Abstract
In liquid crystal (LC) droplets, small changes in surface anchoring energy can produce large changes in the director field which result in readily detectable optical effects. This makes them attractive for use as biosensors. Coating LC droplets with a phospholipid monolayer provides a bridge between the hydrophobic world of LCs and the water-based world of biology and makes it possible to incorporate naturally occurring biosensor systems. However, phospholipids promote strong perpendicular (homeotropic) anchoring that can inhibit switching of the director field. We show that the tendency for phospholipid layers to promote perpendicular anchoring can be suppressed by using synthetic phospholipids in which the acyl chains are terminated with bulky tert-butyl or ferrocenyl groups; the larger these end-group(s), the less likely the system is to be perpendicular/radial. Additionally, the droplet director field is found to be dependent on the nature of the LC, particularly its intrinsic surface properties, but not (apparently) on the sign of the dielectric anisotropy, the proximity to the melting/isotropic phase transition, the surface tension (in air), or the values of the Frank elastic constants.
Highlights
It has long been known that liquid crystals (LCs) are present in a large number of biological systems, but despite this, researchers have only relatively recently realized that the effects of biological stimuli on LCs, in particular the way that they affect LC alignment, could be used in devices
Experiments have shown that the elastic energy, FE, scales linearly as FE ∼ KR for a droplet of radius R, whereas the surface energy Fs scales as Fs ∼ WR2 where W is an anchoring energy coefficient that controls the orientation of LC molecules at a surface
The lipid-coated LC droplets were produced by shaking the LC in an aqueous solution containing phospholipid liposomes rather than by the microfluidic method which we used previously.[16]
Summary
It has long been known that liquid crystals (LCs) are present in a large number of biological systems, but despite this, researchers have only relatively recently realized that the effects of biological stimuli on LCs, in particular the way that they affect LC alignment, could be used in devices. For these LC materials that align in a homeotropic manner at the air interface and/or on OTS glass, in 14 of the 16 types of lipidcoated droplets the director field is radial; the alignment in the droplet is homeotropic For the two LCs that align in a nonhomeotropic manner at the air interface and on OTS glass in six/seven of the eight types of lipid-coated droplets the director field is bipolar: the alignment in the droplet is nonhomeotropic. The only clear exception is for MLC2081 coated with DOPC:DOPG the lipid mixture most favorable to homeotropic anchoring To understand this dependence on the surface properties of the LCs in more detail, we measured their surface tensions at the LC/air interface. There is insufficient experimental data (and in particular we were unable to obtain surface tension data) to be able to understand the factors involved in the radial nature of the MLC2081 droplets coated with DOPC:DOPG
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