Enhanced Deformation of Azobenzene-Modified Liquid Crystal Polymers under Dual Wavelength Exposure: A Photophysical Model

Abstract

Azobenzene-embedded liquid crystal polymers can undergo mechanical deformation in response to ultraviolet (UV) light. The natural rodlike trans state azobenzene absorbs UV light and isomerizes to a bentlike cis state, which disturbs the order of the polymer network, leading to an anisotropic deformation. The current consensus is that the magnitude of the photoinduced deformation is related to the statistical building up of molecules in the cis state. However, a recent experimental study [Liu and Broer, Nat. Commun. 6 8334 (2015).] shows that a drastic (fourfold) increase of the photoinduced deformation can be generated by exposing the samples simultaneously to 365 nm (UV) and 455 nm (visible) light. To elucidate the physical mechanism that drives this increase, we develop a two-light attenuation model and an optomechanical constitutive relation that not only accounts for the statistical accumulation of cis azobenzenes, but also for the dynamic trans-cis-trans oscillatory isomerization process. Our experimentally calibrated model predicts that the optimal single-wavelength exposure is 395 nm light, a pronounced shift towards the visible spectrum. In addition, we identify a range of optimal combinations of twowavelength lights that generate a favorable response for a given amount of injected energy. Our model provides mechanistic insight into the different (multi)wavelength exposures used in experiments and, at the same time, opens new avenues towards enhanced, multiwavelength optomechanical behavior.

Travelling waves on photo-switchable patterned liquid crystal polymer films directed by rotating polarized light

“…The wave speed can be further enhanced by an increase in actuation frequency, which has been observed for a glassy azobenzene-embedded LC polymer by a slight addition of visible light and temperature increase, 38 attributed to a boosted cis-to-trans isomerization. Furthermore, a moderate addition of visible light is found to enhance the total lightinduced strains due to the additional effect of continuous transcis-trans isomerization cycles, 29,45,46 leading to wave amplitudes that can be a factor of 4 higher. Thus, we expect that the modulation and frequency can be further enhanced leading to larger wave speeds and amplitudes and thus a higher flow efficiency for peristaltic microfluidic propulsion.…”

“…25 In the current study, however, we explore their effect on the generation of travelling waves. The attenuation length d t is inversely proportional to the concentration of the azobenzene added to the LC polymer and falls typically in the sub-micrometer range for heavily-dozed polymers, 24 and in the range from a few micrometers 25,29,58 to a few tens of micrometers 59 for systems with relatively low azo-dosages. Thicknesses of LC films fall in the range from a few to tens of micrometers.…”

“…We use liquid crystal (LC) glassy polymers networks as obtained by in situ polymerization of polyfunctional reactive mesogens, with embedded photo-responsive azobenzene chromophores, that undergo dimensional changes in response to ultraviolet (UV) light. [22][23][24][25][26][27][28][29] Compared to LC elastomer systems, these glassy polymers feature a higher crosslink density. 22 The azobenzene molecules are embedded in the polymer network as crosslinker and upon UV exposure transition from a rod-like trans state to a bent-like cis state (see Fig.…”

Microfluidic slug transport on traveling-wave surface topographies by mechanowetting

“…Because the three-phase line dynamically pins to traveling surface undulations, we show that the fluids in the channel can be transported. Such undulations can be generated by using moving light sources over light-responsive liquid crystal polymers [17,18,[22][23][24], by alternating heating and cooling of responsive hydrogels embedded in elastic channel walls [25], or by utilizing vacuum-pressed thin polymer films on a moving structured belt [21].…”

Functional Liquid Crystal Polymer Surfaces with Switchable Topographies

“…Further studies have revealed that the generation of free volume is largely enhanced by the oscillation of the LC polymer network using the dynamic trans–cis–trans isomerization of azobenzene by the illumination of 365 and 455 nm light with proper light intensity regulation (Figure 5a‐ii). [ 100,101 ]…”