An electrically switchable surface free energy on a liquid crystal and polymer composite film

Abstract

An electrically switchable surface free energy on a liquid crystal and polymer composite film (LCPCF) resulting from the orientations of liquid crystal molecules is investigated. By modification of Cassie’s model and the measurement based on the Chibowski’s film pressure model (E. Chibowski, Adv. Colloid Interface Sci. 103, 149 (2003)), the surface free energy of LCPCF is electrically switchable from 36×10−3J/m2 to 51×10−3J/m2 while the average tilt angle of LC molecules changes from 0° to 32° with the applied pulsed voltage. The switchable surface free energy of LCPCF can help us to design biosensors and photonics devices, such as electro-optical switches, blood sensors, and sperm testers.

Liquid Crystals for Bio-medical Applications

“…In order to sustain thermotropic nematic LC materials, LC and polymer complex systems are great candidates to turn the soft LC materials into film-like structures which are easy to apply voltage to further stimulate the LC orientations for biosensing. Recently, we develop a LC and polymer composite film (LCPCF) whose wettability (or surface free energy) is electrically tunable due to the orientation of LC molecules on LCPCF [16][17][18][19][20][21]. LCPCF is a film fabricated through photoinduced phase separation process.…”

“…The four chemical structures of LC (E7) compounds of LCPCF are listed as well. The monomer is a liquid crystalline monomer (4-(3-Acryloyloxypropyloxy)-benzoic acid 2-methyl-1, 4-phenylene ester) [21] balance of three phases, Wenzel's magnification of wettability and Cassie's chemical heterogeneity are the factors that can help us to describe the wetting properties or surface free energy of a surface. However, one more factor, molecular orientations, can also be considered when we design an actively switchable surface, LCPCF for example.…”

“…However, one more factor, molecular orientations, can also be considered when we design an actively switchable surface, LCPCF for example. The surface free energy of the LCPCF in the air (γ LCPCF-air ) with an unit of J/m 2 can be expresses in 15.1 according to the Chibowski's film pressure model [21]:…”

“…The advancing angle (θ a ) and the receding angle (θ r ) are the angles at which the contact line changes when the volume of the fluidic drop increases and decreases, respectively. In experiments, the surface free energy of the LCPCF (γ LCPCF-air ) increases from 36 × 10 −3 to 51 × 10 −3 J/m 2 with the applied pulsed voltage [21]. According to Cassie's model, the surface free energy of LCPCF in the air is a linear composition of the surface free energies of polymer (γ p-air ) and the surface free energy of LC (γ LC-air ).…”

“…γ ph-ter is ~29 × 10 −3 J/m 2 , γ alky-alko is ~29 × 10 −3 J/m 2 , and γ CN is ~154 × 10 −3 J/m 2 [21]. The averaged LC molecules tilt up from 0 to 32° with the applied pulsed voltage under calculations from experiments.…”