Physical mechanisms of dc switching in a liquid-crystal bistable boundary layer display

Abstract

We discuss two mechanisms proposed to explain dc switching of a bistable liquid-crystal boundary layer display. These are (1) biasing by double layer fields that arise from charge of one sign preferentially adsorbed at the walls, and (2) asymmetry in the transient depletion layer fields which result from a difference in mobility of the positive and negative ions. By a quantitative discussion, we argue that both mechanisms are plausible, but crucial parameters remain unknown. For the double layer mechanism the surface density of adsorbed charge is estimated as 8×10-10 C/cm 2 from published measurements of the thickness dependence of the resistivity. For a numerical example of the differential mobility mechanism, we arbitrarily take μ-=3μ+=9×10-6 cm2/V sec. Either mechanism may be made dominant by the choice of material and surface properties. For fast switching, it may prove advantageous to choose interfaces to exploit the double layer mechanism because it begins to act immediately upon application of the voltage, whereas the other mechanism entails a delay before field asymmetry builds up from the differential drifting of the ions. To enhance the double layer effect and obtain fast switching, one should use (1) a thin cell, (2) a highly purified liquid crystal, which keeps the ionic charge density low and the Debye screening length significant in comparison with the cell thickness, and (3) interfaces that strongly absorb ions of one sign so that the double layer field will be strong. On the other hand, fast switching by the differential mobility mechanism calls for a somewhat higher ionic density with ions of one sign rather mobile, and the others relatively sluggish.