Orientational Structure and Electro-Optical Properties of Chiral Nematic Droplets with Conical Anchoring

The polymer dispersed liquid crystals (PDLCs) with conical boundary conditions are considered. PDLC films with different values of the relative chirality parameter <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>N</mi><mn>0</mn></msub></semantics></math></inline-formula> of chiral nematic droplets ranging from 0 to <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>1.32</mn></mrow></semantics></math></inline-formula> are studied experimentally and theoretically. In flattened spheroid-shaped chiral nematic droplets, a twisted axial-bipolar structure is formed whose twist angle increases with rising <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>N</mi><mn>0</mn></msub></semantics></math></inline-formula> value. Two stable states of the structure are revealed: one with the bipolar axis oriented perpendicular to the short axis of the spheroid and another with the bipolar axis oriented parallel to it. Applying a small voltage causes the bipolar axes of the chiral nematic droplets to reorient parallel to the electric field. The structure is unwound in strong electric fields, and the droplet order parameter reaches a high value of nearly <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>0.95</mn></mrow></semantics></math></inline-formula>. These features of the voltage-induced reorientation of the axial-bipolar structure explain the experimentally observed characteristic electro-optical properties of PDLC cells: high transmittance <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msup><mi>T</mi><mi>max</mi></msup><mo>≅</mo><mn>0.90</mn></mrow></semantics></math></inline-formula> in the on-state and low control voltages of less than 35 V. The minimum transmittance of the PDLC cells decreases as the value of <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>N</mi><mn>0</mn></msub></semantics></math></inline-formula> increases; for samples with <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>N</mi><mn>0</mn></msub><mo>≥</mo><mn>0.60</mn></mrow></semantics></math></inline-formula>, the contrast ratio exceeds 145.