Surface anchoring and dynamics of jump-wise director reorientations in planar cholesteric layers

Abstract

A theoretical investigation is made into the dynamics of pitch jumps in cholesteric liquid-crystal layers having finite strength surface-anchoring conditions. A presentation is given of general formulations which connect the dynamics of pitch jumps with the key material parameters such as the viscosity, the specific form of the anchoring potential, and the dimensionless parameter Sd=K22/Wd, where K22 is the elastic modulus, W is the depth of the anchoring potential, and d is the layer thickness. To illustrate the dependence of the pitch jump dynamics upon the shape and strength of the anchoring potential, we investigate two sets of different model surface-anchoring potentials for a jump mechanism that is connected with the slipping of the director at a surface over the barrier of the anchoring potential. Two types of 'narrow' well potentials that are natural extensions of the more familiar 'wide' potentials are considered: one type is based upon the well-known Rapini-Papoular potential and the other upon the B potential, introduced in Belyakov, Stewart, and Osipov, JETP 99, 73 (2004). Calculations for the unwinding (winding) of the helix in the process of the jump were performed to investigate the case of infinitely strong anchoring on one surface and finite anchoring on the other, which is important in applications. The results show that an experimental investigation of the dynamics of the pitch jumps will allow one to distinguish different shapes of the finite strength anchoring potential, and will, in particular, provide a means for determining whether or not the well-known Rapini-Papoular anchoring potential is the best suited potential relevant to the dynamics of pitch jumps in cholesteric layers with finite surface-anchoring strength

An experimental investigation of discrete changes in pitch in a thin, planar chiral nematic device

“…As the temperature is changed, so-called pitch jumps occur which are easily observable as changes in the selective reflection wavelength. There have been several reports of thermal hysteresis in the pitch jumps [2][3][4], as well as theoretical treatments that relate the hysteresis to parameters including surface anchoring and cell thickness [5][6][7][8].…”

Edge Modes in Chiral Liquid Crystals: Options for Low Threshold Lasing

“…Because the lasing frequency is determined by the edge mode frequencies there is an option for some variation of the lasing frequency inside the width of the dye line by changing the CLC pitch by means of the temperature variations [5,6] or by application to the layer of external electric or magnetic field [4]. Note, that some times smooth variations of the external agent may result in jump-like variations of the lasing Edge Modes in Chiral LC 227 frequency [19] connected with the jumps of the CLC pitch which are sensitive to the surface anchoring [20].…”

“…(19,20)). The mentioned relationship between the amplitudes of eigen and incident waves at the specific frequencies shows that the energy of radiation in CLC at the layer thickness for these frequencies is much higher than the corresponding energy of the incident wave at the same thickness.…”

Weak Anchoring Effects in Nematic Liquid Crystals

“…The physical motivation for the B form of surface energy is largely driven by pitch 'jumps' in cholesteric liquid crystals where the twist of the director is dominant and is related to the twist elastic constant K 2 [3][4][5]. By considering simple series expansions, it can be shown that the RP and B energies almost coincide when the actual surface twist angle is close to the easy direction / Ã .…”

“…if the director at the surface coincides with the easy direction. Recently, however, Belyakov et al [3][4][5] proposed an alternative form for a weak anchoring energy density that is particularly relevant to twist geometries. In our notation, this alternative version may be written as…”