Viscoelastic rheology of colloid-liquid crystal composites

Redox-Responsive and Thermoresponsive Supramolecular Nanosheet Gels with High Young's Moduli

Supramolecular gels made from 2D building blocks are emerging as one of the novel multifunctional soft materials for various applications. This study reports on a class of supramolecular nanosheet gels formed through a reversible self-assembly process involving both intramolecular folding and intermolecular self-assembly of poly[oligo(ethylene glycol)-co-(phenyl-capped bithiophenes)]. Such hierarchical self-assembled structure allows the gels to switch between sol and gel states under either redox or thermostimulus. Moreover, the gels illustrate high Young's moduli, compared to their controls that are made from the same oligo(ethylene glycol) and phenyl-capped bithiophenes blocks but have highly covalent-crosslinked structures. The example might open a window for emerging supramolecular 2D materials to develop mechanically robust and stimuli-responsive soft materials without compromising their intrinsic functions.

Tunable dynamic moduli of magnetic elastomers: from characterization by x-ray micro-computed tomography to mesoscopic modeling

Ferrogels and magnetorheological elastomers are composite materials obtained by embedding magnetic particles of mesoscopic size in a crosslinked polymeric matrix. They combine the reversible elastic deformability of polymeric materials with the high responsivity of ferrofluids to external magnetic fields. These materials stand out, for example, for significant magnetostriction as well as a pronounced increase of the elastic moduli in the presence of external magnetic fields. By means of x-ray micro-computed tomography, the position and size of each magnetic particle can be measured with a high degree of accuracy. We here use data extracted from real magnetoelastic samples as input for coarse-grained dipole-spring modeling and calculations to investigate internal restructuring, stiffening, and changes in the normal modes spectrum. More precisely, we assign to each particle a dipole moment proportional to its volume and set a randomized network of springs between them that mimics the behavior of the polymeric elastic matrix. Extending our previously developed methods, we compute the resulting structural changes in the systems as well as the frequency-dependent elastic moduli when magnetic interactions are turned on. Particularly, with increasing magnetization, we observe the formation of chain-like aggregates. Interestingly, the static elastic moduli can first show a slight decrease with growing amplitude of the magnetic interactions, before a pronounced increase appears upon the chain formation. The change of the dynamic moduli with increasing magnetization depends on the frequency and can even feature nonmonotonic behavior. Overall, we demonstrate how theory and experiments can complement each other to learn more about the dynamic behavior of this interesting class of materials.

Quantifying the dispersion quality of partially aggregated colloidal dispersions by high frequency rheology

An important parameter for the performance of nanomaterials is the degree by which the nanoparticles are dispersed in a matrix. Optical microscopy or scattering methods are useful to characterise the state of dispersion, but are not generally applicable to all materials. Electron microscopy methods are laborious in preparation and typically offer only quantitative information on a very local scale. In the present work we investigate how high frequency rheological measurements can be used for partially dispersed suspensions at intermediate to higher particle loadings, even for high viscous matrices. Although the contribution of the particles is particularly visible in the low frequency linear viscoelastic behaviour, a more direct relationship between rheological properties and degree of dispersion can be derived from the loss modulus in the high frequency limit. To this end, a home-built piezo shear rheometer is constructed to extend the frequency range typically accessible by commercial rotational rheometers. Measurements on spherical silica particles, with a varying degree of dispersion in low molecular weight PDMS, are used to demonstrate how high frequency rheometry can be used to quantify dispersion quality. The linear viscoelastic properties are compared to analytical scaling theories to demonstrate that a hydrodynamically dominated regime is reached. The dependence of the relative high frequency loss modulus on volume fraction is then compared to predictions of a hydrodynamic viscosity model for the derivation of a dispersion quality index. It is used to follow the evolution of the dispersion quality as a function of mixing time and consumed power.

Droplet impact on soft viscoelastic surfaces

In this work, we experimentally investigate the impact of water droplets onto soft viscoelastic surfaces with a wide range of impact velocities. Several impact phenomena, which depend on the dynamic interaction between the droplets and viscoelastic surfaces, have been identified and analyzed. At low We, complete rebound is observed when the impact velocity is between a lower and an upper threshold, beyond which droplets are deposited on the surface after impact. At intermediate We, entrapment of an air bubble inside the impinging droplets is found on soft surfaces, while a bubble entrapment on the surface is observed on rigid surfaces. At high We, partial rebound is only identified on the most rigid surface at We≳92. Rebounding droplets behave similarly to elastic drops rebounding on superhydrophobic surfaces and the impact process is independent of surface viscoelasticity. Further, surface viscoelasticity does not influence drop spreading after impact-as the surfaces behave like rigid surfaces-but it does affect drop recoiling. Also, the postimpact drop oscillation on soft viscoelastic surfaces is influenced by dynamic wettability of these surfaces. Comparing sessile drop oscillation with a damped harmonic oscillator allows us to conclude that surface viscoelasticity affects the damping coefficient and liquid surface tension sets the spring constant of the system.

Optovibrometry: tracking changes in the surface tension and viscosity of multicomponent droplets in real-time

An instrument was developed for measuring real time changes in the surface tension and viscosity of multicomponent droplets of miscible liquids and other soft materials. Droplets containing glycerol and water were supported on superamphiphobic surfaces and vibrated by applying a short mechanical impulse. Laser light was refracted through the droplets and allowed to fall on the surface of a photodiode. Time dependent variations in the intensity measured by the photodiode during vibration were used to monitor the decay of the droplet oscillations. The frequencies and spectral widths of the droplet vibrational resonances were then obtained from Fourier transforms of these time dependent intensity signals. A recently developed model of viscoelastic droplet vibration was used along with these values and measurements of the drop dimensions to extract the surface tension and viscosity of the drops as they evaporated. Collection of data was automated and values of frequency, spectral width, drop size, surface tension and viscosity were obtained with a time resolution of three seconds over a period of thirty minutes. The values of surface tension and viscosity obtained were shown to be in good agreement with literature values obtained from bulk glycerol/water solutions; thus validating the technique for wider application to other multicomponent liquids and soft matter systems.

Rheological Properties of Viscoelastic Drops on Superamphiphobic Substrates

The rheological properties of microliter sized drops of polymer solutions were investigated using measurements of their mechanical vibrational response. Drops were suspended on superamphiphobic substrates and vibrated by the application of a short mechanical impulse. Surface vibrations were monitored by refracting laser light through the drops and focusing the refracted light onto the surface of a photodiode. Time dependent variations in the photodiode output were Fourier transformed to obtain the frequency and spectral width of the mechanical resonances of the drops. These quantities were related to the frequency dependent shear storage and loss moduli (G' and G″, respectively) using a simple theoretical model. The resulting rheological properties were found to be in agreement with microrheology measurements of the same solutions. Drop vibration therefore provides a fast and accurate method of quantifying the rheological properties of single drops.

Hierarchical microstructures formed by bidisperse colloidal suspensions within colloid-in-liquid crystal gels

Past studies have reported that colloids of a single size dispersed in the isotropic phase of a mesogenic solvent can form colloid-rich networks (and gels) upon thermal quenching of the system across the isotropic-nematic phase boundary of the mesogens. Herein we report the observation and characterization of complex hierarchical microstructures that form when bidisperse colloidal suspensions of nanoparticles (NPs; iron oxide with diameters of 188 ± 20 nm or poly(methyl methacrylate) with diameters of 150 ± 15 nm) and microparticles (MPs; polystyrene with diameters of 2.77 ± 0.20 μm) are dispersed in the isotropic phase of 4-pentyl-4'-cyanobiphenyl (5CB) and thermally quenched. Specifically, we document microstructuring that results from three sequential phase separation processes that occur at distinct temperatures during stepwise cooling of the ternary mixture from its miscibility region. The first phase transition demixes the system into coexisting MP-rich and NP-rich phases; the second promotes formation of a particle network within the MP-rich phase; and the third, which coincides with the isotropic-to-nematic phase transition of 5CB, produces a second colloidal network within the NP-rich phase. We quantified the dynamics of each demixing process by using optical microscopy and Fourier transform image analysis to establish that the phase transitions occur through (i) surface-directed spinodal decomposition, (ii) spinodal decomposition, and (iii) nucleation and growth, respectively. Significantly, the observed series of phase transitions leads to a hierarchical organization of cellular microstructures not observed in colloid-in-liquid crystal gels formed from monodisperse colloids. The results of this study suggest new routes to the synthesis of colloidal materials with hierarchical microstructures that combine large surface areas and organized porosity with potential applications in catalysis, separations, chemical sensing, or tissue engineering.

Rheology of twist-grain-boundary-A liquid crystals

We report studies on the rheological properties of a liquid crystalline analog of Abrikosov phase in type-II superconductors known as twist-grain-boundary-A (TGB(A)) phase. The TGB(A) phase shows a large apparent yield stress compared to the cholesteric (N*) phase. The storage modulus (G') of the TGB(A) phase is significantly larger than the loss modulus (G''). The dynamic relaxation measurements indicate a solid-like behavior of N*, TGB(A), and smectic-C* phases. The complex shear modulus of the TGB(A) phase exhibits a power-law behavior G*(ω) ∼ ω(α) with α ≃ 0.5. The relative amplitude of G' and G'' at various temperatures indicate that the enhanced elasticity of TGB(A) phase is due to the structural defects.

Laser-speckle-visibility acoustic spectroscopy in soft turbid media

We image the evolution in space and time of an acoustic wave propagating along the surface of turbid soft matter by shining coherent light on the sample. The wave locally modulates the speckle interference pattern of the backscattered light, which is recorded using a camera. We show both experimentally and theoretically how the temporal and spatial correlations in this pattern can be analyzed to obtain the acoustic wavelength and attenuation length. The technique is validated using shear waves propagating in aqueous foam. It may be applied to other kinds of acoustic waves in different forms of turbid soft matter such as biological tissues, pastes, or concentrated emulsions.

Liquid-crystal mediated nanoparticle interactions and gel formation

Colloidal particles embedded within nematic liquid crystals exhibit strong anisotropic interactions arising from preferential orientation of nematogens near the particle surface. Such interactions are conducive to forming branched, gel-like aggregates. Anchoring effects also induce interactions between colloids dispersed in the isotropic liquid phase, through the interactions of the pre-nematic wetting layers. Here we utilize computer simulation using coarse-grained mesogens to perform a molecular-level calculation of the potential of mean force between two embedded nanoparticles as a function of anchoring for a set of solvent conditions straddling the isotropic-nematic transition. We observe that strong, nontrivial interactions can be induced between particles dispersed in mesogenic solvent, and explore how such interactions might be utilized to induce a gel state in the isotropic and nematic phases.

Dynamic arrest of nematic liquid-crystal colloid networks

We report interesting self-assembly structures of nematic liquid-crystal colloid (NLCC) networks, which are arrested during cooling from the isotropic temperature to room temperature. The NLCC is composed of sterically stabilized colloidal particles and a nematic liquid crystal (NLC) with nematic-isotropic transition temperature (T_{NI}) that is much higher than those of previously studied 4-Cyano-4'-pentylbiphenyl and N-(4-Methoxybenzylidene)-4-butylaniline. We find that the structure of NLCCs depends on T(NI), cooling rates, and boundary conditions, varying from cellular network to hierarchical fern structures in different length scales. Our time-lapse study shows that the transition from the cellular network to the fern structure directly corresponds to the transition from a spinodal demixing to a nucleation-and-growth mechanism.

Antibacterial strategies from the sea: polymer-bound cl-catechols for prevention of biofilm formation

Inspired by the amino acid 2-chloro-4,5-dihydroxyphenylalanine (Cl-DOPA), present in the composition of the proteinaceous glue of the sandcastle worm Phragmatopoma californica, a simple strategy is presented to confer antifouling properties to polymer surfaces using (but not releasing) a bioinspired biocide. Cl-Dopamine is used to functionalize polymer materials and hydrogel films easily, to prevent biofilm formation on them.