SOFT MATTER: LIQUID CRYSTALS

Dierking Group
Physics and Astronomy, University of Manchester
United Kingdom
RESEARCH
The examples below give a flavour of the research topics that the group is working on. To investigate these systems, which are mostly based on liquid crystals in one way or the other, we employ a range of different techiques. One of the fundamental pieces of apparatus are several optical polarizing microscopes equipped for transmission and reflection and a digital camera for time resolved image aquisition. One of the microscopes can also be connected to a high resolution, time resolved optical spectrometer.
Electrooptic setups are available to measure transmission and response times as electric fields are applied via function generators and power amplifiers. Electrooptic outputs are measured by photodiodes with different steepness and sensitivity to be visualized on several digital four channel oscilloscopes, all run via LabView.
A polarization and high resolution tilt angle measurement setup is available, just as dielectric spectroscopy and a pyroelectric setup. Other electrical measurements such as conductivity can also be carried out.
A range of other experimental techniques, such as Differential Scanning Calorimetry, Scanning Electron Microscopy, x-ray diffraction, and several techniques used in the characterization of nanoparticles are available through collaboration with the School of Chemistry, the School of Materials Sciences, and OMIC. Terahertz spectroscopy investigations are performed in collaboration with a group in Marburg, Germany. 
Chirality in Liquid Crystals


We are interested in all fundamental effects related to chirality in liquid crystals and beyond. These are for example Ferroelectric Liquid Crystals (FLC), frustrated phases like the Blue Phases (BP) and the Twist Grain Boundary Phases (TGB), but also the well studied cholesteric phases. Chirality manifestation via helical superstructures in cholesterics and SmC* phases are just as interesting as chirality related effects like the selective reflection, which can also be observed for some beetles, or the occurence of a spontaneous polarization and the electroclinic effect. Furthermore, a whole range of interesting applications of chiral liquid crystals have been proposed, from fast switching optical components and displays to heat repellant sheets.  
Polymer Stabilized Liquid Crystals

One of the long standing topics of the group are polymer stabilized liquid crystals. For these systems a small amount of a photosensitive bifunctional monomer is mixed uniformly into the liquid crystal phase and polymerized most often via UV irradiation. The resultant polymer network acts like a template of the liquid crystalline structure it was formed in, like the image besides, where the network is formed in the vicinity of a s= -1 defect, imaging the director field around this defect. Polymer stabilization was originally intented to be employed in the production of electronic paper / ebook readers, but is now superseeded by a different technology. Nevertheless, plenty of applications are being proposed, such as mechanical stabilization of ferroelectric liquid crystal devices, reflective displays, highly reflective cholesteric devices, or for phase stability in Blue Phase displays.


Defect Creation and Annihilation

Liquid Crystals exhibit a wealth of different defects and especially the nematic phase is known for its strength s=+1, s=-1, s=+1/2 and s=-1/2 defects. Processes of how these defects form and how they annihilate, can be related to the Kibble-Zurek mechanism, which originates from a description of a transition in the early universe from a uniform to a defect state. Cosmic strings are the remnance of these defects. The model was extended to solid state materials by Zurek, and a range of scaling laws were proposed. Liquid crystals represent an elegant way to test these scaling laws of defect formation and annihilation in the laboratory. In addition, effects of boundaries, confinement and dimensionality of the system can easily be investigated.
Liquid Crystal - Nanotube Dispersions

The pioneering work on Liquid Crystal - Carbon Nanotube dispersions was performed in our group. Minute amounts of nanotubes are dispersed in a nematic liquid crystal to change and improve the materials properties through exploiting the extraordinary properties of nanotubes, such as conductivity along the tube and insulating across, great mechanical strength, or heat insulation. We demonstrated that the liquid crystal orients the nanotubes along the director field and can even dynamically change their orientation through the electric or the magnetic Freedericksz transition. In this way an electrical or magnetically steered off-on or on-off switch can be realized, which can act as field sensor. The work was later extended to study the dynamics of the reorientation and to develop a percolation model describing the reorientation, as well as using other liquid crystal phases.   
Micro- and Nano-particles Dispersed in Liquid Crystals

​The addition of small amounts of nanoparticles to liquid crystal phases can drastically change their properties, and represent a way to tune threshold voltage, elasticity, viscosity and response times. If the added particles possess ferroelectric, ferromagnetic or superparaelectric properties, these can add functionality to the system through additional interactions with electric and / or magnetic fields.

In a different project we investigate the motion of microspheres and cylindrical microparticles in liquid crystals, caused by the application of electric fields of varying amplitude and frequency. This study is related to linear and non-linear electrophoresis, and exhibits a wealth of different translational and rotational modes to be characterized and described for various different phases.

Graphene Oxide Liquid Crystals

Graphene oxide is an interesting material to add to a liquid crystal. It exists in monoatomic two-dimensional flakes, and has a profound influence on the properties of liquid crystals, again allowing tunability. The field is currently only in its infancy, and we are trying to understand the most basic mechanisms in graphene oxide doped thermotropic liquid crystals.

At the same time, graphene oxide exhibits lyotropic nematic behaviour, when added above certain concentrations and flake sizes to a polar isotropic solvent. The formed phases are very stable and orient well under confinement. They also exhibit a very interesting dielectric relaxation behaviour, which can be attributed to fluctuations of graphene oxide flakes.