New frontiers in anisotropic fluid-particle composites

Electrically controlled dynamics of colloidal particles in liquid crystals

Professor Oleg Lavrentovich, Kent State University, USA

• Audio recording (mp3)

Gold nanoparticles as additives for liquid crystals

Dr Torsten Hegmann, Kent State University, USA

Abstract

Abstract

Gold nanoparticles (Au NPs) have emerged as a promising class of materials with an enormous potential for modulating and improving the characteristics of liquid crystals (LCs) used in device applications. Recent, global research activities including research performed in our lab show that Au NPs induce distinct effects in LCs that may point to new directions for the use of these LC colloids in optical and sensing applications.[1,2] This talk will summarize recent fundamental research performed in my lab on nematic liquid crystals doped with functionalized Au NPs and position these results within the field. We have demonstrated the use of chiral dopant, (S)-naproxen, decorated Au NPs that effectively induce chiral nematic LC phases not only with a stronger CD response (tighter helical pitch) but also with the opposite helical sense in comparison to pure (S)- naproxen doped into the same nematic host.[3] We have established that parameters such as the nature of the interface as well as the concentration and surface modification of the NPs doped into a nematic host can be tuned to either result in the formation of unique defect patterns (i.e. the formation of birefringent stripes surrounded by larger domains with homeotropic alignment)[4] or produce a temperature-dependent alignment change from planar to vertical that could be exploited for sensor applications.[5] We also showed an unprecedented dual alignment/switching mode with drastically reduced values of the threshold voltage (Vth) by doping LCs with gold NPs,[6] and the formation of convection rolls (Williams Kapustin domains).[7] This research is currently expanded to Au nanorods,[8] gold nanostars, and magic-sized quantum dots.[9]

[1] (a) O Stamatoiu, M Mirzaei, X Feng, T Hegmann, Top Curr Chem 2012, in press (b) U Shivakumar, J Mirzaei, X Feng, A Sharma, P Moreira, T Hegmann, Liq Cryst 2011, 38, 1495-1514.

[2] (a) H Qi, T Hegmann, J Mater Chem 2008, 18, 3288-3294; (b) T Hegmann, H Qi, V M Marx, J Inorg Organomet Polym Mater 2007, 17, 483-508.

[3] (a) H Qi, J O’Neil, T Hegmann, J Mater Chem 2008, 18, 374-380; (b) H Qi, T Hegmann, JACS 2008, 130, 14201-14206.

[4] (a) H Qi, T Hegmann, J Mater Chem 2006, 16, 4197-4205; (b) M Urbanski, B Kinkead, H Qi, T Hegmann, H -S Kitzerow, Nanoscale 2010, 2, 1118-1121; (c) B Kinkead, M Urbanski, H Qi, H -S Kitzerow, T Hegmann, Proc SPIE 2010, 7775, 777511

[5] H Qi, T Hegmann, ACS Appl Mater Interfaces 2009, 1, 1731-1738.

[6] H Qi, B Kinkead, T Hegmann, Adv Funct Mater 2008, 18, 212-221.

[7] M Urbanski, B Kinkead, T Hegmann, H -S Kitzerow, Liq Cryst 2010, 37, 1151-1156.

[8] S Umadevi, X Feng, T Hegmann, Ferroelectrics 2012, in press

[9] M Mirzaei, M Urbanski, K Yu, H -S Kitzerow, T Hegmann, J Mater Chem 2011, 21, 12710-12716.

• http://downloads.royalsociety.org/audio/KAV/TM/2012_05/Torsten Hegmann.mp3

Strong coupling between ferromagnetic particles and rod-like particles in aqueous suspensions

Professor Yuriy Reznikov, Institute of Physics of National Academy of Sciences of Ukraine, Ukraine

Abstract

Biography

Yuriy Reznikov, PhD, Professor in Physics, head of the Department of Crystals at the Institute of Physics (Kyiv, Ukraine). His primary interests are photo-induced and surface phenomena in liquid crystals as well as novel LCD technologies and nanophysics of liquid crystals. Yuriy Reznikov is a co-inventor of effect of photoalignment of liquid crystals. He has been focusing on the study and application of liquid crystal nano-colloids, electro-optics, photorefraction and photonics effects in liquid crystals. Yuriy Reznikov is a co-author of 19 USA patents including basic patents on photoalignment technology, more than 170 papers.

Abstract

A two-component dispersion of hard rods, one of which is magnetically sensitive, is studied theoretically and experimentally. It is shown that control of the ordering of the magneto-sensitive component by H-field provides effective way to order the non-magnetic component. We found that magnetically-induced ordering of low concentrated ferromagnetic nanoparticles in isotropic phase of the suspension of V2O5 rods in water results in a strong birefringence of the suspension despite V2O5 rods themselves not being sensitive to magnetic field. Furthermore, ferromagnetic nanoparticles also cause extremely high sensitivity of the suspension in the nematic phase; if the native suspension requires magnetic field, H> 7kGs to be reoriented, the doped suspension effectively responds to the magnetic field, H< 30Gs.

• http://downloads.royalsociety.org/audio/KAV/TM/2012_05/Yuri Reznikov.mp3

Density functional theory for colloidal platelet mixtures

Professor Matthias Schmidt, University of Bayreuth, Germany

Abstract

Density functional theory for inhomogeneous classical liquids is a powerful tool to formulate the Statistical Mechanics of bulk and inhomogeneous classical many-body systems. The central variational principle can be elegantly formulated using Levy's method, and robust approximations for the free energy functional can be constructed using Rosenfeld's fundamental measure theory. After a brief introduction to these general topics, the presentation describes applications of DFT to mixtures of anisometric hard particles, including platelet-platelet and platelet-sphere mixtures. Rich bulk phase diagrams and interfacial effects at substrates and in sedimentation equilibrium are reported. The relevance for describing collective phenomena in corresponding colloidal mixtures is pointed out.

• Audio recording (mp3)

Exploring and applying liquid crystals in new geometries prepared by microfluidics and electrospinning

Professor Jan Lagerwall, Seoul National University, Graduate School of Convergence Science & Technology, Korea

Abstract

We use coaxial electrospinning to prepare composite fibers with a core of LC (nematics or smectics, chiral or non-chiral) inside a polymer sheath, the LC providing functionality and responsiveness [1-3]. With chiral nematics in the core we can produce non-woven textiles with iridescent color that can be tuned (or removed) by heating or cooling [3]. The fibers have wide application possibilities, e.g. as sensors. Using a nested capillary microfluidics set-up we also produce and investigate thin shells of LC, suspended in aqueous host phases [4-6]. The various arrangements of topological defects and other geometrical features developing by self-asesmbly in these shells, and the possibility of tuning the result by modifying boundary conditions, LC phase and thickness and diameter of the shell, make this new LC configuration very attractive.

[1] J P F Lagerwall, J T McCann, E Formo, G Scalia, Y Xia, Chem.Commun, 42, 5420 (2008)

[2] E Enz, U Baumeister, J Lagerwall, Beilstein J Org Chem, 5 (2009)

[3] E Enz, J Lagerwall, J Mater Chem, 20, 6866 (2010)

|4] H-L Liang, S Schymura, P Rudquist, J Lagerwall, Phys Rev Lett 106, 24, 247801 (2011)

[5] H-L Liang, R Zentel, P Rudquist, J Lagerwall, Soft Matter, DOI:10.1039/C2SM07415J, in press (2012)

[6] H-L Liang, E Enz, G Scalia, J Lagerwall, Mol Cryst Liq Cryst, 549, 69 (2011)

• Audio recording (mp3)

X-ray studies of particles in liquid crystals

Professor Robert Richardson, University of Bristol, UK

Abstract

Nematic liquid crystal comprising small elongated molecules have been the basis for the highly successful display technology. However, they do have some limitations. Displays based on director rotation in transparent materials (e.g. twisted nematic) require polarization and colour filters which absorb a significant fraction of the light. Reflective displays based on the absorption light by guest dye molecules could improve on this. However, they tend to have poor contrast because the orientational order parameter of the dye molecules within the nematic host is low. Suspensions of pigment particles have therefore been investigated as materials for future display applications. In principle, rod shaped particles with the transition dipoles of an absorption in the visible spectrum lying along their axes would be an ideal system. Different combinations of plate and rod like particles with different stabilizers in isotropic and nematic solvents have been used. The principle aim of this work is to characterise the electro-optic properties and understand their origins at the nanoscale level. This can then inform the development of better materials. Several experimental methods have been used to elucidate the properties of the pigment suspensions but absorption spectroscopy and X-ray scattering have predominated.

• Audio recording (mp3)

Carbon nanotubes in liquid crystals

Professor Giusy Scalia, Seoul National University, Korea

Abstract

Carbon nanotubes are notoriously difficult to handle, making their efficient dispersion and alignment challenging. Liquid crystals are very effective per se in imposing organization on carbon nanotubes, and orientational alignment can be successfully transferred by both families of liquid crystals, thermotropics as well as lyotropics, onto embedded nanotubes. The two groups are advantageous for different aspects: while thermotropics are very easy to align macroscopically, profiting from the very well developed aligning techniques, and to reorient with external fields, lyotropics are more compatible with common dispersion methods for carbon nanotubes. These aspects and the achieved results using both classes of liquid crystal, and single- as well as multiwall carbon nanotubes with varying characteristics, will be discussed together with the important, general issue of the dispersion of nanotubes. There will be a particular focus on how the details of the LC matrix can strongly affect the result, lyotropics and thermotropics each having their pros and cons and results from nematic and isotropic hosts being quite different. While the molecular structure of the host seems not to play a critical role for the nanotube ordering this aspect becomes crucial for the capacity of the LC to efficiently disperse the CNTs.

• Audio recording (mp3)

Liquid crystal phase transitions in suspensions of mineral colloids: new life from old roots

Professor Henk Lekkerkerker, Utrecht University, The Netherlands

Abstract

One of the most remarkable phenomena exhibited by concentrated suspensions of colloidal particles is the spontaneous transition from fluid-like structures to those exhibiting long-range spatial and/or orientational order (colloidal crystals and colloidal liquid crystals). The fact that such ordering can occur in suspensions in which interparticle forces are purely repulsive provides a dramatic realization of the predictions made by Lars Onsager in the 1940’s and later substantiated by computer simulations. From these studies it is clear, that the ordering is driven by entropy. Liquid crystalline phases in suspensions of mineral colloidal particles have been known for a long time. Zocher reported in 1925 on the observation of a nematic phase in suspensions of V2O5 and a smectic phase in suspensions of -FeOOH. In recent years the number of mineral colloidal liquid crystals has steadily increased. These systems are interesting due to the fact that size and shape of the particles can be tuned by dedicated chemical synthesis methods leading to novel and interesting phase behavior A very important development is the discovery of a number of natural clays(nontronite,beidellite) that show nematic liquid crystal phases at concentrations as low as 2 wt% This opens the way to produce lyotropic liquid crystals with cheap and abundantly available materials. Modern optical microscopy and Synchrotron X-ray scattering techniques allow these systems to be studied in detail. Furthermore the electric magnetic and optical properties can be varied over a wide range leading to interesting phenomena. In this talk I will review the phases formed and their structural characteristics and compare the results with computer simulations and theoretical predictions.

• Audio recording (mp3)

Liquid crystals in natural clay suspensions

Dr Jeroen S van Duijneveldt, University of Bristol, UK

Abstract

Onsager was the first to propose a theory of the isotropic / nematic transition for suspensions of hard rod-like particles, which occurs due to the fact that the gain in translational entropy overweighs the loss in orientational entropy on increase of concentration. The same phenomenon was also predicted and observed in hard platelet suspensions. The scope for observing liquid crystals in suspensions of natural clay particles will be addressed. Sepiolite consists of rod-like particles and indeed displays a nematic phase; the spread in particle lengths however modifies and enriches the phase behaviour compared to the theory for monodisperse rods. Furthermore the mineral is porous which allows incorporation of guest molecules such as dyes. One might expect plate-like smectite clay particles (especially montmorillonite) to be perfect candidates to form colloidal liquid crystals, due to their high aspect ratio (400 nm diameter and 1 nm sheet thickness). However, a nematic phase in montmorillonite suspensions remains elusive, and only recently has a nematic phase been observed in aqueous suspensions of related clays, nontronite and beidellite. Often a sol / gel transition pre-empts the formation of a nematic phase. The effect of adsorbing surfactant on the clay surface will be discussed.

• Audio recording (mp3)

Liquid processing of carbon nanotubes

Professor Philippe Poulin, Centre de Recherche Paul Pascal de Bordeaux, France

Abstract

Processing carbon nanotubes in liquid states offers opportunities to control their spatial organization in films, composites, fibers, etc. The use of molecular or macromolecular dispersants allows the interactions between the nanotubes to be finely tuned. As a consequence different states can be obtained from disordered dispersions with low percolation thresholds to ordered liquid crystalline phases in which the nanotubes exhibit long range orientational ordering. We show in this presentation the effect of attractive interactions on the phase behavior of carbon nanotubes. We discuss the possibility to control the morphology of carbon nanotube networks and their resultant electro-optical properties. In particular it is expected that weak attractive interactions should improve electrical contacts between nanotubes and be of interest to develop efficient conductive inks. At high concentration carbon nanotubes form liquid crystals which are of interest to develop anisotropic conductors. We show the influence of processing conditions on the order parameter of nanotube based liquid crystals. When dried they can be used to form highly conductive films. The conductivity anisotropy as a function of the order parameter reveals much greater variations than that of conventional dielectric liquid crystals in which the conductivity anisotropy arises from the anisotropic mobility of charge carriers.

• Audio recording (mp3)

Pushing, pulling and twisting liquid crystal systems; exploring new directions with laser manipulation

Professor Helen Gleeson, University of Manchester, UK

Abstract

Optical tweezers are exciting tools with which to explore liquid crystal systems; the motion of particles held in laser traps through liquid crystals is perhaps the only approach that allows a low Erickson number regime to be accessed. This offers a new method of studying the microrheology associated with micron-sized particles suspended in liquid crystal media – and such hybrid systems are of increasing importance as novel soft-matter systems.  This talk describes the microrheology experiments that are possible in nematic materials and discusses the sometimes unexpected results that ensue.  It also touches on the inverse system; micron-sized droplets of liquid crystal suspended in an isotropic medium and shows some remarkable light-induced changes in chirality that result in a micron-sized opto-mechanical transducer.  Opportunities for the future are discussed.

Co-authors:
M R Dickinson, J E Sanders and Y Yang, The University of Manchester, UK

• Audio recording (mp3)

Topology and nematic colloids

Professor Igor Muševič, J Stefan Institute and University of Ljubljana, Slovenia

Abstract

Topology has long been considered as an abstract mathematical discipline with little connection to materials science. However, the emergence of a new class of topological materials that includes topological insulators, topological memories and knotted colloidal soft matter, provides strong evidence that topology might play an important role in the design of novel materials with counter-intuitive properties. Here, we discuss the importance of topology in the design and assembly of nematic colloids, where the structural forces between the colloidal particles are much more complex than the forces between the electric charges. This is because the topology of the electromagnetic field has little importance in water-based colloids, but is a novel paradigm in liquid-crystal colloids. In this case the topology of the ordering field is responsible for the striking observations of topological-defect-mediated interactions in 2D colloids, such as the assembly of 2D nematic colloidal crystals and colloidal interactions mediated by entanglement, where knotted topological defect loops form knots and links of arbitrary complexity. In all cases, the colloidal binding energy is of the order of several 1000 kBT. This is several orders of magnitude higher than for water-based colloids and could provide new strategies for topological soft materials and applications in photonics.

• Audio recording (mp3)