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Nucleation of phase-orientation domains in pressure-induced nematic microflows. (A) Schematic illustration of a channel with homeotropic anchorage on the upper and lower surfaces used in the experiment; IR, infrared; ITO, indium tin oxide. (B) The nematic in a channel appears black between crossed polarizers in the absence of flux and acquires a visible birefringence due to a directional distortion driven by the flux that traps a domain of the state flush-aligned (also called dowsing state from now on) n designates the nematic director. The light strongly absorbed by the laser tweezers heats the NLC, creating an isotropic island (Iso) that cools in the nematic phase (N) when the laser is turned off. The dense entanglement of defects grows into a single defect loop that traps a stream-aligned dowsing state, identifiable as a low velocity green zone. (C) Laser-induced nucleation of dowsing domains can be automated and their shape can be dynamically controlled by adjusting flow parameters. The double crossed arrows indicate the orientation of the polarizers. The empty white arrows in the lower left corners indicate the direction and qualitative velocity of the flow in the paper. Credit: Science Advances, doi: 10.1126 / sciadv.aav4283
Oscillating flux and light pulses can be used to create a reconfigurable liquid crystal architecture. Material scientists can carefully design collaborative microfluidic flows and localized optothermic fields to control the nucleation, growth, and shape of such liquid domains. In comparison, pure liquids in thermodynamic equilibrium are structurally homogeneous. Experimental work based on theory and simulations have shown that if liquids are maintained in a state of controlled non-equilibrium, the resulting structures can be stabilized indefinitely.
Sculpted liquids can find applications in microfluidic devices to selectively encapsulate solutes and particles in optically active compartments to interact with external stimuli for various medical, medical and industrial applications. In a recent study published in Progress of scienceTadej Emeršič and collaborators in Slovenia and the United States have developed pure nematic liquid crystals (CLN), in which they dynamically manipulate defects and reconfigurable states of materials by the simultaneous application of multiple external fields.
Solid materials can simultaneously exhibit distinct structural phases, a property that can be manipulated to design features. However, in pure equilibrium liquids, such structural phases corresponding to grain boundaries and defects do not occur. Although liquids have a number of attractive features, including their ability to wet surfaces, high diffusion coefficients, and absolute compliance, it is difficult to include additional features in liquids because of their inherent homogeneity. Complex behavior is observed in multicomponent synthetic and biological mixtures and the resulting structures are difficult to manipulate because they occur in out-of-equilibrium situations. Such situations generally involve several components with clear miscibility and gradients between the hydrophilic and hydrophobic domains.
Expansion and contraction of laser nucleate domains in a moderate nematic microflow. The life of the domain is proportional to the critical speed and initial size. Recorded under cross polarizers at 30 fps, the field of view is 480 μm × 120 μm. Credit: Science Advances, doi: 10.1126 / sciadv.aav4283
Scientists have developed active material in the form of live colonies and bioinspired synthetic homologs. They printed hydrophobic / hydrophilic domains on liquid mixtures by relying on surfactant nanoparticles and off-equilibrium controlled systems to demonstrate the motion and transition between different rheological regimes. Liquid crystals (LC) are an ideal system for studying phenomena of interest, such as spontaneous symmetry breaks, topological defects, orientation orders, and phase transitions based on external stimuli.
Nematic liquid crystals (CLNs) are the simplest form of liquid crystal molecules without ordered positions and differ from pure liquids in molecular orientation. NLCs have a range of properties that allow them to serve as microreactors and conduct inherent polymerization reactions for intriguing future applications. Ongoing fieldwork is still experimental, for example, nematic fluxes in microfluidic environments, which highlight possible interference between topological defects in different fields of velocity and molecular orientation.
In this work, scientists observed for the first time the phase interface with NLCs, performed experimentally by generating controlled polar phase domains by practically combining microfluidic confinement, fluid flow rates and laser pulses. . Emeršič et al. used pentyl-cyanobiphenyl (5CB), a one-component nematic material, in all experiments carried out in linear microfluidic channels of rectangular cross-section. Scientists fabricated the channels with glass substrates coated with polydimethylsiloxane (PDMS) and tin-indium oxide (ITO) using standard soft lithography procedures. They then filled the microfluidic channels with 5 CB in its heated isotropic phase and allowed it to cool down to the nematic phase, before starting the flow experiments. Scientists also chemically treated the microchannel walls to develop a powerful homeotropic surface for anchoring 5CB molecules.
To cultivate and reduce dowsing domains in digitally simulated nematic microflows. Simulation of a laser-induced fault loop in an expanding or narrowing channel, subjected to a flow controlled by low or high pressure. Top: Top view of the canal showing the defective loop. Bottom: side view showing the evolution of the dowser's structure. The elastic constants of 5 CB are adopted in the calculation. Credit: Science Advances, doi: 10.1126 / sciadv.aav4283
The work represented an ideal experimental model of a material phase of quasi-two-dimensional (2D) orientation. In the initial stationary state in a microfluidic channel, the heated material appeared black. When the flux was activated, depending on the flow velocity, the birefringent appearance changed from black to glossy. The flow-aligned domains evolved in this way to grow or annihilate with the flow velocity.
Material scientists have termed the flow regime "bowser state" because of its curved profile and the aligned state of the flow as a result. "state of sourcier" because of its analogy with the so-called "sourcier" field in nematostatics, where the charge is nematostatic. density of elastic nematic materials, similar to electrostatic. The state of the dowser has an anisotropic orientation with its own elastic behavior, topological defects and solitons (a solitary wave pack that retains its shape while propagating at a constant speed). In comparison, the bowser state is actually isotropic and simple in the simplified 2D view. Scientists were able to control the shape, division, and coalescence of these phase domains.
Emeršič et al. conducted all experiments at room temperature, controlling and controlling the flow of fluid in the microchannel with the aid of a pressure-controlled microfluidic flow control system. They studied flow regimes, reorientation dynamics, and 5CB deformations induced by flow in microchannels using polarized light microscopy. Scientists built laser forceps around the inverted light microscope with an IR fiber laser running at 1064 nm as a light source and a pair of acoustic-optical wind deflectors controlled by a computerized system to accurately manipulate the beam.
Produce a steady stream of dowsing fields by cutting out the state of bulk dowsing with a moving laser dot. By moving a laser-heated nematic isotropic island transversely across the phase boundary between the flange and looper (black) states, it is possible to produce a uniform stream of coil domains. Recorded under cross polarizers at 30 fps, the field of view is 480 μm × 120 μm. Credit: Science Advances, doi: 10.1126 / sciadv.aav4283
In the study, the state of the stream-aligned dowser was stable under strong flows but unstable under a weak flow. Depending on the flow velocity, dowsing domains could grow and shrink in experiments, as shown by numerical simulations. Scientists calculated the criteria for growth and reduction of domains over time and indicated how these domains grew, shrank or were annihilated along the canal.
By carefully applying the tweezers to the laser, scientists have shown that a continuous stream of domains can be produced by dissecting the original mass sourcer with a moving laser point, where the laser melts the sides of the phase boundary of the material. . A growing domain at a higher flow rate could thus be divided longitudinally in two, with a static laser beam at low light intensities.
The laser forceps allowed dynamic control of the size, number, and life of the generated dowsing domains, which were then manipulated by modulating the periodic flow rate. For example, under a uniform flow, the field of the dowser aligns uniformly in the direction of flow to grow or contract, depending on the rate of velocity. Scientists were able to actively adjust and control the flux as a constant size domain that can be stably maintained for more than ten seconds.
Systematic remodeling of the areas of the dowser under the action of the laser and oscillatory flows. (A) Moving the laser beam transversely across the bulky dowsing eliminates a uniform "train" of the domains. (B) A static beam at a low power of 80 mW generates a small isotropic region that cuts a large area of souring longitudinally by half. (C) The shape and size of the domain can be conserved over long time and length scales by periodically modulating the driving pressure around the value that induces the desired average flow rate. (D) Under an alternative flow, a dowsing domain changes orientation whenever the direction of flow is changed. Reorientation creates spot defects on the surface and realigned edges, visible under the microscope as a rapid color change. The energetically unfavorable "old" orientation narrows into a narrow soliton of 2π and pinches the domain boundary (black arrows). (E) A sufficiently fast flow inversion creates pairs of point faults connected by solitons. When the flow is off, the characteristic length goes to infinity and the solitons develop, thus revealing their signature profile in transmitted light intensity (encrusted). In a slow residual flow, parts aligned with the flow contract more slowly than parts with an unfavorable orientation. Scale bars, 20 μm. Credit: Science Advances, doi: 10.1126 / sciadv.aav4283
Furthermore, in the model developed by Emeršič et al., They showed how the direction of flow could be reversed for the dowser domain, which led to a rapid reversal of the orientation relative to the direction of the dowser. previous equilibrium state. In addition, the dowser field could be coupled to external magnetic and electrical fields and channel thickness gradients to determine the control, flow direction and optical adjustment of the 5CB nematic material. Scientists clearly observed the direct response to external stimuli by birefringence during the study and determined that it was an appropriate method to measure the viscoelastic and rheological properties of the material .
Emeršič et al. consider the possibility of conducting chemical reactions in such confined volumes, as previously shown with liquid crystal models. In addition to this, based on the principles stated by Emeršič and his collaborators, a 3D printing system can be designed to contain liquids, within which complex and out of equilibrium structures can be created and stabilized . The experimental models developed in this study using standard thermotropic LCs are also transferable to active and biological materials with nematic behavior. The proposed and demonstrated method is a technical tool in materials science, with potential applications in biophysics, chemistry and chemical engineering.
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More information:
Tadej Emeršič et al. Sculpting stable structures in pure liquids, Progress of science (2019). DOI: 10.1126 / sciadv.aav4283
Xiaoguang Wang et al. Topological defects in liquid crystals as models for molecular self-assembly, Nature Materials (2015). DOI: 10.1038 / nmat4421
Anupam Sengupta et al. Microfluidic liquid crystal for adjustable flux shaping, Letters of physical examination (2013). DOI: 10.1103 / PhysRevLett.110.048303
Gareth P. Alexander et al. Editor's note: Colloquium: disclosure loops, point defects and all this in the nematic liquid crystals [Rev. Mod. Phys.RMPHAT0034-686184, 497 (2012)], Reviews of Modern Physics (2012). DOI: 10.1103 / RevModPhys.84.1229
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