Direct measurement of the propagation of the phase-transition region of liquid crystals

Abstract

Many types of active matter, such as biological cells, have liquid-crystalline membranes, which are soft and flexible in their interactions with their surroundings and sometimes allow molecular-structural or -orientational changes to extend for long distances, owing to long-range molecular interactions. Despite the technological and fundamental importance of these long-range changes, there is no good physical property with which to express them for the liquid crystal. Here, we show direct measurements of the propagation of structural or orientational changes due to long-range molecular interactions in liquid crystals. We induced a patterned phase transition in a liquid crystal via illumination with a fringe pattern and observed the propagation of the phase-transition region. We determined that the propagation occurred in a ballistic manner with a velocity of 80–110 m/s and that two types of propagation—side-by-side and head-to-tail molecular interactions—were found.

Analysis of Molecular Disordering Processes in the Phase Transition of Liquid Crystals Observed by Patterned-Illumination Time-Resolved Phase Microscopy

“…From the overall responses shown in the insets of Figure 3 , the signal intensity for each probe polarization increased until several microseconds, followed by a decay until a millisecond. The rising component corresponds to the disordering and the subsequent decay component corresponds to the ordering processes, with the result agreeing well with the previous results [ 35 , 36 ]. At any pump intensity, responses started to increase within the time resolution (3 ns).…”

“…We developed a heterodyne transient grating technique, where the irradiation light was homogeneous in a broad region and could provide a reproducible response, and the multiple processes depending on the light intensity were clarified [ 34 ]. The phase transition and photo-induced orientation change were distinguished and the anisotropic phase transition/recovery was clarified by the observation of the extraordinary and ordinary refractive index change [ 35 ], and also revealed that the front surface of phase transition moved at ~100 m/s [ 36 ]. In recent years, we have studied the origin of the optical nonlinearity from the dynamics of the LCs by using time-resolved methods and found that the enhancement occurred on the millisecond time scale [ 37 ].…”

Origin of optical nonlinearity of photo-responsive liquid crystals revealed by transient grating imaging

“…Since the isotropic region became dark in this configuration, the dark regions correspond to the isotropic phase. The formation of the dark regions indicates that the phase transition from nematic to isotropic had already occurred within a millisecond, which agrees with the previous studies showing that the phase transition from the nematic to isotropic finished within hundreds of nanoseconds 20,21 . On the other hand, the bright regions were formed at the non-irradiated regions by the pump light, and the brightness was probably increased because of the temperature rise.…”

“…In the reorientation process for azobenzene-doped 7-Heptyloxy-4-cyanobiphenyl (5CB), the reorientation time depended on the director axis, which was explained by the formation of the anisotropic domain 20 . Recently, we found that the molecular disordering during the phase transition propagates in a ballistic manner, with a speed of ~100 m/s 21 . In our previous work, the nonlinear refractive index change was observed when the temperature or the pump intensity were adjusted so that the phase was very close to the isotropic condition, and a drastic change in the dynamics was observed, which involved a long-lasting diffraction intensity on the order of 100 ms 22 .…”

Aggregation-Induced Expansion of Poly-(N-isopropyl acrylamide) Solutions Observed Directly by the Transient Grating Imaging Technique

“…A detailed principle on the laser temperature-jump method is provided elsewhere. 20 Although in previous studies we utilized the heterodyne TG method, 21 in which the refractive index change was measured via the heterodyne signal, the homodyne TG method was used here; 26 Figure S5a in the Supporting Information reports a schematic representation of the setup. In detail, an excitation pulse was made to pass through a transmission grating and, thus, the grating pattern produced was projected onto the sample using a 4f configuration of two lenses with different focal lengths, and the fringe space on a sample was reduced to half of the original one.…”