Surface Tension γL Research Articles

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 γL of the DPPC-coated water-air interface is varied in the range of 2.5 ≲ γL ≲ 72 mN/m. From the measured hysteresis loop of the capillary force for the three coated fiber surfaces with varying γL, we observe a sharp transition from partial to complete wetting when γL is reduced to a critical value (γL)c. The obtained values of (γL)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 (γL)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 γL is reduced to (γL)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.

An Equation-of-State approach to measure the surface free energy (SFE) of bituminous binders

To overcome the liquid set dependency of existing surface free energy (SFE) component approaches, this paper presents an Equation-of-State (EQS) approach to measure the SFE of bituminous binders. A succession of water-ethanol mixtures with varying surface tensions are first prepared as the test liquids. The Wilhelmy plate method is then adopted to measure the surface tension γL of each liquid as well as the contact angle θ between bituminous binder and each test liquid. A quadratic polynomial function is subsequently employed to fit the plot of γLcosθ versus γL. Combining Young’s equation and Good’s interaction parameter and then imposing a limiting condition in which the contact angle between bituminous binder and test liquid is exactly 0°, the SFE of a bituminous binder can be finally determined. This approach is verified to have capability to accurately measure the SFE of bituminous binders with a solid thermodynamic basis.

Fabrication of highly oleophobic and superhydrophobic surfaces on microtextured Al substrates

Theoretical calculations suggest that creating highly oleophobic surfaces would require a surface energy lower than that of any known materials. In the present work, we demonstrate microtextured Al substrate surfaces with vein-like micro-/nanostructures displaying apparent contact angles of >120°, even with nitromethane (surface tension γl = 37 mN m−1). The Al substrate was microtextured by a chemical solution mixed by zinc nitrate hexahydrate, hexamethyltetramine and a little hydrofluoric acid. A fluoroalkylsilane agent was used to tune the surface wettability. The Al substrates were microtextured by vein-like micro-/nanostructures and generated a solid/liquid/vapour composite interface. Combined with fluoroalkylsilane modification, the Al surfaces resulted in an oleophobicity with contact angle of 126·3° for nitromethane (152·7° for diethylene glycol and γl = 45·2 mN m−1). In addition, the Al surfaces demonstrated a low rolling off angle with <6° even for diethylene glycol. However, the nitromethane droplet favoured to pin on the sample surface even when the sample stage is tilted to 90°. It is noted that this highly oleophobic behaviour is induced mainly by topography, which forms a composite surface of air and solid with oil drop sitting partially on air. The results are expected to promote the study on self-cleaning applications, especially in the condition with oil contaminations.

Studies of the wetting kinetics of liquid drops on solid surfaces

The viscosityη L and the surface tensionγ L of the liquid as well as the equilibrium contact angleθ e are essential parameters governing the wetting kinetics of liquids on solids. By means of a contact angle apparatus with video image digitization, the dynamic contact angle θ and the radiusr of the contact area of sessile drops on solid surfaces have simultaneously been determined in dependence on time after drop application between about 3·10−2 s and long times. The measurements were performed with series of liquids: polydimethylsiloxanes with different molecular masses and solutions of polyisobutylene in decalin and polyacrylic acid in water, covering a wide range of concentrations. The liquids in each series have a constant surface tension, but viscosities ranging over about four orders of magnitude, allowing the influence ofη L andγ L to be studied independently. Solids such as glass, polyethylene and polytetrafluoroethylene were chosen so that the cases of complete wetting (spreading) and partial wetting (θ e) could be studied. The curves of cosθ andr/R 0 vs. time for the different liquids of a series can be superimposed to a master curve by plotting them againstγ L·tη L·R 0, whereR 0 is the radius of the original drop. All these master curves coincide at small wetting times, with exception of the data for the polysiloxanes. That means that the early stage of the wetting process is determined only by the properties of the wetting liquid. The influence of the solid surface, characterized by the equilibrium contact angleθ e becomes significant only at the end of the wetting process.

溶融ポリエチレンのぬれと接着性について ＩＩ

In the previous paper, the keinetics of wetting and adhesion of various adherends by molton polyethylene in argon gas were investigated. In this paper, the keinetics of wetting and adhesion of glass plate by molten polyethylene containing 0.5 mm length glass fibers were investigated. Relation among wettability, adhesiveness and surface tension γL and viscosity η of this composite polyethylene was also studied.The results are summarized as follows:(1) It was experimentally confirmed that the time constant τ of cos θ-t curve obtained on plain polyethylene or the polyethylene which contains glass fibers is in proportion to η/γL, where θ is contact angle, and τ is mainly affected by the change η than that in γL, when the temperature changes.(2) According as polyethylene contains more glass fibers, τ and η increase and the rate of the increase of τ is far greater than that of η.(3) Because of the existence of glass fibers in polyethylene, wettability is reduced but the adhesive strength considerably increases and this is due to the fact that the cohesive failure occurs in the polyethylene layer and the polyethylene is reinforced by glass fibers.