Scholarly article on topic 'Manipulation of colloids by optical and electrical control of disclination lines in liquid crystals'

Manipulation of colloids by optical and electrical control of disclination lines in liquid crystals Academic research paper on "Chemical sciences"

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Academic research paper on topic "Manipulation of colloids by optical and electrical control of disclination lines in liquid crystals"

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Optofluid. Microfluid. Nanofluid. 2016; 3:59-62

Research Article

Open Access

D. Kasyanyuk*, P. Pagliusi, A. Mazzulla, S. Tomylko, V. Reshetnyak, Yu. Reznikov, C. Provenzano, M. Giocondo, M. Vasnetsov, O. Yaroshchuk, and G. Cipparrone

Manipulation of colloids by optical and electrical control of disclination lines in liquid crystals

DOI 10.1515/optof-2016-0008

Received November8, 2016; revised December5, 2016; accepted December 7, 2016

Abstract: We report two viable strategies to assemble and manipulate arrays of nano- and micro-particles by means of topological defects (TDs) in anisotropic fluids. Exploiting different boundary conditions, single TD, 1D arrays of TDs are tailored in liquid crystal twist cells. In a first approach, light-guided control of particles captured in discli-nation lines is demonstrated involving the use of a photosensitive chiral dopant within a nematic host. In the second one, an applied voltage enables a continuous displacement and deformation of the particles arrays. The reported results open up new possibilities for managing nano- and micro-metric objects over large distances.

Dedicated to the late of Professor Yuri Reznikov

Periodic assembling and manipulation of micro- and nanoparticles are of increasing interest in biotechnology, nanophotonics and material sciences, enabling lab-on-a-chip solutions for sensing, imaging and drug delivery [15]. In addition, tunable periodic organization of nanoparticles (NPs) allows production of active metamaterials

Corresponding Author: D. Kasyanyuk: Institute of Physics, National Academy of Sciences of Ukraine, pr. Nauky 46, Kyiv 03028, Ukraine, E-mail: deniskasyanyuk@hotmail.com P. Pagliusi, G. Cipparrone: Physics Department, University of Calabria, Ponte P. Bucci, Cubo 33B, 87036 Rende (CS), Italy and CNR-NANOTEC, LiCryL and Centre of Excellence CEMIF. CAL, Ponte P. Bucci, Cubo 33B, 87036 Rende (CS), Italy S. Tomylko, Yu. Reznikov, M. Vasnetsov, O. Yaroshchuk: Institute of Physics, National Academy of Sciences of Ukraine, pr. Nauky 46, Kyiv 03028, Ukraine

C. Provenzano: Physics Department, University of Calabria, Ponte P. Bucci, Cubo 33B, 87036 Rende (CS), Italy A. Mazzulla, M. Giocondo: CNR-NANOTEC, LiCryL and Centre of Excellence CEMIF. CAL, Ponte P. Bucci, Cubo 33B, 87036 Rende (CS), Italy

V. Reshetnyak: Taras Shevchenko National University of Kyiv, Kyiv 01601, Ukraine

which are very promising for future applications [6, 7]. At the same time, it is a fertile ground for new fundamental science at the micro- and nanoscale. Different techniques were developed to produce ordered particle structures, such as molecular imprinting technique, preparation of particles-filled polymer matrices, etc. [8-13]. However, the possibility to control the internal structure or shape of designed architectures has a great influence on collective photonic, electronic and magnetic properties of the composite materials and allows developing switchable and tunable devices [14-16]. Optical tweezers are mainly involved in trapping and manipulation of micro-particles, but the displacement range is typically restricted to the micro-scale [17,18]. The capability to assemble micro- and nanoparticles particles into 1D and 2D structures and to manipulate them at larger scales remains challenging. In this regard, liquid crystals (LCs) appear as perfect materials for assembling and manipulation of particles, because of their molecular order and mobility. Typically TDs appear in LCs regions where the stresses associated with continuous variation of the LC director orientation are relieved by local loss of nematic order and the molecular director becomes undefined [19]. They occur in the form of isolated points or disclination lines. At the same time the high sensitivity of LCs to external stimuli allows to modify the orientational order with consequent displacement of TDs over macroscopic scale. Micro- and nanopar-ticles tend to localize in TDs core due to the gradient of LC director and order parameter fields, respectively [2023]. The interaction force between the micro-particles and TDs can be roughly estimated by F = KR3 /l2 , where K is the average Frank elastic constant, R is the radius of the micro-particle and l is the distance between TD and micro-particle [20]. Concerning the nanoparticles, the mechanism of nanoparticles-TDs interaction is described only qualitatively in some works [22, 23]. Therefore, exploiting of controllable TDs in LCs looks promising for assembling and manipulation micro- and nanoparticles at large scales.

Here we report examples of trapping and manipulation of micro- and nanoparticles by means of single TD,

© 2016 D. Kasyanyuk et al., published by De Gruyter Open.

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License.

1D arrays of TDs generated in LCs, demonstrating rotation, translation and deformation of the particles assemblies over large scales (up to centimeters). We explore two feasible manipulation strategies, exploiting low intensity light or DC/AC voltage as control parameters. Orientational TDs (disclinations) with the desired configuration are reliably produced in LCs by proper design of the anchoring condition on the orienting substrates. In particular, in nematic and chiral nematic LCs a disclination line appears between two regions where inhomogeneous boundary conditions impose opposite twist domains.

In order to obtain a single TD line or arrays of linear disclinations, we use two different types of LC twist cells [24]. In the first case, the cells (type I) are manufactured from two anchoring substrates imposing unidirectional and circular planar alignments. When the nematic LC (NLC) is infiltrated between the substrates, provided opposite twist deformations of NLC are separated by the disclination line [25] (Figure 1a). In the second case, LC cells (type II) are assembled using a unidirectional planar and a planar periodic orienting substrates. A polarization hologram recording is exploited to impose the periodic planar anchoring on a polarization sensitive pho-toaligning substrate. Here, opposite twist configurations of the LC director are realized periodically, thus providing the condition for an array of disclinations (Figure 1b).

tial state, while the irradiation by light with wavelength A =400 nm transforms the chiral dopant to cis form. Conformational change of the Li-7 molecules results in variation of the helical twisting power and consequently of the LC cholesteric pitch. Following irradiation by light with wavelength A =532 nm induces reversible transformation of chiral dopant molecules to trans form.

Figure 1: Micrographs of the disclinations produced in 20 ^m thick twist cells (a) unidirectional-circular alignment and (b) unidirectional planar- planar periodic alignment. In the latter case the periodicity of the disclinations array is A =75 ^m.

In the following, we describe rotation, displacement or deformation of the disclinations lines resulting from variation of the equilibrium LC director distribution, triggered by light or an electric field. In the first approach, we use light-induced modification of LC chirality, using a not-commercial photosensitive switchable chiral dopant (ch. d.) Li-7, whose chemical structure is reported in Fig. 2. The Li-7 is a bistable azobenzene based material that can undergo a reversible trans-cis isomerisation upon irradiation with visible light. Trans form corresponds to the ini-

Figure 2: Chemical structure of light-driven chiral dopant Li-7 switched in trans form.

A mixture of nematic LC 5CB (4-pentyl-4'-cyano-biphenyl) and Li-7 (c = 0.5 wt.%) is used in our experiments. Uniform irradiation of the samples with low-power incoherent light at A = 400 nm results in continuous rotational movement of the single disclination in cells with circular alignment (type I) and translational movement of the disclinations arrays in cells with planar unidirectional-periodic alignment (type II). Rotation angles of a centimeter-long disclination can reach hundreds of degree depending on the initial concentration of the dopant, while displacements of tens of micrometers are achieved for the linear TDs arrays [24]. Following irradiation by light at A = 532 nm results in backward movement of the disclinations to the initial position in both cases. The intensity of irradiation for each wavelength is I«15 W/m2.

Particles of different sizes are used in our experiments: CdSe/ZnS fluorescent quantum dots (QDs) with diameter d«3 nm, SiO2 nanocrystals with d«6 nm and colloidal SiO2 particles with d«3 ^m. The particles are mixed with the LC/chiral dopant solution at concentrations up to 0.01 wt% for SiO2 micro- and nanoparticles and 0.007 wt% for CdSe/ZnS QDs. Localization of particles inside the LC cells is investigated by confocal fluorescence microscopy (for QDs) and bright field microscopy (for SiO2 particles).

Effective trapping of QDs as well as SiO2 colloidal particles by TDs is demonstrated. Confocal fluorescence analysis of twist cells with periodic linear alignment (type II) confirms that the CdSe/ZnS QDs are confined in narrow lines with a transversal width 1.5 ± 0.5 ^m and are uniformly distributed over the whole centimeter-long TD lines (see Fig. 3(a)). The QDs lines are located close to the middle of the LC film. In addition, optical microscopy between

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Manipulation of colloids by optical and electrical control of disclination lines 61

crossed polarizers at 100X magnification (inset of Fig. 3(a)) shows that the collection of NPs inside the disclinations does not lead to detectable modification of the LC director field, supporting the assumption that NPs are located in the disclination core (estimated size about 10 nm).

Analysis of the cells with circular alignment (type I) using bright field microscopy reveals a high concentration of SiO2 micro-objects (microparticles and their aggregates) around the TD, Fig. 3(b).

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Figure 3: (a) An array of QDs chains designed by TDs. (b) SiO2 microparticles trapped by a single TD.

We demonstrate large angle rotation and translation of QDs chains and complete reversibility of the movement with a good fatigue resistance [24]. The same effect is observed for SiO2 micro- and nanoparticles and their aggregates which transversal width can reach up to 100?m. During photoinduced motion, the disclinations hold strongly both micro- and nano-sized particles, and efficiently transport them. Furthermore, displacement (rotation or translation) of the disclinations "brushes and swipes" particles which are located in a plane of TDs or close to it. Bright field microscope images of type I cell presented on Fig. 4 demonstrates transportation of SiO2 nanoparticles aggregates after irradiation by light with A = 400 nm and dose D= 290 mJ/cm2. The rotation of the disclination line approximately on 200 deg. allows trapping of the SiO2 particles in the surrounding region, thus reducing the particle concentration in the LCs bulk. Such TD rotation allows transportation of trapped micro- and nano-objects on a few micrometers in the center of the sample up to few centimeters on a periphery.

In the second approach we use an external voltage to perform the movement of the particles-charged disclina-tions. Experiments are carried out in type II cells (i.e., planar uniform - planar periodic alignment) filled with positive (5CB) or negative (MBBA) dielectric anisotropy NLCs, both mixed with nanoparticles. The materials features enable to investigate two geometries: in the first one a DC

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Figure 4: Rotational movement of nanoparticles-rich TD line captures SiO2 nanoparticles and their aggregates dispersed in the bulk.

electric field is applied in-plane along the LC film (inset of Figure 5a), in the second one an AC electric field is applied across the LC film (inset of Figure 5c). We observe linear displacement of a TDs array filled with aggregates of SiO2 nanoparticles under in-plane DC voltage (Figure 5a and 5b). In this case twist cells were filled with 5CB and electric field was applied along the film and perpendicular to TDs lines. A DC electric field E = 0.1 V/^m induces a linear shift l «40 ^m which is approximately half of the period A (Figure 5a and 5b). Application of an AC electric field (E = 4.3 V/^m @ 1 kHz) across the cell filled with MBBA produces harmonic distortion of TDs probably due to the involvement of hydrodynamic processes in the LC matrix [26, 27] (Figure 5c). Confocal fluorescent analysis reveals that distorted TDs still hold the QDs. Increasing the electric field strength results in a decreasing of the TDs curvature radius, from Ri = 11 ^m to R2 = 2 ^m, until at the E = 4.6 V/^m TDs are destroyed. Switching off the electric field restores the initial arrangement of the particles-charged TDs.

To conclude, we demonstrate micro- and nanoparti-cles assembling and transportation on large scales exploiting topological defects in anisotropic fluids, namely liquid crystals. The shape and location of the disclinations in LCs depend on the geometry of twisted domains induced in the cell, controlled by the proper alignment configuration at the confining substrates. Some geometries have been investigated, demonstrating large rotation, translation and deformation of TDs lines filled with micro- and nanoparticles. Based on the flexibility of the material composition and on their high sensitivity to external agents, light and electric field controlled TDs enable transportation of nano-sized particles as well as large aggregates of colloidal particles with transversal width up to ~100 ^m. We demonstrate moving of particles on distances ranging from few micrometers up to several centimeters. The coupling of control parameters and the occurrence of hydrodynamics processes, pave the way towards new strategies of managing nano- and micro-objects based on anisotropic fluids.

Figure 5: (a), (b) Translational movement of SiO2 nanoparticles aggregates induced by lateral DC electric field induced. (c) Deformation of TDs array induced by AC electric field perpendicular to LC film. (d) Corresponding behavior of QDs chains in AC field. Geometries of both experiments are presented on (e), (f) respectively. Electric field is generated between indium tin oxide electrodes marked on the sketches as blue layer.

Acknowledgement: COST Action MP1205 for financial support of Short Term Scientific Missions.

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