Controlled Placement of Microparticles at the WaterâLiquid Crystal

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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 15007−15013

Controlled Placement of Microparticles at the Water−Liquid Crystal Elastomer Interface Greta Babakhanova,†,‡ Hao Yu,†,‡ Irakli Chaganava,§,∥ Qi-Huo Wei,†,‡,⊥ Paul Shiller,# and Oleg D. Lavrentovich*,†,‡,⊥

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†

Advanced Materials and Liquid Crystal Institute, ‡Chemical Physics Interdisciplinary Program, and ⊥Department of Physics, Kent State University, Kent 44242, United States § Institute of Cybernetics of Georgian Technical University, Tbilisi 0186, Georgia ∥ Georgian State Teaching University of Physical Education and Sport, Tbilisi 0162, Georgia # Civil Engineering-Timken Engineered Surface Laboratory, The University of Akron, Akron 44325, United States

ABSTRACT: Controlled placement of microparticles is of prime importance in production of microscale superstructures. In this work, we demonstrate the remote control of microparticle placement using a photoactivated surface profile of a liquid crystal elastomer (LCE) coating. We employ light-responsive LCEs with preimposed patterns of molecular orientation (director) in the plane of coating. Upon UV illumination, these in-plane director distortions translate into deterministic topographic change of the LCE coating. Microparticles placed at the interface between the LCE coating and water, guided by gravity, gather at the bottom of photoinduced troughs. The effect is reversible: when the substrates are irradiated with visible light, the coatings become flat and the microparticle arrays disorganize again. The proposed noncontact manipulation of particles by photoactivated LCEs may be useful in development of drug delivery or tissue engineering applications. KEYWORDS: liquid crystal elastomer photoresponsive coatings, disclinations, self-assembly

1. INTRODUCTION Placing microparticles in predetermined locations is a challenging problem of microtechnology.1−4 Many current approaches are based on heterogeneous patterning of substrates.5 One of the issues is the strength of binding forces and reversibility of particle placement. In this work, we propose an approach that offers reversible light-controlled predesigned placement of microparticles. The technique is based on photoresponsive liquid crystal elastomer (LCE) coatings with predesigned molecular orientation. Ultraviolet (UV) (λ = 365 nm) photoactivation of the LCE translates inplane patterns of molecular orientation into variable surface topography.6−14 When such a photoactivated LCE elastomer coating with spatially varying profile serves as a substrate in contact with a water dispersion of microparticles, the particles are guided by gravity toward preprogrammed locations with the lowest potential energy. When UV irradiation is terminated, the LCE recovers its flat profile and the positions of particles are randomized. We consider different geometries © 2019 American Chemical Society

of LCE patterning and discuss the advantages of reversibility of the proposed technique.

2. MATERIALS AND METHODS 2.1. Preparation of the Liquid Crystal Cells. Indium tin oxidecoated glass substrates were cleaned in an ultrasonic bath and subsequently rinsed with isopropyl alcohol. The glass substrates were then dried in an oven at 90 °C to evaporate the solvent. It is very important for the surface of the glass substrate to be free from organic residuals for better spreading of the polyimide layer which was used to achieve planar anchoring. Thus, the substrates are treated with UV− ozone for 5 min to remove organic residuals and increase the hydrophilicity of the glass for better spreading of the polyimide layer.15 The uniform planar alignment layer was achieved by spin coating the polyimide layer PI2555 (HD Microsystems) onto a cleaned glass and baked in an oven at 275 °C for 1 h, after which the Received: December 19, 2018 Accepted: March 26, 2019 Published: March 26, 2019 15007

DOI: 10.1021/acsami.8b22023 ACS Appl. Mater. Interfaces 2019, 11, 15007−15013

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ACS Applied Materials & Interfaces

Figure 1. Chemical structures of LC monomers (a) RM82, (b) RM23, (c) RM105, (d) azo dye, A3MA; (e) photoinitiator Irgacure 819; and (f) azobenzene dye, SD1.

Figure 2. Schematic representation of assembled cells consisting of (a) identical linear “V-stripes” at the top and bottom glass boundaries, (b) linear photopatterned “V-stripes” and unidirectional planar substrates, and (c) circular “C-stripes” and unidirectional planar glass substrates. Scale bar 40 μm. substrates were rubbed 10 times with a velvet cloth in a unidirectional fashion. The photosensitive alignment layer was deposited by spin coating a solution of the photosensitive azo-dye SD1 in N,Ndimethylformamide at 0.5 wt % concentration onto the glass, and the substrates were baked in a hot oven at 95 °C for 30 min. The SD1-coated glass substrates were used to prepattern a spatially varying liquid crystal (LC) director field using plasmonic metamask (PMM) composed of 200 × 100 nm nanoaperture arrays in the Al film.16,17 The PMM was exposed to an unpolarized metal halide light source with emission spectrum λ = 300−750 nm (EXFO X-Cite series 120). After passing through each nanoaperture in the PMM, the outgoing beam of light becomes linearly polarized. The locally polarized light incident onto the SD1 coating causes the azo dye molecules to reorient their long axes perpendicularly to the polarization of light.18 Azo dyes such as SD1, Figure 1f, absorb UV light strongly when their transition dipole is parallel to the polarization of light, experiencing trans-to-cis-isomerization of the azobenzene moieties.18 During isomerization and relaxation back to the trans-state, orientation of the molecule changes. A sequence of trans-to-cis and cis-to-trans transformations eventually brings the molecules to the state with the transition dipole perpendicular to the local polarization of light, making further changes in orientation highly unlikely.19−22 The orientational pattern of nanoapertures in the PMM produces the desired pattern of azo dye orientation that is followed by the LC molecules that are in direct contact with the azo dye molecules at the bounding plates.16

The LC mixture (Figure 1) was injected into the assembled sandwich-type cells (Figure 2a) via capillary action to create disclination-free LCEs, whereas the assembly in Figure 2b,c was used to create LCEs with periodic disclinations. The cell spacing was controlled using micropearl microspacers with a diameter (D) of ∼8 μm (Sekisui Chemical Co. Ltd.) premixed with NOA71 UV glue. 2.2. Preparation of the Light-Responsive LCE Coatings. The composition of the photosensitive mixture presented in Figure 1 included monomers RM23 (24.8 wt %), RM105 (46.9 wt %), and RM82 (23.9 wt %), which were homogeneously mixed with a photoinitiator Irgacure 819 (0.9 wt %) (Merck) and azo dye A3MA (3.5 wt %) (Synchom) in a dichloromethane solvent (Sigma-Aldrich). The monomeric mixture was injected in the cell in an isotropic phase (T = 80 °C) using capillary action, after which the system was brought to 25 °C using Linkam PE 94 temperature controller and the LTS 150 hot stage with precision of 0.01 °C (Linkam Scientific Instruments, Ltd.). The aligned monomeric mixture was photopolymerized using a metal halide unpolarized light source (EXFO XCite series 120) with intensity of 18.3 mW/cm2 for 10 min at T = 25 °C. Additionally, a 400 nm high-pass filter was used to avoid exposure to the UV light to prevent premature isomerization of the photoresponsive A3MA molecules.23 After curing, one substrate was removed leaving a light-responsive LCE coating on the remaining substrate; a force was applied using a razor blade, which was positioned in between two glass plates in one corner (far away from the photopatterned area). Careful separation of the two plates leaves an LCE coating, especially the photopatterned area, intact. 15008


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Figure 3. LS system. (a) 3D visualization of an actuated LCE with LS pattern showing alternating linear hills and valleys via a Zygo optical profilometer, (b) light microscopy image showing the random distribution of microparticles at the initial deposition on an LCE surface; scale bar 100 μm, (c) histogram showing the 80 μm periodicity of the microparticle separation after UV light illumination [the image in panel (d) was used to generate the histogram], (d) light microscopy image showing linear chains of assembled microparticles at the disclination sites; scale bar 100 μm, (e) 2D image of a dry, initial LCE surface before UV photoirradiation visualized via a digital holographic microscope (DHM); scale bar 50 μm, and (f) height profiles of line VV′ shown in panel (e) before and after UV light exposure obtained via DHM.

3. RESULTS AND DISCUSSION

n̂ = (|cos πy/p| , sin πy/p , 0)

We use a special class of LC molecules with polymerizable acrylate groups (Figure 1a−c). When using such reactive LC mesogens, the LC molecular order can be fixed via UV photopolymerization.24−27 Anisotropic properties such as orientational order parameter are coupled to the rubber-like elasticity of the LCE coatings. Such coupling presents an opportunity to control the elastomer deformations in predesigned locations.26,28,29 In particular, in-plane patterns of molecular orientation specified by the so-called director, n̂, control the local thickness of the LCE coatings activated by temperature.7,17 In this work, we employ photoactivation of the LCE coatings. The LCE coatings were created using reactive mesogens (Figure 1a−c), mixed with a photoresponsive azobenzene monomer, A3MA (Figure 1d), and a photoinitiator Irgacure 819 (Figure 1e). The director patterning of the LCE was achieved by the technique of PMM photoalignment, as described in Section 2.1.16 The mixture of monomers was confined between two glass plates with a predesigned surface alignment pattern (Figure 2). We used three different patterns of the director at the bounding plates. The first is a uniform director aligned along a single direction in the xy-plane of the cell, which we call the y-axis, n̂ = (nx, ny, nz) = (0, 1, 0). The second represents a one-dimensional periodic system of stripes with V-shaped director field, written as

(1)

where p is half the period. The third represents circular stripes with the director resembling a letter “C” and written as n̂ = (cos πr /p , sin πr /p , 0)

(2)

where r is the distance to the center of the pattern. After the monomeric mixture is filled in the space between the two glass plates, the surface-anchoring forces control the bulk director of the monomer mixture. The bulk director structure depends on the type of bounding plates used. If both plates are predesigned as V-stripes in phase with each other, then the bulk director forms a periodic linear nonsingular (LN) splay-bend structure, Figure 2a. When the cell comprises one plate treated uniformly and the second plate with Vstripes, then a linear singular (LS) system forms, with LS disclinations of strength 1/2 running along the x-axis in the bulk of the sample. The disclinations form along the lines where the director orientations at two plates are perpendicular to each other and separate the right- and left-handed twist domains of the director.30−32 In a similar fashion, a combination of circular stripes and a uniform plate, shown in Figure 2c, produces a system of concentric singular disclinations, which we call a circular singular (CS) pattern. After the cells were filled with the monomeric mixtures and adopted director fields specified by the boundary conditions, the material was polymerized using a metal halide lamp with λ = 300−750 nm emission spectrum to form a flat ∼8 μm thick 15009


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ACS Applied Materials & Interfaces LCE slab. During photopolymerization of the monomers, a high pass filter was placed after the light source to avoid the premature photoisomerization of A3MA photoresponsive molecules.23 At this stage, the LCE elastomer slab is of a uniform thickness, which is defined by the gap thickness between the two plates. Finally, one of the bounding plates was removed to produce an LCE coating. The bottom surface of the LCE coating is affixed to the glass substrate. A water dispersion of microparticles is deposited onto the free top surface of the coating. The LCE coating remains almost flat until it is irradiated with UV light at λ = 365 nm, Figure 3f. Irradiation results in the trans−cis isomerization of the A3MA azo dye, which reduces the scalar order parameter of the LCE and triggers activation forces that lead to expansion/shrinkage of the coating, as described in ref 17. Previously, we demonstrated that upon decreasing the orientational order, splay deformation of the director causes the coating to develop valleys, and bend develops protrusions.17 The activation force, f, that links the director field gradients to the topographical changes of the surface profile17 f = α(n̂ div n̂ − n̂ × curln̂ )

(3)

where α is an activation parameter, increases as the director gradients become stronger. The activation forces cause transport of the material, which produces surface deformations of the coating that are determined by the prescribed director pattern (Figures 4a, 5a, 6a). As the experiments reveal, these surface topography changes are sufficiently strong to control placement of microparticles, Figures 3−6. Deposition of spherical resin microparticles (Sekisui Chemical Co. Ltd.) of a radius R = 5 μm dispersed in water onto a flat LCE coating yields a random distribution (Figures 3b and 6a). Water prevents sticking of the microspheres to the dry LCE coating. Upon UV irradiation at λ = 365 nm, locations with y = 80, 160, 240 μm (Figure 2a), at which the director deformations are predominantly of the splay type, develop valleys (Figures 3a, 4a, and 5a).17 As a result, the microparticles move toward the valleys, driven by gravity (Figures 3d, 4b, 5b, and 6b). Figure 7 illustrates the rate of surface deformations as a function of UV light photoirradiation time. Under UV light irradiation, the dynamic changes of the surface profile start to saturate after about ∼10 s. The microparticles organize at the valleys after about 50−60 s since the start of irradiation. The assembly/disassembly remains reversible after at least 10 cycles of UV/blue light irradiation, Figure 8. The temperature of an LCE coating after irradiating the sample with UV light for 20 min measured using a thermocouple remained constant at 25 °C, within 1 °C. A small temperature increase is not expected to alter the photoinduced profiles, as we did an experiment in which the sample was heated from 25 to 30 °C and the surface profile remained unchanged, with profile variations on the order of 1−2 nm or less. The microparticles form chain-like assemblies, spatially separated by 80 μm period, matching the period of the inscribed photopattern (Figures 3c,d, 4b,c, 5b,c, and 6b). Note here that the final distribution of the microparticles is affected by their random initial distribution in water. Additionally, the light scattering due to surface roughness, microparticles, and water might cause slight uneven distribution of the light intensity that may be responsible for slight variations of surface topographies, visible, for example, in Figure 5a.

Figure 4. CS system. (a) 3D visualization of an actuated LCE showing circular surface topography via DHM, (b) light microscopy image showing the microparticles assembling at the disclination sites after UV irradiation, and (c) histograms showing periodic 80 μm separation of microparticles upon UV light irradiation [the image in panel (b) was used to generate the histogram]. The number of particles increases with distance, R, as the radius and length of each trough increase with the distance. Scale bar 50 μm.

The disclinations located in splay regions of the LS and CS coatings are clearly visible under light microscope (Figures 3b,d, 4b, 6, and 8). Upon UV illumination at λ = 365 nm, these regions develop periodic valleys (Figures 3a and 4a) that collect microparticles (Figures 3d, 4b, and 6b). The histograms in Figures 3c and 4c show 80 μm spatial separation of the microparticles matching the periodicity of the photopatterned director field. The disclinations should yield sharper tilts of the LCE coatings, as suggested by eq 3 because the director gradients near the disclinations are stronger than those in a smoothly deformed director. The experiments confirm this expectation: the amplitude of the protrusions in Figure 3a is ∼20% greater than that in Figure 5a. Thus, the disclinations provide a deeper potential minimum for the assemblies of microparticles. Microparticle assembly at the troughs is guided by gravity. In our system, the component of the gravitational force acting on microparticles, Fg = (ρparticleVparticle − ρwaterVwater)g sin ψ, is in the range of (2−6) × 10−14 N, where ρparticle = 1190 kg m−3 and Vparticle = 4πR3/3 are the density and the volume of the 15010


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surface topography as ψ ≈ h/d, where h ≈ 130 nm is the average amplitude of deformations of the coating’s profile, and d = (10−40) μm is the distance between the minima and maxima of the profile. The driving force, Fg overcomes the friction, f = (ρparticleVparticle − ρwaterVwater)μrg cos ψ force, where μr is the coefficient of rolling friction. A typical value of rolling friction is μr ≈ 10−5.33 Because ψ > μr, the microparticles migrate toward the troughs. The difference in potential energy of the microspheres between the elevated regions and valleys, calculated as ΔUg = Vparticle(ρparticle − ρwater)gh ≈ 1.3 × 10−19 J, is significantly higher than the thermal energy, kBT = 4.1 × 10−21 J, thus providing a robust trapping at the troughs. Obviously, as the particles become smaller, the trapping becomes weaker; for radii smaller than about ∼1.5 μm, the Brownian motion could even detach the particles from the substrate. Applicability of the method to control positions of larger particles is limited by the period of the pattern. Another factor that might limit the approach is sticking of the particles to the substrate. Note here that the initial state of the coating shows nanoscale profile variations of amplitude about hnano ≤ 10 nm and an angle of about 10−4 rad, Figure 3f. Although these tilts are still larger than μr, they do not cause any visible repositioning of the microparticles, apparently, because of a very small potential energy, ΔUghnano/h ≈ 10−20 J and finite adhesion of the particles to the substrate. The discussion above considers only the gravitational force, rolling friction, and thermal fluctuations. A more realistic model should also account for substrate−particle anchoring or absence of thereof, hydrophobic and hydrophilic forces, and so forth. These interactions could be potentially used for further control of particles assembly; the corresponding studies are planned for future work. Note here also that similar manipulation of microparticles through a dynamic surface of LCEs can be achieved by means other than photoirradiation, such as temperature17 or electric field.34 The light-controlled mechanism demonstrated in our work has an advantage of remote application and also of potentially local character, as different regions of the substrate can be irradiated with different light beams. Optical control is also often less harmful to living matter as compared to heat and electric field.

Figure 5. NS system. (a) 3D visualization of an actuated LCE showing linear, periodic hills and valleys via DHM, (b) linear Vstriped prepatterned LCE showing microparticles forming chain-like assemblies in the splay regions of the LCE coating, where valleys are formed upon UV irradiation, and (c) histogram showing spatial distribution of the microparticles upon UV light irradiation exhibiting the 80 μm periodicity [the image in panel (b) was used to generate the histogram]. Scale bar 100 μm.

4. CONCLUSIONS To conclude, we demonstrated that light-responsive LCE coatings can be used for a controlled placement of microparticles. Contact-free UV irradiation creates a nonflat surface topography that is preprogrammed through in-plane molecular alignment of the LC during the preparation of coatings. Splay deformations of molecular alignment result in depressions of the topography which collect the microparticles, thanks to gravity. Assemblies of microparticles are reversible, as the elastomer coatings restore their initial profile under visible light

microsphere, respectively, ρwater = 997 kg m−3 is the density of water at 25 °C, g = 9.8 m s−2 is the gravitational acceleration, ψLN ≈ 0.0035 rad and ψCS ≈ 0.01 rad are the slope angles of the LN and CS coatings, respectively, measured from the

Figure 6. CS system. (a) initial deposition of microparticles dispersed in water onto a flat LCE surface showing random distribution, (b) microparticles aggregated at the disclination sites, where the valleys were formed upon UV illumination for 3 min, and (c) blue light illumination for 3 min yielded random distribution of microparticles, thus, showing the reversibility of the system. Scale bars 50 μm. 15011


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Figure 7. CS system. (a) 2D visualization of an LCE coating during UV light irradiation showing three disclinations via DHM; scale bar 50 μm; regions (b) #1 and (c) #2 indicated in panel (a) show the rate of an LCE surface deformation while irradiating it with UV light.

Figure 8. Bright field microscopy observations of the photoinduced assembly/disassembly of microparticles dispersed in water showing the reversibility and reproducibility of the CS system. Scale bars 50 μm.

migration, tissue formation, and in designing targeted/sitespecific drug delivery systems.11,35−37

illumination. We demonstrated both linear and circular assemblies of the particles driven by linear defects-disclinations and by disclination-free coatings. In both cases, the microparticles migrate to regions of the lowest potential energy. LCE coatings of the LS/CS type containing disclinations with stronger gradients of the molecular alignment produce stronger topography variations and valleys that are almost three times steeper than those produced in the disclination-free LN coatings. Thus, they may serve as better traps. More complex director field patterns could be used for desired particle assembly patterns. For example, periodic lattice of point defects in molecular alignment presented in ref 17 can be used to create disk- and aster-like clusters of microparticles. Different periodicities of the director patterns presented in this work may also allow one to trap particles of different sizes because stronger gradients due to smaller periodicities yield steeper valleys and, thus, may attract particles with smaller diameter. It would be of interest to explore whether the proposed noncontact photocontrol of surface topography can be expanded to biological applications to control cell growth,

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Greta Babakhanova: 0000-0002-5027-5673 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

The work was funded by the Office of Sciences, DOE, grant DE-SC0019105. Notes

The authors declare no competing financial interest. 15012


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ACKNOWLEDGMENTS The authors thank Dr. Albert P. H. J. Schenning and Dr. Dirk J. Broer for providing the LC monomers; Dr. Frank Liu and Lyncée Tec for providing the DHM for characterization of the LCE coatings; Dr. Barbara Tury for her help with the Zygo profilometer, and Dr. Jagat Budhathoki and Taras Turiv for helpful discussions.

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ABBREVIATIONS LCE, liquid crystal elastomer UV, ultraviolet LC, liquid crystal LN, linear nonsingular LS, linear singular CS, circular singular PMM, plasmonic metamask

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REFERENCES

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Controlled Placement of Microparticles at the WaterâLiquid Crystal

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Controlled Placement of Microparticles at the WaterâLiquid Crystal