Sculpting stable structures in pure liquids

Abstract

Pure liquids in thermodynamic equilibrium are structurally homogeneous. In liquid crystals, flow and light pulses are used to create reconfigurable domains with polar order. Moreover, through careful engineering of concerted microfluidic flows and localized optothermal fields, it is possible to achieve complete control over the nucleation, growth, and shape of such domains. Experiments, theory, and simulations indicate that the resulting structures can be stabilized indefinitely, provided the liquids are maintained in a controlled nonequilibrium state. The resulting sculpted liquids could find applications in microfluidic devices for selective encapsulation of solutes and particles into optically active compartments that interact with external stimuli.

Fig. 1. Nucleation of orientational phase domains in pressure-driven nematic microflows.

(A) Schematic illustration of a channel with homeotropic anchoring on the top and bottom surfaces used in the experiment (see Materials and Methods). IR, infrared; ITO, indium tin oxide. (B) The nematic in a channel looks black between crossed polarizers in the absence of flow and gains visible birefringence due to flow-driven director distortion that traps a domain of the flow-aligned state (also called the dowser state from here on); n denotes the nematic director. Strongly absorbed light of the laser tweezers heats the NLC, creating an isotropic (Iso) island that is quenched into the nematic (N) phase when the laser is switched off. The dense tangle of defects coarsens into a single defect loop that traps a flow-aligned dowser state, identifiable as a green area at low velocity. (C) The laser-induced nucleation of dowser domains can be automated and their shape can be dynamically controlled by tuning the flow parameters. Crossed double arrows indicate the orientation of the polarizers. White empty arrows in the bottom left corners indicate direction and qualitative velocity of the flow throughout the paper. Scale bars, 20 μm.

Fig. 2. Dynamic evolution of dowser field domains in stationary nematic microflows.

(A to D) Growing and shrinking flow-aligned dowser domains in experiments and numerical simulations, captured at two different flow velocities. In (B), one can observe the varying profile of the half-integer disclination loop in the xz plane, which serves as a phase boundary and stabilizes the dowser domain. Empty white arrows indicate the qualitative magnitude and direction of flow. (E) Loop lifetime, determined from numerical simulations in the shrinking regime (D). The lifetime diverges at a certain critical pressure gradient that is proportional to the critical velocity. Note that the scale in simulations is orders of magnitude smaller than that in experiments. (F) Time dependence of the loop radius for different values of flow velocity. For shrinking loops, a theoretical model (Eq. 3) is fitted to the data points. The fitting function is shown by the bold lines. The theoretically predicted growth does not apply to growing loops, as their growth is confined by the channel walls. (G) Critical velocity extracted from the fit parameter 1/r_c, obtained for loop annihilation at different velocities. A linear fit is used to determine the critical velocity at (56.4 ± 1.4) μm/s. (H) Phase diagram for shrinking (blue) and growing (orange) loops, separated by the curve for r_c as obtained from the fit in (G). Some shrinking loop data points lie above the critical curve, due to loops that are still in the transition process after the quench and were thus omitted from the fit in (F). Scale bars, 20 μm.

Fig. 3. Systematic reshaping of dowser domains under laser action and oscillatory flows.

(A) Moving the laser beam transversely across the bulk dowser pinches off 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 dowser domain longitudinally in half. (C) The shape and size of the domain can be maintained 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 alternating flow, a dowser domain reverses orientation every time the flow direction is changed. The reorientation creates surface point defects and realigning fronts, visible under the microscope as a rapid color change. The energetically unfavorable “old” orientation shrinks into a narrow 2π soliton and pinches the domain boundary (black arrows). (E) Sufficiently rapid flow reversal creates point defect pairs connected by solitons. With the flow turned off, the characteristic length goes to infinity, and the solitons expand, revealing their signature profile in transmitted light intensity (inset). In a slow residual flow, flow-aligned parts shrink more slowly than parts with unfavorable orientation. Scale bars, 20 μm.