Confined fluid orientational order

Short-time Brownian motion was simulated and compared with experiments [108]. The structural evolution and dynamics [109] and the translational and bond-orientational order [110] were simulated with Brownian dynamics (BD) for dense binary colloidal mixtures. The short-time dynamics was investigated through the velocity autocorrelation function [111] and an algebraic decay of velocity fluctuation in a confined liquid was found [112]. Dissipative particle dynamics [113] is an attempt to bridge the gap between atomistic and mesoscopic simulation. Colloidal adsorption was simulated with BD [114]. The hydrodynamic forces, usually friction forces, are found to be able to enhance the self-diffusion of colloidal particles [115]. A novel MC approach to the dynamics of fluids was proposed in Ref. 116. Spinodal decomposition [117] in binary fluids was simulated. BD simulations for hard spherocylinders in the isotropic [118] and in the nematic phase [119] were done. A two-site Yukawa system [120] was studied with... 

Figure 3. (a) Two-dimensional, bond orientational order parameter average values in the molecular fluid layers of LI ecu confined in a multi-walled carbon nanotube of diameter D=9norder parameter values for the contact, second, third and fourth layers, respectively. The dotted line represents the bulk solid-fluid transition temperature, (b) Positional and orientational pair correlation functions in the unwraiqred contact layer of U CCU confined in a multi-walled carbon nanotube of diameter D=9.1< (5 nm) showing liquid phase at 7=262 K and crystal phase at 7=252 K. 

As for simple fluids that have only translational degrees of freedom the structure of the confined DSS fluid is inhomogeneous on account of stratification (see Section 5.3.4). Because of the additional rotational degrees of freedom, however, the structure of the DSS fluid may be more complex as snapshots from the MC simulations in Fig. 6.7 illustrate. In the left part of that figure, a snapshot is presented for a globally isotropic system, whereas the right part shows a snapshot for an orientationally ordered phase. For the sake of clarity only molecules in one contact layer (i.e., the layers of molecules closest to one of the walls) are plotted. 

An important difference between the confined system and the bulk, however, concerns the thermodynamic conditions related to the onset of long-range parallel order. In fact, based on the data plotted in the two parts of Fig. 6.8, we conclude that in the confined system the onset of order occurs at somewhat lower pressures/densities, indicating that the walls promote rather than inhibit spontaneous orientational order. This result is, at least at first sight, rather surprising, because the substrates in the current system do not couple directly to the fluid particle dipole moments. A rationale for this shift of the onset of spontaneous order in the confined relative to the bulk fluid is offered in Ref. 257 where we basically employ entropic arguments. 

Klapp SHL, Schoen M (2002) Spontaneous orientational order in confined dipolar fluid films. J Chem Phys 117(17) 8050-8062... 

Gramzow M, Klapp SHL (2007) Capillary condensation and orientational ordering of confined polar fluids. Phys Rev E 75(1) 011605... 

Near strongly attractive surfaces, liquid structure differs noticeably from the bulk one. This is caused by the packing effect due to the localization of molecules in a plane(s) parallel to the wall and by specific fluid-wall interactions, such as H-bonds. Density oscillations of liquids near solid substrates were observed in experiments [143, 144, 417-419] and in numerous computer simulations of confined fluids. Besides, fluids with strongly anisotropic interactions (such as water) unavoidably undergo orientational ordering near the wall. It is important to know the character of this ordering and its intrusion into the bulk liquid. In the present section, we consider structural properties of adsorbed water layers in the liquid, bilayer, and monolayer phases. 

Multilamellar bilayers in the fluid phase are also ordered in the sense that they are smectic liquid crystals. Of great interest is the range of molecular order this is long-range in the sense that the molecules are confined to two dimensions there is also some kind of short-range order in molecular orientations and conformations, but the range of this latter ordering is not known at present. 

The film enters a solid state as T or h decrease or Pi increases. Bulk films of spherical molecules tend to crystallize (27,34,35). Walls may fiiistrate or enhance crystalline order in molecularly thin films, depending on the relative size of wall and fluid atoms and the relative orientation of the crystalline axes of the walls. Recent simulations of rigid short-chain alkanes indicate that they also crystallize when confined between hydrocarbon walls (36) although crystallization is not observed with metal wall potentials (37-39). The more flexible linear molecules used here enter a glassy state. As we now illustrate, the nature of the changes in the dynamical response near the glass transition is the same whether T, h, or Px is varied. 

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