Forces in the isotropic phase of a confined nematic liquid crystal 5CB

Abstract

Using a temperature controlled atomic force microscope, we have measured the temperature dependence of the force between a flat silanated glass surface and a silanated glass microsphere, immersed in the isotropic phase of the nematic liquid crystal 5CB (4'-n-pentyl 4-cyanobiphenyl). At separations of several nanometers, we observed a weak, short range attractive force of the order of 100 pN, which was increased by decreasing the temperature. The temperature dependence of the amplitude and the range of this attractive force can be described by a combination of van der Waals and a mean-field prenematic force due to the surface-induced nematic order. This is supported by ellipsometric study and allows for the determination of the surface coupling energy of 5CB on a silanated glass surface.

Liquid-crystal-mediated force between a cylindrical nanoparticle and substrate

“…When the separation between the cylinder and the substrate decreases, a bridge of nematic fluid forms between the substrate and the cylinder. Such capillary bridging has been seen experimentally [15,16] and has been studied for similar systems using phenomenological theories [11]. This gives rise to an attractive interaction between the cylinder and substrate.…”

“…At short range the variation of Ω(z c ) for this larger radius is similar to that for R = 15b with a more pronounced tail. It may be expected that with increasing cylinder radius (with R very much greater than the nematic coherence length) the z substrate, in isotropic 5CB and 8CB [15,31], show similar behaviour. It should be noted that on increasing µ from βµ = 1.2 to βµ = 1.3 the slope in Ω(z c ) (hence force) decreases, which differs from the experimental results.…”

“…Here, that work is extended to study such nanoparticles in the vicinity of solid substrates in both the nematic and isotropic phases of the solvent. Experimental [15,16] studies of similar systems have shown a rich phenomenology. Considering the interaction between a colloid and a substrate also offers insights into interactions between colloidal particles [17,18].…”

Forces in nematic liquid crystals: from nanoscale interfacial forces to long-range forces in nematic colloids

“…The surface-induced pre-nematic force was actually observed in our experiments with 5CB nematic liquid crystal on silanated surfaces inducing strong homeotropic alignment (22) and is shown in Figure 4. The force is attractive, as expected for a pre-nematic force, and it is very small.…”

“…The magnitude of the measured force, normalised to the radius of the microsphere, F R, is typically 6 · 10 À6 Nm À1 , which for 2R = 7 m corresponds to a force of about 20 pN. This attractive force can be well described as a sum of an attractive pre-nematic force and usual van der Waals attraction across the liquid crystal medium (22).…”