Surfactant-Induced Ordering and Wetting Transitions
of Droplets of Thermotropic Liquid Crystals “Caged”
Inside Partially Filled Polymeric Capsules
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Abstract
We
report a study of the wetting and ordering of thermotropic liquid
crystal (LC) droplets that are trapped (or “caged”)
within micrometer-sized cationic polymeric microcapsules dispersed
in aqueous solutions of surfactants. When they were initially dispersed
in water, we observed caged, nearly spherical droplets of E7, a nematic
LC mixture, to occupy ∼40% of the interior volume of the polymeric
capsules [diameter of 6.7 ± 0.3 μm, formed via covalent
layer-by-layer assembly of branched polyethylenimine and poly(2-vinyl-4,4-dimethylazlactone)]
and to contact the interior surface of the capsule wall at an angle
of ∼157 ± 11°. The internal ordering of LC within
the droplets corresponded to the so-called bipolar configuration (distorted
by contact with the capsule walls). While the effects of dodecyltrimethylammonium
bromide (DTAB) and sodium dodecyl sulfate (SDS) on the internal ordering
of “free” LC droplets are similar, we observed the two
surfactants to trigger strikingly different wetting and configurational
transitions when LC droplets were caged within polymeric capsules.
Specifically, upon addition of SDS to the aqueous phase, we observed
the contact angles (θ) of caged LC on the interior surface of
the capsule to decrease, resulting in a progression of complex droplet
shapes, including lenses (θ ≈ 130 ± 10°), hemispheres
(θ ≈ 89 ± 5°), and concave hemispheres (θ
< 85°). The wetting transitions induced by SDS also resulted
in changes in the internal ordering of the LC to yield states topologically
equivalent to axial and radial configurations. Although topologically
equivalent to free droplets, the contributions that surface anchoring,
LC elasticity, and topological defects make to the free energy of
caged LC droplets differ from those of free droplets. Overall, these
results and others reported herein lead us to conclude that caged
LC droplets offer a platform for new designs of LC-droplet-based responsive
soft matter that cannot be realized in dispersions of free droplets