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Detection of alignment changes at the open surface of a confined nematic liquid crystal sensor
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View: Figures


Image of FIG. 1.
FIG. 1.

(a) A schematic depicting the experimental setup. Microwells contain the LC material with an open (active) upper surface. (b) Physical dimension of the well were studied using a white light interferometer to ensure square sidewalls and accurate dimensions. (c) SEM images of the square wells, formed with SU-8 photoresist.

Image of FIG. 2.
FIG. 2.

A schematic presentation of the simulation box. The nematic bulk region is shown in white, the side and bottom surface mesh points (with fixed anchoring strength) are presented in black, and the active top surface mesh points, which impose differing strengths of either homeotropic or degenerate (nonuniform) planar anchoring, are represented by red. The left frame (in grayscale) indicates that the polarization micrographs are calculated by considering only the phase shift between the ordinary and extraordinary polarization; is the ordinary refractive index, whereas is the extraordinary refractive index dependent on the local orientation of the director given by and .

Image of FIG. 3.
FIG. 3.

(a) Experimental optical micrographs showing strong homeotropic anchoring at the upper surface of the sensor wells. The image evolution on the right shows the sensor rotated between crossed polarizers. (b) Theoretically reproduced polarization micrographs for different homeotropic top surface anchoring strengths , at 0° (top row) and 45° (bottom row) between crossed polarizers. (c) Tilt angle of the nematic along the well diagonal (dashed line in the inset) for different anchoring strengths. The curve is below the curve. The inset indicates the local director in the corners of the wells: corresponds to the field of a radial hedgehog defect, as to the hyperbolic hedgehog.

Image of FIG. 4.
FIG. 4.

(a) Planar upper surface produced by the introduction of an aqueous solution to the top of the sensor. Experimental micrographs show the “cross structure” at 0° and through crossed polarizers (sequence of images on right). (b) Theoretical calculations of the director field variations for different top planar anchoring strengths with corresponding polarization micrographs at 0° (upper frames) and 45° (lower frames). Central top surface point defect is visualized as an isosurface of the nematic degree of order . Note an interesting structural transition between anchoring strengths and .

Image of FIG. 5.
FIG. 5.

Structural transitions of the LC orientation through the introduction of SDS, a homeotropic surfactant. Varying concentrations of the surfactant solution were added, while the transition from planar to homeotropic was observed for both increasing and decreasing concentrations of solvent. (a) DI water, (b) SDS, (c) SDS, (d) SDS, (e) SDS, (f) SDS.

Image of FIG. 6.
FIG. 6.

The structural transition of the nematic LC for two different sized sensor wells: , shown in the left column, , shown in the right. The concentrations of SDS at which the nematic transitions from a planar alignment to homeotropic decreases with a decrease in well depth.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Detection of alignment changes at the open surface of a confined nematic liquid crystal sensor