Research ArticleMATERIALS SCIENCE

Twist again: Dynamically and reversibly controllable chirality in liquid crystalline elastomer microposts

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Science Advances  27 Mar 2020:
Vol. 6, no. 13, eaay5349
DOI: 10.1126/sciadv.aay5349
  • Fig. 1 Schematic of the system and calibration of light penetration parameter.

    (A) Schematic of the polymer network. The micropost is composed of a network of polymer chains. Mesogens (gray) are attached side-on to the polymer backbone (black), and the chains are cross-linked by an azobenzene derivative (green). The orientation of the nematic director is indicated by the double-headed red arrow. Under UV light, the cross-linkers undergo a trans-cis isomerization, disrupting the local nematic order. This effect attenuates deeper into the material. (B) Schematic of the post. The elastomeric micropost is modeled with a set of initially cubic finite elements, at a scale of one element per 2.5 μm. The incident light source can be varied in both the polar (θ) and azimuthal directions (ϕ). (C) Calibration of light-scattering distance. The penetration of light was assumed to fall off exponentially, with a length scale chosen to match experimental results of an 18° deflection for light incident at 30° from vertical, for a micropost of height 75 μm and width 25 μm. From this comparison, we obtain a value of 6 μm for the penetration depth. The inset shows the bending angle (δ) measured by the deflection of the surface normal at the top of the post relative to its initial vertical orientation.

  • Fig. 2 Effect of post aspect ratio for director orthogonal to substrate.

    (A) Effect of micropost height on bending response. The height ranges from 75 to 150 μm, while width is kept at 25 μm. Increasing the height over which the post can bend leads to a larger bending response. This is bounded above by the angle of the incident light source from vertical, indicated by the dashed line. (B) Effect of micropost width on bending response. At a fixed height of 75 μm, we vary the post width between 10 and 35 μm. Thinner posts have a lower bending modulus and are thus able to track the light more closely, although the self-shadowing of the post prevents it from bending past the incident angle of the light.

  • Fig. 3 Light-phobic bending for director in the plane of the substrate.

    (A) Schematic of light-phobic bending. Disruption of the nematic state near the illuminated face will cause that region to expand vertically when the director is in the plane of the substrate. This will, in turn, cause the post to bend backward, away from the light. (B) Rotational variation of response. An overhead view of the post shows that we now have an additional degree of freedom as the light source is rotated relative to the director. To test the dependence on this coordinate, we simulate multiple configurations where the nematic director (red arrows) is oriented along the x axis and the light source (yellow arrows) is rotated around it. (C) Dependence of bending on incident azimuthal angle. Simulations represent posts of height 150 μm and width 25 μm, responding to a light source 30° from vertical. The light-averse response is strongest when the light is orthogonal to the programmed nematic director. (D) Post at incident azimuthal angle of 5°. Here, the light is incident at an oblique angle to the nematic director and induces a small backward bending. (E) Post at incident azimuthal angle of 85°. Here, the light is nearly orthogonal to the nematic director and induces a larger degree of backward bending than for an azimuthal angle of 5°.

  • Fig. 4 Twisting behavior for director at 45° from the substrate.

    (A) Schematic of illuminated post. When the illuminated region transitions from a nematic to an isotropic state, the angled orientation of the director will cause it to deform into a parallelogram. This will produce a shearing at the top of the post, causing it to twist. For this particular angle of the incident light, the induced twist is right-handed, as indicated by the curved arrow in the top view. (B) Micropost twist angle versus azimuthal angle of incident light. As the light source is moved counterclockwise around the post, it first produces right-handed (positive) twisting and then left-handed (negative) twisting, with no twisting at 0° or 180° from the x axis. (C) Overhead and side view of the micropost for an incident azimuthal angle of 125°. Here, light incident on the +y face produces right-handed twisting. This corresponds to the data point outlined in purple in (B). (D) Overhead and side view of micropost for an incident azimuthal angle of 245°. Here, light incident on the −y face produces left-handed twisting. This corresponds to the data point outlined in green in (B). (E) Experimental observations of twisting of surface-anchored LCE microposts. For the director orientation of 45° from the vertical, the LCE microposts reversibly twist clockwise and counterclockwise, with handedness controlled by the direction of incident light, as predicted by the simulations.

  • Fig. 5 Effect of director tilt angle on twisting and bending.

    (A) Twist angle versus nematic orientation. As the director orientation is varied between 0° (vertical) and 90° (planar), we see that the twisting behavior of the micropost measured at an azimuthal angle of 125° from the x axis first increases and then decreases. The maximum twist is achieved for a nematic director near 45° from the vertical axis. (B) Deflection of posts toward and away from light. The bending response of the micropost moves from light-seeking to light-avoiding as we consider nematic director orientations ranging from vertical (purple) to planar (green). The cross-over point, between 45° and 60° from vertical, coincides with the maximum twisting orientation.

  • Fig. 6 Twisting and bending behavior in chimeric microposts.

    (A) Schematic of chimeric microposts. Bi-domain microposts are constructed with a vertical director at the base and a tilted director in the top half (green), as well as the reverse geometry, with the tilted director at the base (blue). Tilted (yellow) and vertical (purple) monodomain microposts are presented for comparison. (B) Micropost twisting angle versus incident azimuthal angle. The light source is at a polar angle of 86° from vertical. A tilted nematic in the bottom half (blue points) reduces the total twisting by 50% relative to the homogeneous tilted nematic (yellow points), as only half of the post is twisting. Placing the tilted nematic in the top half (green points) further reduces twisting as the bending at the base moves the top half out of the path of incident light. (C) Post with vertical director in anchored bottom half. The incident azimuthal angle is 125°. The post bends toward the light at the base and twists in the top half. (D) Bending angle as a function of azimuthal angle of incident light. The deflection angle at the top of the post is larger when the vertical director is in the free top half versus when it is in the constrained bottom half. (E) Deflection in the xy plane, measured at the top of the post. Bending at the base (green points) leads to a greater deflection than bending that begins halfway up the post (blue points). Both cases result in greater deflection than for a homogeneously tilted director (yellow points) but less than that of a vertical director (purple points). (F) Post with tilted director in anchored bottom half. The incident azimuthal angle is 125°. The post twists at its base and bends toward the light in the top half.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/13/eaay5349/DC1

    Fig. S1. Selection of mesh size.

    Fig. S2. Time evolution of bending.

    Fig. S3. Twisting versus height for a post with a tilted nematic director in the xz plane.

    Fig. S4. Diagram of effect of shadowing on twist.

    Movie S1. Top view of light-responsive deformation of LCE micropost.

  • Supplementary Materials

    The PDF file includes:

    • Fig. S1. Selection of mesh size.
    • Fig. S2. Time evolution of bending.
    • Fig. S3. Twisting versus height for a post with a tilted nematic director in the xz plane.
    • Fig. S4. Diagram of effect of shadowing on twist.
    • Legend for movie S1

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    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). Top view of light-responsive deformation of LCE micropost.

    Files in this Data Supplement:

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