Probing the Stoichiometry Dependence of Enzyme-Catalyzed Junction Zone Network Formation in Aiyu Pectin Gel via a Reaction Kinetics Model.

We investigate the enzymatic self-catalyzed gelation process in aiyu gel, a natural ion crosslinked polysaccharide gel. The gelation process depends on the concentration ratio (Rmax) of the crosslinking calcium ions and all galacturonic acid binding sites. The physical gel network formation relies on the assembly of calcium-polysaccharide crosslink bonds. The crosslinks are initially transient and through break-up/rebinding gradually re-organizing into long, stable junction zones. Our previous study formulated a reaction kinetics model to describe enzymatic activation, crosslinker binding, and crosslink microstructural reorganization, in order to model the complex growth of elasticity. In this study, we extend the theory for the time-dependent profile of complex moduli and examine the interplay of enzyme conversion, crosslink formation, and crosslink re-organization. The adjusted model captures how the gelation and structural rearrangement characteristic times vary with the polymer and calcium concentrations. Furthermore, we find that calcium ions act as both crosslinkers and dopants in the excess calcium ion scenario and the binding dynamics is determined by Rmax. This study provides perspectives on the dynamic binding behaviors of aiyu pectin gel system and the theoretical approach can be generalized to enzyme-catalyzed ionic gel systems.

Wavelength-scanning second-harmonic generation for determining absolute charge density at aqueous interfaces.

We develop a new, to the best of our knowledge, optical scheme based on second-harmonic generation (SHG) at multiple wavelengths for unequivocal separation of the second-order and the electric-field-induced third-order nonlinear optical contributions from aqueous interfaces. The third-order SHG originating from the field-induced reorientation order of water molecules in the electrical double layer offers an optical label-free and inherent probe to the surface charge density and surface potential in the absolute scales. We verify this wavelength-scanning SHG scheme both theoretically and experimentally, and show that the approach is applicable to water interfaces with bulk ionic strength below 500 µM and can achieve a detection sensitivity for a surface charge density of ∼10-4C/m2.

Granular flow from silos with rotating orifice.

For dry granular materials falling through a circular exit at the bottom of a silo, no continuous flow can be sustained when the diameter D of the exit is less than five times the characteristic size of the grains. If the bottom of the silo rotates horizontally with respect to the wall of the silo, finite flow rate can be sustained even at small D. We investigate the effect of bottom rotation to the flow rate of monodisperse plastic beads of d=6mm diameter from a cylindrical silo of 19 cm inner diameter. We find that the flow rate W follows Beverloo law down to D=1.3d and that W increases with the rotation speed ω in the small exit regime. If the exit is at an off-center distance R from the axis of the silo, W increases with the rate of area swept by the exit. On the other hand, when the exit diameter is large, W decreases with increasing ω at small ω but increases with ω at large ω. Such nonmonotonic dependence of flow rate on rotation speed may be explained as a gradual change from funnel flow to mass flow due to the shear at the bottom of the silo.

Modeling shear-induced particle ordering and deformation in a dense soft particle suspension.

We apply the lattice Boltzmann method and the bead-spring network model of deformable particles (DPs) to study shear-induced particle ordering and deformation and the corresponding rheological behavior for dense DP suspensions confined in a narrow gap under steady external shear. The particle configuration is characterized with small-angle scattering intensity, the real-space 2D local order parameter, and the particle shape factors including deformation, stretching and tilt angles. We investigate how particle ordering and deformation vary with the particle volume fraction ϕ (=0.45-0.65) and the external shear rate characterized with the capillary number Ca (=0.003-0.191). The degree of particle deformation increases mildly with ϕ but significantly with Ca. Under moderate shear rate (Ca = 0.105), the inter-particle structure evolves from string-like ordering to layered hexagonal close packing (HCP) as ϕ increases. A long wavelength particle slithering motion emerges for sufficiently large ϕ. For ϕ = 0.61, the structure maintains layered HCP for Ca = 0.031-0.143 but gradually becomes disordered for larger and smaller Ca. The correlation in particle zigzag movements depends sensitively on ϕ and particle ordering. Layer-by-layer analysis reveals how the non-slippery hard walls affect particle ordering and deformation. The shear-induced reconfiguration of DPs observed in the simulation agrees qualitatively with experimental results of sheared uniform emulsions. The apparent suspension viscosity increases with ϕ but exhibits much weaker dependence compared to hard-sphere suspensions, indicating that particle deformation and unjamming under shear can significantly reduce the viscous stress. Furthermore, the suspension shear-thins, corresponding to increased inter-DP ordering and particle deformation with Ca. This work provides useful insights into the microstructure-rheology relationship of concentrated deformable particle suspensions.

Three-dimensional fibroblast morphology on compliant substrates of controlled negative curvature.

Traditionally, cell biological investigations have mostly employed cells growing on flat, two-dimensional, hard substrates, which are of questionable utility in mimicking microenvironments in vivo. We engineered a novel scaffold to achieve cell culture in the third dimension (3D), where fibroblasts lose the strong dorsal-ventral asymmetry in the distribution of cytoskeletal and adhesion components that is induced by growth on flat substrates. The design principle of our new 3D substrate was inspired by recent advances in engineering cellular microenvironments in which rigidity and the patterning of adhesion ligands were tuned on two-dimensional substrates; the engineered substrates enable independent control over biochemical and mechanical factors to elucidate how mechanical cues affect cellular behaviours. The 3D substrates consisted of polyacrylamide scaffolds of highly ordered, uniform pores coated with extracellular matrix proteins. We characterized important parameters for fabrication and the mechanical properties of polyacrylamide scaffolds. We then grew individual fibroblasts in the identical pores of the polyacrylamide scaffolds, examining cellular morphological, actin cytoskeletal, and adhesion properties. We found that fibroblasts sense the local rigidity of the scaffold, and exhibit a 3D distribution of actin cytoskeleton and adhesions that became more pronounced as the pore size was reduced. In small pores, we observed that elongated adhesions can exist without attachment to any solid support. Taken together, our results show that the use of negatively curved surfaces is a simple method to induce cell adhesions in 3D, opening up new degrees of freedom to explore cellular behaviours.

Optically probing nanoemulsion compositions.

Many types of colloids, including nanoemulsions, which contain sub-100 nm droplets, are dispersed in molecular and micellar solutions, especially surfactant solutions that confer stability. Since it would be desirable to measure the droplet volume fraction ϕ and surfactant concentration C of a nanoemulsion non-destructively, and since the droplet and surfactant structures are significantly smaller than the shortest wavelengths of visible light, optical refractometry could provide a simple and potentially useful approach. By diluting a silicone oil-in-water nanoemulsion having an unknown ϕ and C with pure water, measuring its refractive index n(ϕ,C) using an Abbé refractometer, and fitting the result using a prediction for n that treats the nanoemulsion as an effective medium, we show that ϕ and C can be deduced accurately over a relatively wide range of compositions. Moreover, we generalize this approach to other types of nanoemulsions in which a molecular constituent partitions in varying degrees between the dispersed and the continuous phases.