Rolando Castillo

The linear and nonlinear rheological behavior of alginate/Ca2+ gels made through slow solubilization of CaCO3 using gluconic acid-δ-lactone was studied. Rheological modulus follows a power law at the critical point; exponents (n = 0.60–0.65) decrease slightly, increasing Ca2+ ions. These values agree with those reported in the literature with similar mannuronic/guluronic residue ratios and total polymer concentration. The strain-hardening behavior of matured gels was investigated using large amplitude oscillatory shear. The Blatz–Sharda–Tschoegl scaling model was used to estimate their fractal dimension, whose values were 1.23–1.31; the fractal dimension is not sensible to Ca2+ concentration. The Lissajous–Bowditch curves show a higher nonlinearity and constitute a rheological fingerprint of these gels. The geometrical decomposition of intracycle strain in terms of strain thickening and stiffening ratios shows a weak formation of temporary network junctions during the breaking process due to the convexity of their curves. Diffusion wave spectroscopy was used to determine the mean square displacement of microspheres embedded in polymer solution exhibiting a subdiffusive process, with two slopes: m ∼ 0.3 at short times and m ∼ 1 at long times. The diffusive region section decreases as the gelation progresses due to Ca2+ and disappears when gels are formed. Gelation time was estimated to identify a terminal relaxation time, whose evolution follows almost the same exponential curve for gels with high Ca2+ concentration due to the formation of many multiple egg-box structures. After 24 h of gelation, mean square displacement curves show an apparent plateau, indicating important particle confinement.

Colloidal suspensions made of smart core–shell structures are of current interest in many fields. Their properties come from the possibility of varying the core and shell materials for modifying the composite particles’ chemical, biological, and optical properties. These particles are formed with a material with a constant refractive index core and a shell with a refractive index decaying until it matches the solvent refractive index. Poly(N-IsoPropyl AcrylaMide) (PNIPAM) is a typical example of materials forming shells. In this report, we present how to apply Mie scattering theory to predict and understand the static light scattering of large nonhomogeneous colloidal particles with spherical symmetry whose size is comparable with or larger than the light wavelength used for developing scattering experiments, where the Rayleigh–Gans–Debye approximation is not valid. Here, the refractive index decay was approximated by a Gaussian RI profile numerically evaluated through a multilayer sphere. We calculated the form factor functions of suspensions of PNIPAM microgels previously reported and core–shell suspensions made of polystyrene/PNIPAM at 20 and 40 °C synthesized by us. In all the cases, our method succeeded in providing the scattering intensity as a function of the angle. The software for using the numerical method is fairly straightforward and is accessible as an open-source code. The results can not only help predict and understand the photonic properties of microgels with large core–shell structures but also for any particle with a refractive index distribution with spherical symmetry, as in the case of microgels with super chaotropic agents, hollow microgels, or microparticles.

Depending on how they form their linkages, biopolymer gelatin gels are commonly classified as physical, chemical, or hybrid; in gelatin hybrid gels, the physical and chemical crosslinking mechanisms occur simultaneously. The viscoelastic behavior of gels following different gelation processes was determined around the gel point. Their gel fractal dimensions were obtained using the BST-scaling model from large amplitude oscillatory shear results. The fractal dimension of hybrid gels is between 1.46 and 1.60, depending on the dominant crosslinking process. The main features of the Lissajous–Bowditch curves were determined for maturated gels that follow different gelation processes, and it is possible to observe the dominant gelation mechanism. The gelation kinetics process is followed by measuring the mean squared displacement (MSD) of microspheres embedded in gelatin solutions using diffusion wave spectroscopy, which in turn allows evaluating G′(ω) and G′′(ω), the persistence length, and the mesh size as a function of time throughout the gelation process. The MSD, as a function of elapsed time from the start of the gelation process, follows a behavior that depends on the gelation processes. As time elapses after gelation starts, the persistence length of the unstructured, non-bonded flexible polymer sections decreases due to the formation of bonds. In the hybrid case, it is not a mixture of both processes; they are not independent when occurring simultaneously. The time evolution of the gel network's mesh size roughly follows an exponential decay.

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.

The capillary interaction force between spherical Janus particles trapped at the air–water interface is measured using a time-sharing optical tweezer (bond number ≪ 1). One face of the particles is hydrophilic, and the other one, hydrophobic. Measured force goes from almost pure quadrupolar to almost pure hexapolar interaction due to the three-phase contact line corrugation. Measured force curves are modeled as a sum of power laws, Ar^−α + Br^−β + Cr^−γ, obtained from an expansion in capillary multipoles. The mean values for the exponents of particle pairs of 3 μm are 〈α〉 = 5.05 ± 0.12, 〈β〉 = 7.02 ± 0.03, and 〈γ〉 = 5.96 ± 0.03. For particles pairs of 5 μm, we find 〈α〉 = 5.02 ± 0.04, 〈β〉 = 6.94 ± 0.06, and 〈γ〉 = 5.80 ± 0.05. In both cases, A < 0, B < 0, and C > 0.