Ordering transitions of weakly anisotropic hard rods in narrow slitlike pores

Abstract

The effect of strong confinement on the positional and orientational ordering is examined in a system of hard rectangular rods with length L and diameter D (L>D) using the Parsons-Lee modification of the second virial density-functional theory. The rods are nonmesogenic (L/D<3) and confined between two parallel hard walls, where the width of the pore (H) is chosen in such a way that both planar (particle's long axis parallel to the walls) and homeotropic (particle's long axis perpendicular to the walls) orderings are possible and a maximum of two layers are allowed to form in the pore. In the extreme confinement limit of D H 2  , where only one layer structures appear, we observe a structural transition from a planar to a homeotropic fluid layer with increasing density, which becomes sharper as L H. In wider pores (2D<H<3D) planar order with two layers, homeotropic order, and even combined bilayer structures (one layer is homeotropic, while the other is planar) can be stabilized at high densities. Moreover, first order phase transitions can be seen between different structures. One of them emerges between a monolayer and a bilayer with planar orders at relatively low packing fractions.

Fromn-layer planar ordering to the monolayer homeotropic structure of confined hard rods: The effect of shape anisotropy and wall-to-wall separation

“…We studied the confined hard rod particles with a rectangular cross-section and edge lengths L, D, and D into a slit-like pore with flat and parallel hard walls where the walls are perpendicular to the z-axis and placed at = 0 and = and spread in the plane. We use the Onsager theory with the Parsons-Lee modification [27,52] within three-state Zwanzig approximation [53], which the orientation freedoms of the particles are along the x, y, and z directions. Therefore, the local density of each direction ( where = , , ) is a function of the z-coordinate only, and we can achieve the packing fraction () from the local densities ( ( ), = , and ) as below:…”

“…The nematic planar phase confined between two walls could be uniaxial (U) or biaxial (B). The U-B transition induced by a substrate(s) has been predicted to be second order for confined hard rod-like [27][28][29][30] and plate-like [31] particles. The biaxial nematic phases have been studied widely experimentally [32][33][34] and theoretically [35][36][37] due to their fast response time to the applied electric field, which is an essential factor in display technologies [38].…”

“…The ordering of the rectangular hard rod particles in contact with a single wall or between two hard surfaces has been investigated through the theory [27][28][29][30]43]. Their results can be summarized as (i) a wall-induced surface transition from uniaxial to biaxial symmetry, (ii) the nematic film wets the wall completely, (iii) the critical value of the pore width at which the capillary nematization (I-N) terminates is about twice the length of the rod, (iv) the P-H transition occurs at strongly confined nonmesogenic rods (i.e.…”

Connectedness percolation in the random sequential adsorption packings of elongated particles

“…The density profiles showed planar ordering and damped oscillatory behavior [35]. For non-mesogenic particles with small aspect ratios (ε < 3) in the extreme confinement limit -small distances between the walls (h/d ≤ 2), a structural transition from a planar (particle's long axis parallel to the walls) to a homeotropic (particle's long axis perpendicular to the walls) layer with increasing density was observed [37]. For hard discorectangles between the two parallel walls in strongly confined systems (1 < h/d ≤ 2), a rich phase behavior with dependence on the value of the particles' aspect ratio has been observed [38].…”

“…It was shown that confined particles tend to align their long axes parallel to the confining walls with the effects being more pronounced for smaller separations between those walls. Strongly confined systems between two parallel walls have been examined, theoretically, for rectangular particles [35][36][37]. The density profiles showed planar ordering and damped oscillatory behavior [35].…”