Modeling of surface waves in photo-responsive viscoelastic liquid crystal thin films under a moving light source

In this paper we present a computational model to simulate surface waves on photo-responsive liquid crystal thin films under dynamic illumination. The influence of light attenuation is included to model the dynamic photo-mechanical response as a result of the forward (trans to cis) and the backward (cis to trans) isomerization of the embedded azobenzene molecules. The viscoelastic nature of the liquid crystal films is also estimated and incorporated in this study. The temporal response of the material under static spot illumination is analyzed first to understand the underly- ing mechanisms behind the observed response. The relation between the fraction of transformed molecules and the macroscopic light induced strains and stresses through the thickness is investi- gated to establish the significance of light attenuation. Subsequently, the model liquid crystal film is exposed to a rectangular illumination waveform to simulate a moving light source and thereby generate surface waves. Results are presented that relate the wave attributes, (i.e., the amplitude, frequency and phase lag) to the time scales associated with the moving light source, the molecular trans-cis isomerization reaction and the viscoelastic response of the material. A design guideline for producing surface waves with specific features (amplitude, phase) is presented.