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The line energy associated with the triple phase contact line is a function of local surface defects (chemical and topographical); however, it can still be calculated from the advancing and receding contact angles to which those defects give rise. In this study an expression for the line energy associated with the triple phase contact line is developed. The expression relates the line energy to the drop volume, the interfacial energies, and the actual contact angle (be it advancing, receding, or in between). From the expression we can back calculate the equilibrium Young contact angle, θ0, as a function of the maximal advancing, θA, and minimal receding, θR, contact angles. To keep a certain maximal hysteresis between advancing and receding angles, different line energies are required depending on the three interfacial energies and the drop's volume V. We learn from the obtained expressions that the hysteresis is determined by some dimensionless parameter, K , which is some normalized line energy. The value of K  required to keep a constant hysteresis (θA − θR) rises to infinity as we get closer to θ0 = 90°.

We have studied the properties of layers of end-functionalized poly(ethylene glycol) (PEG) grafted by one end to mica surfaces, and, using a surface force balance, the interaction between two such layers. PEG of molecular weight 3.4 kg/mol, functionalized at both ends with an N-hydroxysuccinimide carbamate (NHS) group, to form NHS−PEG−NHS, does not adsorb on mica. When a trimethylammonium group ((CH3)3N⁺) replaced one of the NHS groups to form ((CH3)3N⁺)−PEG−NHS, a surface layer formed on the mica from its aqueous solution, showing that the functionalized PEG had been attached by its (CH3)3N⁺ end. The properties of the ((CH3)3N⁺)−PEG−NHS layer were examined primarily by surface force measurements as well as by atomic force microscopy, contact angle measurements, and confocal fluorescence microscopy. The layer was found to be very smooth, and the NHS group was found to retain its reactivity also when attached to the surface, demonstrating that the end-functionalized PEG chains can be used as flexible surface-active spacers for binding amino-containing molecules to the surface. Refractive index measurements yield the adsorbance and hence mean interanchor spacing s of the PEG chains, while force profiles revealed the role of both electrostatic double layers and steric components of the normal interactions at different surface separations D. Shear forces between the sliding surfaces were first observed at compression ratios (2L0/D) ~ 1.2, increasing markedly for (2L0/D) > 3, where L0 is the unperturbed PEG-brush thickness. This may be attributed both to the relatively low (L0/s) ratio, which allows high interpenetration and therefore high segmental density and viscous shear dissipation between opposite layers, and to bridging effects.

Poly(γ-benzyl-l-glutamate) (PBLG) is a synthetic polypeptide with a rigid helical conformation in several solvents. Small-angle X-ray and neutron scattering measurements were used to study the reversible gelation process in semidilute isotropic solutions of PBLG in benzyl alcohol. Time-resolved measurements of the scattering signal due to aggregation of PBLG helices into a network of microfibrils show characteristics of a nucleation and growth process with an incubation period which is extremely sensitive to the gelation temperature. Analysis by the Avrami equation yields an exponent of about 2. A mechanism of homogeneous nucleation and one-dimensional growth of fibrils, which incidentally merge or branch to form the network, is proposed. The thermodynamic transition temperature controlling fibril nucleation seems to be below the gel melting temperature. This transition may be an incongruent melting point of an ordered crystalline phase, which lies within the wide biphasic region of the phase diagram.

We have used a new design of the mica surface force balance (SFB), with extreme sensitivity in measuring normal and particularly shear or frictional forces between two surfaces sliding past each other, to measure the forces between two mica surfaces across a confined 4-cyano-4‘-hexabiphenyl nematogen (6CB). In an earlier study (Langmuir 1997, 13, 4466, paper 1 of the series) we investigated the normal force−distance profiles and the orientation of the confined liquid crystal (LC). Here we extend this to study the shear forces Fs between the sliding mica surfaces across the 6CB nematogen as a function of orientation of the confined LC, the applied normal load Fn, the separation D of the mica surfaces, the shear velocity vs, and the relative shear direction of the confining surfaces (which may be varied using the new SFB design). Our results, where the shear forces are measured down to levels that are some orders of magnitude more sensitive than in earlier studies, show that the highly confined nematogen (D in the range from 16 to ca. 100 Å) behaves under shear in a quasi-solidlike fashion for all three orientations studied:  planar, planar twisted, and homeotropic. There is a linear relation between Fs and Fn for each of the three orientations, with the effective friction coefficient (dFs/dFn) largest for the homeotropic orientation and lowest for the planar twisted one. Intriguingly, for the planar orientation a clear increase in the friction could be observed when the initial shear direction was changed by 90°. We attribute this to the effect of the initial shear in orienting the confined nematogen layer in the initial direction, so that subsequent sliding motion at right angles to this encounters greater resistance. We also find that within a range of some 40-fold in vs, there was little change in the shear force, in line with what is generally observed for solid−solid friction, and consistent with the fact that little relaxation in Fs is observed over macroscopic times on applying a step strain to the confined LC.

Oleic acid and stearic acid are similar surfactants which, however, lead respectively to stability and to precipitation of ferrofluid suspensions:  to understand this, the forces between layers of oleic-like surfactants and between layers of stearic-like surfactants across a hexadecane (HD) medium were measured using a surface force balance (SFB). Separate measurements reveal that only the oleic layers are solvated by HD, while the SFB results reveal that for both surfactants a marked net attraction is present between the surfaces. Simple considerations based on these observations explain why, despite this attraction, ferrofluid dispersions are stabilized by oleic but not by stearic surfactants.

Using a surface force balance, we have examined the orientation of a nematic liquid crystal (6CB) confined between mica surfaces. By simultaneous measuring of the force and the refractive index profile, we were able to distinguish between planar, planar twisted, and homeotropic orientation of the nematic. When the mica was exposed to air for short periods (<1 h) prior to introducing the 6CB, either planar or twisted planar orientations resulted, depending on the relative orientation of the mica axes. For longer exposure periods (≥3−4 h) prior to introducing the nematic, a homeotropic orientation was observed, presumably because the mica surfaces became sufficiently hydrophobized. Slow transitions (over a period of days) from a planar twisted to a planar orientation and from a homeotropic to a planar orientation could be observed in certain conditions. We propose that these are associated with anchoring transitions on the mica surfaces, resulting from adsorption of water molecules penetrating by diffusion from the ambient atmosphere.