Читать книгу Liquid Crystals - Iam-Choon Khoo - Страница 53

Consider here the simplest case of flows in which the director axis orientation is held fixed. This may be achieved by a strong externally applied magnetic field (see Figure 3.11), where the magnetic field is along the direction . Consider the case of shear flow, where the velocity is in the z‐direction, and the velocity gradient is along the x‐direction. This process could occur, for example, in liquid crystals confined by two parallel plates in the y‐z plane.

In terms of the orientation of the director axis, there are three distinct possibilities involving three corresponding viscosity coefficients:

1 η1: is parallel to the velocity gradient, that is, along the x‐axis (θ = 90°, ϕ = 0°).

2 η2: is parallel to the flow velocity, that is, along the z‐axis and lies in the shear plane x‐z (θ = 0°, ϕ = 0°).

3 η3: is perpendicular to the shear plane, that is, along the y‐axis (θ = 0°, ϕ = 90°).

These three configurations have been investigated by Miesowicz [19], and the ηs are known as Miesowicz coefficients. In the original paper, as well as in the treatment by deGennes [3], the definitions of η₁ and η₃ are interchanged. In deGennes notation, in terms of η_a, η_b, and η_c, we have η_a = η₁, η_b = η₂, and η_c = η₃. The notation used here is attributed to Helfrich [6], which is now the conventional one.

To obtain the relations between η_1,2,3 and the Leslie coefficients α_1,2,…,6, one could evaluate the stress tensor σ_αβ and the shear rate A_αβ for various director orientations and flow and velocity gradient directions. From these considerations, the following relationships are obtained [3]:

(3.62)

In the shear plane x‐z, the general effective viscosity coefficient is actually more correctly expressed in the form [20]

(3.63)

in order to account for angular velocity gradients. The coefficient η_1,2 is related to the Leslie coefficient α₁ by

(3.64)