Control of liquid crystals combining surface acoustic waves, nematic flows, and microfluidic confinement.

The optical properties of liquid crystals serve as the basis for display, diagnostic, and sensing technologies. Such properties are generally controlled by relying on electric fields. In this work, we investigate the effects of microfluidic flows and acoustic fields on the molecular orientation and the corresponding optical response of nematic liquid crystals. Several previously unknown structures are identified, which are rationalized in terms of a state diagram as a function of the strengths of the flow and the acoustic field. The new structures are interpreted by relying on calculations with a free energy functional expressed in terms of the tensorial order parameter, using continuum theory simulations in the Landau-de Gennes framework. Taken together, the findings presented here offer promise for the development of new systems based on combinations of sound, flow, and confinement.

Structure, rheology, and microrheology of wormlike micelles made of PB-PEO diblock copolymers.

A diblock copolymer made of poly(1,4-butadiene)-block-polyethylene oxide, with a degree of polymerization of the polybutadiene and polyethylene oxide blocks of 37 and 57, respectively, self-assembles in water as worm-like micelles determined by small angle neutron scattering with an average diameter of ∼12.7 nm, a core radius of ∼2.7 nm, a shell radius of ∼3 nm, and an estimated persistence length of >225 nm. Worm-like micelles of almost the same diblock copolymer, but with a smaller polyethylene oxide block (degree of polymerization 45) were also measured. The worm-like micelles were also observed with negative staining using low energy electron microscopy. The boundary between dilute and semidilute regimes was estimated to be ∼0.8 wt%. The viscoelastic spectra at low and intermediate frequencies do not follow the Maxwell model. These micelles do not present the same rheological behavior of worm-like micelle solutions of conventional surfactants. The slow dynamics of the self-assembly explains this uncommon behavior for the system. Any micellar rearrangement is impeded due to the extremely high hydrophobicity of the polybutadiene block; stress mainly relaxes by the reptation mechanism. Using diffusive wave spectroscopy, we measured the mean square displacement of particles in the micellar solution. From the mean square displacement, we obtained G'(ω) and G''(ω) at high frequencies. |G*| exhibits a power law behavior showing the stress relaxation changes as frequency increases, first dominated by the Rouse-Zimm modes and then by the bending modes of the Kuhn segments. This allowed us to estimate the worm-like micelle persistence lengths that depend on the copolymer concentration.

Elastic Instability of Cubic Blue Phase Nano Crystals in Curved Shells.

Many crystallization processes, including biomineralization and ice-freezing, occur in small and curved volumes, where surface curvature can strain the crystal, leading to unusual configurations and defect formation. The role of curvature on crystallization, however, remains poorly understood. Here, we study the crystallization of blue phase (BP) liquid crystals under curved confinement, which provides insights into the mechanism by which BPs reconfigure their three-dimensional lattice structure to adapt to curvature. BPs are a three-dimensional assembly of high-chirality liquid crystal molecules arranged into body-centered (BPI) or simple cubic (BPII) symmetries. BPs with submicrometer cubic-crystalline lattices exhibit tunable Bragg reflection and submillisecond response time to external stimuli such as an electric field, making them attractive for advanced photonic materials. In this work, we have systematically studied BPs confined in spherical shells with well-defined curvature and boundary conditions. The optical behavior of shells has also been examined at room temperature, where the cholesteric structure forms. In the cholesteric phase, perpendicular anchoring generates focal conic domains on the shell's surface, which transition into stripe patterns as the degree of curvature increases. Our results demonstrate that both higher degrees of curvature and strong spatial confinement destabilize BPI and reconfigure that phase to adopt the structure and optical features of BPII. We also show that the coupling of curvature and confinement nucleates skyrmions at greater thicknesses than those observed for a flat geometry. These findings are particularly important for integrating BPs into miniaturized and curved/flexible devices, including flexible displays, wearable sensors, and smart fabrics.

Measuring mesoscopic scales in complex fluids embedded with giant cylindrical micelles with diffusing wave spectroscopy micro-rheology.

This review paper presents a procedure for measuring the mesoscopic scales in micellar solutions embedded with giant cylindrical micelles using the mean square displacement determined with a quasi-elastic multiple light scattering method (diffusing wave spectroscopy) and theory. The mesoscopic scales of interest are the micelles' total contour length, persistence and entanglement lengths, and the mesh size of the entangled micellar network. All of them depend on the physicochemical parameters of the solutions and determine the rheological behavior. We present an assessment of the whole procedure, the scattering experiments performance, the recovery of optical parameters, which includes dealing with the light absorption and its treatment, and how to develop the micro-rheology for obtaining the mesoscopic scales in these complex fluids.

Structural Changes in Wormlike Micelles on the Incorporation of Small Photoswitchable Molecules.

Chromophores susceptible to light-induced trans-cis isomerization embedded in cylindrical micelles can modify micelles and their light-responsive performance. A small chromophore (4-(phenylazo)benzoate ion) is embedded in cylindrical micelles made of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) in water. The microstructure is examined by scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR). Rheological behavior and the length scales of the micellar network are determined by rheology and microrheology. The chromophore substantially modifies the micelles even without UV irradiation. The larger is the chromophore concentration, the smaller is the micellar length. Additional length scales of the micellar network do not substantially vary even when NaCl is added. Chromophore incorporation also modifies the rheology of the micellar solution, although gradient shear banding is preserved. Viscosity decreases as the chromophore concentration increases, and viscoelastic spectra are modified, but when they are correctly rescaled, they can be superimposed. The addition of the chromophore makes the fluids more Maxwellian, particularly when NaCl is also added. When the chromophore is incorporated into the micelles, there is a response after UV irradiation, although it does not produce a significant rheological change.

Tuning the Viscoelastic-Gel Transition of Single-Wall Carbon Nanotubes Embedded in pH-Responsive Polyelectrolyte Solutions.

We present the detailed rheological changes that occur when small quantities of single-wall carbon nanotubes are dispersed in a poly(acrylic acid) water solution, around the overlap polymer concentration, up to the gel point. Here, pH is used to tune the gel formation. Suspensions of nanotubes at pH ≤ 5 are exfoliated and dispersed by the polymer. Contacts between the nanotubes are mainly through polymer entangling, and the suspension is viscoelastic. At pH > 5, the polymer is charged, and the solution is not a good solvent for the nanotubes anymore. Nanotube bundles covered with polymer are formed and mechanically percolate along the fluid until they become arrested. As a consequence, the rheological behavior is dominated by a mesoscale superstructure formed by nanotubes and polymer, where viscoelasticity is lost and the suspension becomes elastic. At pH ≥ 9, the surroundings for the nanotubes are worse, bundles and flocs grow to a larger extent, and they can be observable by scanning microscopies. When the suspension becomes a critical gel, the relaxation moduli can be modeled by a power law in the frequency domain in agreement with the model developed by Winter and co-workers.