The effect of temperature on contact angles and wetting transitions for n-alkanes on PTFE.

The aim of this paper is to present a method for predicting the effect of temperature on contact angles and wetting transitions for n-alkanes on PTFE. The analysis is based on the effect of temperature on two closely related phenomena, which are critical in the determination of contact angles: intermolecular forces and the thickness of an adsorbed film in the region adjacent to the three-phase contact. Considering solely van der Waals forces, it is possible to reproduce the experimental temperature dependence of contact angles. At low temperature values, contact angles show a small and linear decrease with temperature. For higher temperature values, substantially larger decreases are exhibited by the more volatile alkanes. In the case of n-octane, a single transition from partial to total wetting is found at 443K. This transition, which arises from the vanishing of the effective Hamaker constant at 430K, is characterized by a surface specific heat exponent close to one, indicating the existence of a first order wetting transition. For the less volatile alkanes, the contact angle decrease is progressively less pronounced as the volatility decreases in such a way that for n-hexadecane the contact angle remains approximately constant throughout the temperature range under study.

Prediction of static contact angles on the basis of molecular forces and adsorption data.

At a three-phase contact line, a liquid bulk phase is in contact with and coexists with a very thin layer of adsorbed molecules. This adsorbed film in the immediate vicinity of a liquid wedge modifies the balance of forces between the liquid and solid phases such that, when included in the balance of forces, a quantitative relationship emerges between the adsorbed film thickness and the static contact angle. This relationship permits the prediction of static contact angles from molecular forces and equilibrium adsorption data by means of quantities that are physically meaningful and measurable. For n-alkanes on polytetrafluoroethylene, for which there are experimental data available on adsorption and contact angles, our computations show remarkable agreement with the data. The results obtained are an improvement on previously published calculations-particularly for alkanes with a low number of carbon atoms, for which adsorption is significant.

Estimating the mechanical properties of retinal tissue using contact angle measurements of a spreading droplet.

When a drop of liquid is placed on the surface of a soft material, the surface deformation and the rate of spreading of the triple contact point is dependent on the mechanical properties of the substrate. This study seeks to use drop spreading behavior to infer the mechanical properties of soft biological materials. As an illustration of the value of this technique we have compared the spreading behavior of a liquid droplet on two viscoelastic, soft materials, namely, an elastomer and a low concentration agar gel. The ratio of the mechanical properties of these soft materials obtained in this way is confirmed by atomic force microscopy (AFM) nanoindentation. By comparing the spreading behavior of a liquid on the retina with that of the same liquid on each of two viscoelastic materials, we can then estimate the elastic moduli of the retina: an estimate that is extremely difficult to carry out using AFM.

Surface characterisation and biomechanical analysis of the sclera by atomic force microscopy.

The sclera is an important collagenous based connective tissue that gives the eye its shape and protects the sensitive layers within the globe. The elasticity and resilience of the sclera mainly come from the stroma, which contains a dense network of collagen fibrils comprising 90% of the thickness of the tissue. However, the outermost layer of the scleral tissue (∼10 µm) is known as the episclera, which is mostly uncharacterised and seldom investigated. Here, we use AFM scanning of porcine eyes to show that the surfaces of these two distinctive layers are structurally different. Furthermore, we use AFM nanoindentation to show that the episclera has significantly different mechanical properties than the stroma. The mechanical properties of the stroma are shown to be distributed between its two component parts (proteoglycan matrix and collagen fibrils).

An analytical solution for a partially wetting puddle and the location of the static contact angle.

A model is formulated for a static puddle on a horizontal substrate taking account of capillarity, gravity and disjoining pressure arising from molecular interactions. There are three regions of interest--the molecular, transition and capillary regions with characteristic film thickness, hm, ht and hc. An analytical solution is presented for the shape of the vapour-liquid interface outside the molecular region where interfacial tension can be assumed constant. This solution is used to shed new light on the static contact angle and, specifically, it is shown that. (i) There is no point in the vapour-liquid interface where the angle of inclination, theta, is identically equal to the static contact angle, theta(o), but the angle at the point of null curvature is the closest with the difference of O(epsilon2) where epsilon2 = ht/hc is a small parameter. (ii) The liquid film is to O(epsilon) a wedge of angle theta(o) extending from a few nanometers to a few micrometers of the contact line. A second analytical solution for the shape of interface within the molecular region reveals that cos theta has a logarithmic variation with film thickness, cos theta=cos theta-ln[1-h2(m)/2h2]. The case, hm = 0, is of special significance since it refers to a unique configuration in which the effect of molecular interactions vanishes, disjoining pressure is everywhere zero and the vapour-liquid interface is now described exactly by the Young-Laplace equation and includes a wedge of angle, theta(o), extending down to the solid substrate.

Hysteresis during contact angles measurement.

A theory, based on the presence of an adsorbed film in the vicinity of the triple contact line, provides a molecular interpretation of intrinsic hysteresis during the measurement of static contact angles. Static contact angles are measured by placing a sessile drop on top of a flat solid surface. If the solid surface has not been previously in contact with a vapor phase saturated with the molecules of the liquid phase, the solid surface is free of adsorbed liquid molecules. In the absence of an adsorbed film, molecular forces configure an advancing contact angle larger than the static contact angle. After some time, due to an evaporation/adsorption process, the interface of the drop coexists with an adsorbed film of liquid molecules as part of the equilibrium configuration, denoted as the static contact angle. This equilibrium configuration is metastable because the droplet has a larger vapor pressure than the surrounding flat film. As the drop evaporates, the vapor/liquid interface contracts and the apparent contact line moves towards the center of the drop. During this process, the film left behind is thicker than the adsorbed film and molecular attraction results in a receding contact angle, smaller than the equilibrium contact angle.