Hot article: Electrically controlled topological micro cargo transportation

Did you know that you have liquid crystals at home? They sound like an outer space material, but they are the basis of the main technology behind the images you see on your TV or computer screen – Liquid Crystal Display (LCD). Their behaviour is quite unique, as they show both solid- and liquid-like features. Liquid crystals present some degree of ordering of their molecules as solids do, but at the same time the ease and speed at which they can rearrange such ordering belongs to the realm of liquids.     

Schematic and optical micrographs of nematic layer

In this publication, the authors take advantage of these features by designing a liquid crystal system which has two areas with different molecular orientations. They show how microparticles can be trapped at the interface between these two areas, and that such an interface can be moved by applying an external voltage to the system. They achieve a precise control over the interface movement, enabling a controlled translation and precision positioning of the microparticle. The insights presented in this work are of high relevance to the design of novel lab-on-chip devices for sensing applications as well as optoelectronic devices.

Comments from the authors:

  • This paper describes how we achieved programmable and directional transport of microparticles immersed in a fluid, referred to as colloidal microparticles.
  • The fluid was a nematic liquid crystal, which has anisotropic physical properties. We used different alignments of the nematic liquid crystal molecular orientation at the two opposing plates that confined the fluid – molecular orientation was parallel (planar) to the lower confining plate whilst the molecular orientation was perpendicular (homeotropic) to the upper confining plate.
  • This “hybrid” nematic alignment broke symmetry so that there were two possible hybrid aligning states, with a topological defect line between them.
  • We applied an in-plane electric field by applying an A.C. voltage V to stripe electrodes to distort the topological defect line and we used the dynamic evolution of the tortuous line to collect, trap and transport the colloidal particles.
  • We had also proscribed the alignment at the spacers around the layer so that the topological defect formed between the two possible hybrid aligning states was initially diagonal compared to the electrode direction, allowing fine control over the shape of the topological defect line.
  • The application of an A.C. voltage V triggered dynamic growth of the topological defect line and movement of the particle in a positive direction when V > Vc, or contraction of the topological defect line growth and movement of the particle in a negative direction when V < Vc; so we referred to Vc as the critical stabilising voltage. Hence the symmetry breaking in the system enabled voltage controlled microparticle movement in either a positive or negative direction within the layer and perpendicular to the resultant electric field direction.
  • We always looked at our device with the planar alignment side at the bottom. We did not observe sedimentation because our particle surfaces were treated to give perpendicular (homeotropic) molecular surface orientation. Therefore elastic levitation opposed the tendency for sedimentation.
  • There are two further observations that are not reported in the paper. The critical stabilising voltage and velocity range of domain wall and micro-particle movement can be altered by varying the thickness of the layer. However, the exact shape of the domain wall did not have a significant effect on the growth velocity.
  • Higher voltages, applied for significant amount of time, can be used to create alternating domain walls across the entire active area of the device. This will be the subject of another follow on paper.

Citation to the paper: Electrically controlled topological micro cargo transportation, A. S. Bhadwal, N. J. Mottram, A. Saxena, I. C. Sage, and C. V. Brown, Soft Matter, 16, 2961–2970 (2020). DOI: 10.1039/c9sm01956a.

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About the web writer

Dr Nacho Martin-Fabiani (@FabianiNacho) is a Vice-Chancellor’s Research Fellow at the Department of Materials, Loughborough University, UK.

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