## 3 Results  
### Analysis of Insufficient HTAB Concentration  
If [HTAB] was insufficient, the LC alignment was not uniform (Figure 2a). The use of polarizer films enables easy verification of the alignment state of the entire cell. The LC molecules did not align at all in the cell where pure 5 CB was used, therefore black regions that imply vertical LC alignment were randomly distributed. However, the LC molecules  
Figure 1: Schematics of (**a**) chemical structure of HTAB, (**b**) principle of vertical alignment of LC molecules mixed with HTAB, and (**c**) cell fabrication.  
started to be aligned along the HTAB in the cell where the surface-active agent HTAB is mixed. The results show that as the LCs mixture is injected by the capillary method, LC molecules are vertically aligned successfully near the injection spot, but LCs are not perfectly vertically aligned at the far region from the injection spot with the insufficient HTAB concentration (Figure 2b). Because the bromide ion in HTAB has a high affinity with ITO, HTAB can attach to the ITO glass substrate. Therefore, HTAB likely moves onto the ITO glass substrate [23]. If the concentration of HATB is insufficient, HTAB is out of stock before HTAB covers the entire ITO glass substrate, hence nonuniformity occurs. If the [HTAB] is insufficient, HTAB is depleted before it can cover the entire ITO glass substrate, so nonuniformity occurs. We confirmed this tendency with all the partially homeotropic-aligned samples (Figure 4, green triangles). Therefore, a method to minimize this non-uniformity must be developed; we leave this task for future work. This result also implies that although we used the same [HTAB] for LC solutions, the result can vary depending on the size of the LC filling region due to the non-uniform adhesion of HTAB to the ITO glass surface. This nonuniformity has been reported in other studies of surface-active agents for homeotropic alignment, and they support that explanation of the non-uniformity [23,36]. To avoid such a size-dependent influence, our LC-injection region was fixed at 2 cm by 2 cm for all cases.  
Figure 2: Image of region near LC injection with the test cell between the two cross-polarized polarizer films; (**a**) injected pure LCs, insufficient [HTAB] for homeotropic alignment, and LCs mixture with sufficient [HTAB] to induce fully-vertical alignment at the cell gap = 20 \(\upmu\)m. LC mixtures with insufficient [HTAB] develop nonuniform LC orientation; (**b**) nonuniform LC cell with an insufficient [HTAB]. (**A**) polarized optical microscopy images of vertically well-aligned part near the LC injection point, (**B**) division area, and (**C**) imperfectly-aligned part far from the LC injection point.  
The non-uniformity of VA can be checked by measuring the OT (Figure 3a). By adjusting the placement of the samples, we can change the area that the light passes through. Under crossed polarizers, the vertically fully-aligned LC molecules with OT was \(<\)-0.83%, whereas the pure 5 CB cell in which alignment was random had 15 \(\leq\) OT \(\leq\) 22%, depending on the measurement location. The non-uniformity was caused during the LC injection process. OT was lowest near the LC injection site (Figure 3 bA), intermediate near the middle of the sample (Figure 3 bB), and lowest farthest from the injection site (Figure 3 bC). We double-checked the non-uniformity with the crossed-polarizers film image and transmittance with all of the partially homeotropic-aligned samples denoted with the green triangle case (Figure 4).  
Figure 3: (**a**) Experimental setup image to evaluate Optical transmittance. Cross-polarization is set for testing the homeotropic alignment. (**b**) Overall image with insufficient [HTAB] for fully-vertical alignment (Cell gap = 10 μm, [HTAB] = 0.01 wt%). White dotted line: area where white light is applied to the sample. Lower graph: transmittance non-uniformity caused by insufficient [HTAB].