Supplementary Material for Applied Physics Letters.
Anomalous temperature-dependent anchoring in liquid crystals mediated by thermodynamic smectic wetting sheets
Satoshi Aya* and FumitoAraoka*
RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa,Wako, Saitama 351-0198, Japan
*E-mail: ,
These notes supplement our paper “Anomalous temperature-dependent anchoring in liquid crystals mediated by thermodynamic smectic wetting sheets”, providing a number of additional experimental methodologies and observations. These are intended to helpwith the understanding of our experiments and the interpretations in the main manuscript.
1 Measurement of anchoring torque by dynamic light scattering
In uniaxial nematic liquid crystals (LCs) aligned by a surface, the anchoring torque or extrapolation length can be estimated from the fluctuation state of the LCs near the surfaceS1-S3. This is because the fluctuation of the LCs can be suppressed (released) by strong (weak) anchoring toque. The detailed theoretical considerations can be found in Refs.[1-3]. Figure S1 shows the variation of the relaxation of the splay mode, associated with the splay elastic constant, with the thickness of the LC material at three temperatures. Parabolic fitting was applied to derive the extrapolation length plotted in Fig. 1 in the main text.
2 Characterization of surface physicochemical properties of CYTOP film
The temperature variation of anchoring is mainly determined by either the temperature variation of the physicality of surface aligner or the temperature variation of interactions between CCN47 and CYTOP molecules. The latter causes creation/deconstruction of interfacial smectic wetting sheets as already mentioned in the main text. In this work, we ruled out the former possibility by contact angle measurement and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR).
2.1 Contact angle measurement
We put a water droplet of 2–5 µl onto a CYTOP-coated substrate on a heating stage equipped with a temperature controller.At equilibrium, the contact angle images were captured and analyzedby the half-angle method. The temperature dependence of contact angle and a typical contact angle image at 50 °Care shown in the left of Fig. S2.
2.2 ATR-FTIR
ATR-FTIR spectroscopy is a surface-sensitive method that enables the recognition and identification of the existence and the orientational state of functional groups on surfaces. In this work, we employedATR-FTIR to monitor how the physical properties of the CYTOP surface changes with temperature. As shown in the right of Fig. S2, the ATR-FTIR absorption spectrum changes little upon temperature variation between 30 and 70 °C. This manifests as the CYTOP surface itself never changing the orientation of functional groups (C-–F, at 800–1000 cm-1) both in the isotropic liquid phase and in the nematic phase, indicating that CYTOP alone cannot drive the orientational transition in question.This is consistent with the fact that similar orientational transition cannot be observed when LC materials other than CYTOP are used.
3 Optical setup of reflective second harmonic generation
Figure S3 shows the optical geometry of reflective second harmonic generation (SHG). We used the fundamental beam from a Ti:sapphire laser (Coherent Vitesse800) with a central wavelength of 800 nm, maximum power of 280 mW, pulse duration of 120 fs, and 80MHz repetition. The fundamental beam was incident on the LC cell with p- or s-polarization and detected by a photomultiplier tube (PMT, Hamamatsu H7732P-01) with p- or s-polarization.
4 Simulation of the temperature dependence of extrapolation length
We estimated the extrapolation length of the three-layer structureconsidered in the main text(bulk LC/transient LC layer/surface) by using the following three procedures. First,is numerically calculated according to the definition by varying to obtain a dataset of foreach . The interfacial free energy is then written as: , where (, ) and (, ) are the sets of anchoring coefficients for the V-alignment surfaceand the P-alignment surface, respectively,and the subscripts 2 and 4 represent the second- and the fourth-order Rapini-Papoular polar anchoring coefficients, respectively. Second, the variation in with temperature is obtainedfrom reflectance spectroscopic ellipsometryand SHG. Finally, the temperature dependence of is determined by combining these data. FigureS4 shows the simulated as a function of temperature, and showsqualitativeconsistencywith the result obtained from the dynamic light scattering experiment.
5 Thickness dependence of orientational transition temperature
Several prior reports have suggested a number of possible explanations for the different types of orientational transitions (e.g., surface biaxiality, cybotacticity, and formation of interfacial electric double layer) S4-S8.Therefore, we briefly compare these works with ours and show the difference in the thickness dependence of orientational transition temperature. The works by Senyuk, Kim, and co-workers addressed plane-tiltanchoring transitions in H-shaped and bent-core LCs. In bent-core LCs, the cybotactic clusters can potentially form smectic layers on the surface to trigger an anchoring transitionS4,S5,S6,S8. Dielectric and polar interactions as well as the formation of a double layer between LCs and surface is known to be important for the occurrence of anchoring transitionsS9. When a double layer is formed on a surface, the anchoring energy is dependent on thicknessS9. In contrast to this, theorientational transition temperature in the current system is hardly changed by altering the cell thickness dependence. We previously reported a high-resolution differential scanning calorimetry study and found that the orientational transition temperature was not changed by the thickness, at least in the range of 7−54 μmS9. Moreover, by observing the orientational transition by polarizing optical microscopy, it was again found that no change in the orientational transition temperature occurs with changing sample thickness, up to 100 µm. It is worth noting that thisbehavior can be reasonably explained by only our modified Rapini-Papoular approach by considering the coverage of surface smectic wetting sheets (see Ref. [17] in the main manuscript).
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