Uniform planar anchoring

Figure 4.6: Photoalignment technique to generate uniform planar surface anchoring in mi-crochannels. (a) When UV light (polarized perpendicular to flow direction) was incident on microchannel coated with PVCN-F, effective anchoring along the flow path was obtained. (b) Uniform planar surface anchoring along the channel width is achieved when the incident UV light was polarized along the flow direction. (c) Degenerate planar anchoring is obtained when unpolarized UV light was used.

A multi-step approach was followed for rendering uniform planar surface anchoring to the microchannels. PVCN-F was deposited on the channel walls by running the solution through freshly prepared microchannels for≈ 2 min. The thickness of the deposited PVCN-F

Figure 4.7: Uniform planar anchoring in microchannels using photoalignment method. (a) POM micrograph showing realization of LC anchoring along the channel. The sample was imaged between crossed polarizers, with one of the polarizers parallel to the resultant director orientation. (b) POM micrograph when the sample is rotated byϕ =π/4 shows a bright signal.

(c) FCPM imaging of the director field in the xy plane. The uniformly high fluorescence signal confirms the LC orientation along the laser polarization (indicated by the double-headed arrow). (d) The signal is reduced to minimum on changing the laser polarization byπ/2. (e) Characteristic conoscopy micrograph obtained from the functionalized channel. The scale bar corresponds to 20µm

.

film was characterized using spectral interferometry techniques after it was stabilized via ther-mal treatment (150^◦C for 1 h). The reflection spectrum, obtained by illuminating the coated layer with a white light source, was analyzed with the help of a spectrometer (AvaSpec-2048-USB2, Avantes). Table 4.2 shows the thickness of the deposited PVCN-F on glass and PDMS.

The layer thickness however could be altered by varying the time over which the solution was run through the channel. The microchannels were subsequently irradiated with polarized UV light (≈ 60 min in Polylux PT UV cabinet). By varying the polarization of the UV light, alignment either parallel or perpendicular to the flow path was achieved, as is represented in Fig. 4.6a and b, respectively. In absence of any preferred direction of polarization of the UV light, degenerate planar anchoring is obtained on the channel walls (Fig. 4.6c).

For uniform planar anchoring we expect a single birefringent domain possessing a homo-geneous appearance throughout the sample. Figure 4.7a - e show optical micrographs of the equilibrium orientation of the LC molecules within a functionalized microchannel. The chan-nel in this particular example was irradiated with UV light polarized along the chanchan-nel width, thus generating alignment parallel to the length of the microchannel. The uniform planar ori-entation of the LC molecules was observed between crossed polarizers (ϕ = 0 andϕ = π/4)

Polymer - Substrate Deposited thickness (nm)

PVCN - PDMS 34±4

PVCN - Glass 96±10

PVA - PDMS 54±6

PVA - Glass 120±10

Table 4.2: Thickness of polymer deposition on different substrates calculated from reflection spectrum. PVA thickness on glass was obtained using AFM measurements.

as shown in Fig. 4.7a and b. A high fluorescent intensity was obtained with an excitation laser polarized along the channel length (Fig. 4.7c). This however reduced to minimum when the polarization direction was rotated byπ/2, shown in Fig. 4.7d. Furthermore, the characteristic hyperbolic isogyres, corresponding to uniform planar anchoring, were observed in conoscopy (Fig. 4.7e). The observations confirmed the uniform planar orientation of the LC molecules along the channel length.

Figure 4.8: FCPM intensity distributions for functionalized microchannels. Normalized flu-orescence intensity is plotted as a function of normalized channel depth. An overall high in-tensity uniformly distributed along thezdirection describes the homogeneity of the molecular orientation. Intensity distributions corresponding to degenerate planar (DP) and homeotropic (H) channels are represented comparatively.

The spatial orientation and uniformity of the alignment of the LC molecules within the functionalized channel were resolved by performing FCPM imaging along the cross-sections (yzplane). The results are presented as fluorescence intensity distributions along the channel

Figure 4.9: Uniform planar anchoring using pneumatic buffing. (a) POM images of mi-crochannel functionalized by pneumatic buffing of PVA layer. (b) Conoscopic micrograph of the planar channel.

depth (Fig. 4.8). The variation of the intensity distribution was used as a measure of the spatial homogeneity of the equilibrium orientation within the functionalized microchannels. The dis-tribution in the present case indicates a uniformly high intensity, except for regions very close to the walls, where local roughness leads to fluctuations of the signal. The distribution within the bulk shows low fluctuations, typically within 10% of the average fluorescence intensity.

This is comparable with the range of intensity fluctuations obtained from a uniform planar LC cell. Combining POM and FCPM imaging, it is concluded that the LC anchoring is uniform planar within the microchannel. The anchoring has been found to be stable for the entire span of observation lasting for weeks.

Alternatively, a uniform planar anchoring could be achieved by a pneumatic buffing of a PVA coating on the channel surfaces. Freshly prepared microchannels were filled with an aqueous solution of PVA. The hydroxyl groups of PVA bond covalently to silanol groups on the glass surface [175]. Due to the presence of silanol groups on plasma treated PDMS as well, a similar mechanism attaches the PVA molecules to the PDMS surface. The bonding is additionally stabilized by baking the filled channel at 80^◦ C for 15 min (soft baking) and followed by baking at 110^◦ C for 1 h (hard baking). During the baking process, air was pumped into the channel at a low flow rate of ≈ 50 µl/min to assist the elimination of any residual moisture from the inner walls of the channel. On completion of the baking process, the random polymer chains could be aligned by continued infusion of air into the channel at high flow rate (≈10 ml/min) for 30 mins, producing an effect similar to mechanical buffing [176] of the polymeric chains. In contrast to the mechanical buffing, where shearing forces deform the polymer film, the viscous forces of air in pneumatic buffing aligned the PVA polymer chains in the direction of the air flow. The channel was thereafter filled with 5CB in the isotropic phase and left to equilibrate. Figure 4.9 presents the resulting anchoring within the microchannel.

This method however encounters a number of limitations. Since the anchoring is induced due to the infusion of air, the resulting NLC orientation is necessarily constrained only along the channel length. Besides, the spatial homogeneity of the resulting alignment was relatively lower. Moreover, the pneumatic buffing produced a rather weak planar anchoring: Subsequent flow of 5CB along a direction opposite to that of the buffing frequently disturbed/reversed the anchoring induced by air-flow. Due to an especially low viscosity of air, the buffing action is envisaged to be rather mild.