My current research interests are mainly in the area of liquid crystals and related materials, with particular emphasis on their optical properties. I am part of the Soft Photonics Systems group of the University of Southampton. This is a cross-disciplinary group that combines modelling and experiments to develop and use new controllable optical materials, mostly liquid-crystal based.
Liquid crystals are asymmetric molecules that have some form of long range order, intermediate between the perfect order of a crystal and the complete disorder of a liquid. The most important property from my point of view is that they have a very strong nonlinear response to the light passing through them: this makes them ideal materials to study the strong coupling limit of light and matter.
Below are some projects I am involved with. For information about past research activity please look at the Completed research projects page.
- Multiscale models of liquid crystals suspensions of nanoparticles
- Optical characterisation of liquid crystal materials
- Optical alignment of liquid crystals
- Fast Q-tensor algorithms for liquid crystal alignment away from defects
For information about past research activity please look at the Completed research projects page.
Multiscale models of liquid crystals suspensions of nanoparticles
Very dilute suspensions of nanoparticles (e.g. gold) in liquid crystals seem to have remarkable effects on the liquid crystal properties. Dr Keith Daly (Engineering Sciences) and I are running a long term project on using homogenisation theory to develop macroscopic models of these suspensions. This project has involved my ex-PhD student Dr Tom Bennett and my current PhD, Jordan Gill.
Suspensions of nano particles in liquid crystals have been studied by means of molecular simulations and macroscopic models. Simulations of rod like molecules are computationally expensive and are limited to small systems. On the other hand macroscopic models contain unknown phenomenological parameters that attempt to capture the coupling between liquid crystal and its dopant. Unfortunately there is often no way to self consistently compute the value of these parameters. The aim of this project is to use homogenisation theory obtain a macroscopic description of a doped nematic system that allows a consistent computation of the new dopant dependent terms.
The outcome of the homogenisation process is that all of the complicated geometric information encoded in the original problem has been moved into effective material parameters. This is the crucial advantage over more phenomenological models. The microscopic detail is retained in the form of a cell problem, which is used to compute the effective material parameters. The end results of the process is two-fold: (i) we obtain equations that are less computationally intensive to solve and (ii) we can compute the new material parameters from the underlying geometry of the system.
The first results of this project focused on nematic liquid crystals with fixed inclusions. They have appeared in Physical Review E:
T.P. Bennett, G. D'Alessandro and K.R. Daly, Multiscale models of colloidal dispersion of particles in nematic liquid crystals, Phys. Rev. E 90(6), 062505 (2014)
We have now extended this work to rotating inclusions in two dimensions. The derivation of the model has appeared in the SIAM Journal on Applied Mathematics:
T.P. Bennett, G. D'Alessandro and K.R. Daly, Multiscale models of metallic nanoparticles in in nematic liquid crystals, SIAM J. Appl. Math. 78, 1228-1255 (2018)
We are currently working at relaxing some of the conditions on these models and comparing the results of homogenisation with other effective medium theories.
This project is in collaboration with Prof Malgosia Kaczmarek and a variety of past and present members of the Soft Photonics Systems group. The aim is to develop an all-optical characterisation of the properties of pure and doped liquid crystals. We have focused on three different aspects of this topic:
J. Bateman, M. Proctor, O. Buchnev, N. Podoliak, G. D'Alessandro and M. Kaczmarek, Voltage transfer function as an optical method to characterize electrical properties of liquid crystal devices, Opt. Lett. 39(14), 3756-3759 (2014)
|Comparison of measured (left column) crosspolarized intensity as a function of voltage and frequency for the E7 cell with that (right column) given by the nonlinear filter model in Eq. (3), with τVTF=1.2 s and τLC=0.7 s. The top row shows the average intensity while the bottom row shows the range. See Bateman et al (2014) for details.|
T.P. Bennett, M.B. Proctor, M. Kaczmarek and G. D'Alessandro, Lifting degeneracy in nematic liquid crystal viscosities with a single optical measurement, J. Colloid Interface Sci. 497, 201-206 (2017) - [PDF]
|Graphical abstract of Bennett et al (2017), J. Colloid Interface Sci. - We measure the cross-polarised intensity through a nematic liquid crystal cell at different frequencies of the applied voltage (high, top right; low, bottom right; intermediate, bottom left). Statistical analysis of these curves give the parameters listed on the top left.|
T.P. Bennett, M.B. Proctor, J.J. Forster, E. Perivolari, N. Podoliak, M. Sugden, R. Kirke, T. Regrettier, T. Heiser, M. Kaczmarek and G. D'Alessandro, Wide area mapping of liquid crystal devices with passive and active command layer, Appl. Optics 56, 9050-9056 (2017) - [PDF]
|PI planar cell filled with E7: spatial map of the liquid crystal (a) thickness and (c) pretilt angle; (b) and (d) are the corresponding errors. Circles are the fitted values, and the background color map is a piece-wise cubic interpolation between them.|
To function correctly many liquid crystal devices require specific alignments of the liquid crystals at the cell surface. In some cases it is possible to use very specialised molecules whose orientation is controlled by light to align the liquid crystal. One class of such molecules are the PAAD molecules developed by Beam Co. (Florida, USA). In general, these molecules are oriented when the liquid crystal device is built and are then polymerised so that they can no longer change orientation. We are, instead, interested in the feasibility of time reversible alignment of liquid crystals using PAAD molecule. In this paper,
E. Perivolari, J.R. Gill, N. Podoliak, V. Apostolopoulos, T.J. Sluckin, G. D'Alessandro and M. Kaczmarek, Optically controlled bistable waveplates, J. Mol. Liq. (2017) - [PDF]we have shown that it is possible to twist and untwist a liquid crystal cell in a stable and repeatable manner.
|Schematics of the optical setup for the (a) single beam and (b) pump-probe experiments. In both experiments, the PAAD-LC cell is placed between a pair of parallel polarizers (P1 and P2). The alignment is detected by measuring the transmitted intensity of the probe beam (B1) with the photodiode (PD). In the single beam experiments B1 is used also to realign the PAAD layer. In the pump-probe experiments, the pump beam (B2) is used to control the PAAD layer alignment. P3 is the pump beam polariser and L1 and L2 are lenses used to expand the pump beam, if required.|
This project was started with my ex-PhD student, Dr Keith Daly and was continued with my EPSRC funded post-doc, Dr Yogesh Murugesan, and Dr Giovanni de Matteis.
Modelling liquid crystals in optical devices requires us to determine the alignment of the liquid crystal molecules as a function of applied external fields. The equations for the liquid crystal alignment can be "stiff", i.e. they may contain terms that evolve on vastly different time scales. This property makes them very hard to integrate numerically. To overcome this problem we have exploited its causes and have used the vastly different time scales to develop an approximation to the liquid crystal equations that is non-stiff and very accurate. The draw-back of this approximation is that it is valid only in regions away from defects. The approximation, called Defect Free Q-Tensor Approximation (DFQTA), is described fully in
K.R. Daly, G. D'Alessandro and M. Kaczmarek, An efficient Q-tensor based algorithm for liquid crystal alignment away from defects SIAM Journal on Applied Mathematics 70(8), 2844-2860 (2010)An introductory explanation and a Matlab code of the equations is available from the DFQTA Homepage. We are now working on including fluid flow in this model.
Pages of some of my collaborators are:
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