The Design of Novel Liquid Crystal Droplets for Biosensing Devices

This thesis describes novel, microfluidically produced liquid crystal
(LC) droplets for use as potential biosensing devices. The focus specifically
is the alignment and stability of LC droplets in various phases,
stabilised utilising a range of mechanisms.

The ability to modify the resultant alignment through careful interplay
between the surface anchoring conditions and the bulk elastic
constants is determined to be possible through modification of the
surfactant coating the droplet surface, and by utilising LCs with different
properties. It is discovered that 8CB has the ability to induce
homeotropic alignment even when the lipid monolayer is comprised
of 50% 1,2-bis(12’ferrocenyl)-dodecanoyl-snglycero- 3-phosphocholine
(DFPC), a lipid designed to suppress homeotropic anchoring. However,
observations note that a lack of electrostatic or steric component
within the surfactant coating can cause poor droplet formation. Further
investigation into the stability of LC droplets determines that
a minimum quantity of electrostatic or steric stabilising surfactant
must be present to produce monodisperse, long-term (> 24hrs) stable
droplets. For phospholipids, this is a minimum of 40mol% anionic
lipid or 3mol% PEGylated lipid with an overall lipid concentration of
5mg/ml.

Further, it is determined that a novel approach to droplet stabilisation
in the form of microgels from waste potato protein is capable
of creating monodisperse LC droplets that are even more stable than
the phospholipid-coated droplets (3+ days) at ambient room temperature.
Additionally, since microgels do not influence the alignment
of the LC, droplets remain bipolar until a surfactant or analyte of
interest is added and thus any change in the LC alignment can be
attributed directly to the presence of said analyte or surfactant. Such
an approach is extremely valuable in the design of LC droplet biosensors,
given their excellent storage capabilities, and may prove to be
an excellent alternative to the use of phospholipids as a stabilising
mechanism.

An exploratory investigation into droplets made with a ferroelectric
nematic (NF) LC demonstrate the ability to access unique alignment
types by changing the type of LC used in droplet formation to a more
complex phase. NF materials in particular appear to have more complicated
alignment structures than typical nematics, and can have
analogues with chiral nematic droplets. Samples utilising novel LC
materials may find use in various applications.

The novel and varied experimental work produced in this thesis demonstrates
the many different avenues that are yet to be explored in the
field of LC droplets for biosensing, and even initial observations can
provide a wealth of information on such systems.