of bubbles, drops, 13,14 nanoparticles, or selfassemblies at the interface of two liquids 15 and to the study of the wetting dynamics of coated air−water interfaces. 16 At the same time, several teams have successfully studied the possibility of using an AFM as a microrheometer. 17−19 Working in a liquid environment greatly reduces the sensitivity of standard oscillating AFM probes.…”

“…For example, they have opened the way to the exploration of wetting and capillarity on a local scale, , of surface forces between a colloid and a liquid interface, of the structure of liquids close to a surface, and of the imaging of living cells . During the past decade, they have been applied to the imaging of bubbles, drops, , nanoparticles, or self-assemblies at the interface of two liquids and to the study of the wetting dynamics of coated air–water interfaces . At the same time, several teams have successfully studied the possibility of using an AFM as a microrheometer. − …”

Wetting Kinetics of Molten Carbonate on Carbon

“…The wettability between a liquid media and a solid substrate is ubiquitous in nature − and used in industrial applications such as electrochemical devices, − chemical reactors, , and metallurgical furnaces. , For example, the wettability of electrolyte on electrode and internal cell component materials is the key factor to improve the power density and lifetime of molten carbonate fuel cells (MCFCs); thus, it is widely investigated in the last 10 years. , As a typical type of MCFCs using carbon as fuel, direct carbon fuel cells (DCFCs) can efficiently convert carbon energy into electricity without the efficiency limitation of the Carnot cycle. − The DCFCs involve the wetting between the liquid molten carbonate electrolyte and the solid carbon electrode. − In a typical DCFC, the carbon anode is in contact with the molten carbonate and oxidized at the carbon/molten carbonate (C/MC) interface . The wettability of molten carbonate on the carbon electrode defines the C/MC interface that allows the mass transfer between the anodic product and electrolyte. , Thus, a good wettability between the molten carbonate and the carbon anode ensures good electrochemical performances in terms of power density and efficiency. ,, The wettability of a liquid medium on a solid substrate is defined by the contact angle and the rate of the wetting process .…”