Density and Phase State of a Confined Nonpolar Fluid

Abstract

Measurements of the mean refractive index of a spherelike nonpolar fluid, octamethytetracylclosiloxane (OMCTS), confined between mica sheets, demonstrate direct and conclusive experimental evidence of the absence of a first-order liquid-to-solid phase transition in the fluid when confined, which has been suggested to occur from previous experimental and simulation results. The results also show that the density remains constant throughout confinement, and that the fluid is incompressible. This, along with the observation of very large increases (many orders of magnitude) in viscosity during confinement from the literature, demonstrate that the molecular motion is limited by the confining wall and not the molecular packing. In addition, the recently developed refractive index profile correction method, which enables the structural perturbation inherent at a solid-liquid interface and that of a liquid in confinement to be determined independently, was used to show that there was no measurable excess or depleted mass of OMCTS near the mica surface in bulk films or confined films of only two molecular layers.

Confinement-induced demixing and crystallization

“…Confinement has also been reported to have a strong impact on the structural relaxation of supercooled liquids [6,11,12] since it restricts the range of accessible length scales, which becomes increasingly important as the glass transition is approached [11]. The effect of confinement on the properties of simple liquids or colloids has, therefore, been carefully studied with experiments [13][14][15][16][17][18][19][20][21][22][23], theory [1,3,7,8,[24][25][26], and simulations [2,[9][10][11][12][27][28][29][30][31][32][33][34]. The results indicate that the rich phenomenology of confined hard spheres indeed accurately applies to liquids and colloids in nanoscopic confinement.…”

Solidification and superlubricity with molecular alkane films

“…In light of such strong relevance it is surprising that one of the simplest model systems of confined fluid -a non-polar molecular liquid confined between two atomically smooth surfaces -still evades full explanation. It is frequently reported that simple molecular liquids confined to films below 10 nm or so undergo a transition to solid-like behaviour (1,(7)(8)(9)(10)(11) however the nature of this transition continues to motivate discussion (12)(13)(14)(15). Furthermore, reported measurements of the shear stress sustained by these films span a wide spectrum with friction coefficients ranging from exceptionally low (for squalane (12)), through intermediate values (µ ∼0.1) for linear alkanes (16,17), and even much higher values (µ > 1) (8); all measured using similar apparatus employing a single-asperity contact.…”

Switching the Structural Force in Ionic Liquid-Solvent Mixtures by Varying Composition

“…[41]). Future work using experimental setups that can track the composition of the fluid in the mica-mica slit [42] will shed light on whether the composition of the electrolyte changes abruptly as a function of bulk concentration.…”