Seth Lindberg

Rheology and microrheology of a microstructured fluid: The gellan gum case

Particle tracking microrheology is used to study the effect of a constant applied shear during gelation of aqueous gellan gum with a monovalent salt. Shear modifies the gellan gum hydrogel microstructure and the bulk rheological properties of the system, depending on whether shear is applied during gelation or afterwards. The microstructure determines the linear elastic response of the gel, as well as the critical strain and stress above which the response becomes nonlinear. Bulk oscillatory rheology is used to study microstructured gellan gum hydrogels at different polymer and salt concentrations. The similarity between our system and concentrated microgel particle suspensions can be explained by considering the microstructured gellan system to be composed of microgel particles whose size is set by the applied shear stress magnitude during gelation. Polymer concentration and ionic strength control the individual microgel particles’ elastic properties. We also find the gellan system exhibits an isoenerget...

Two-Dimensional Raman Correlation Spectroscopy Study of an Emulsion Copolymerization Reaction Process

The emulsion copolymerization of styrene and 1,3-butadiene using an oligomeric nonionic surfactant as an emulsifier to make a styrene–butadiene rubber (SBR) copolymer latex was monitored by real-time in situ Raman spectroscopy. Time-resolved Raman spectra collected during the early stage of the polymerization reaction were subjected to a series of data analysis techniques, including two-dimensional (2D) correlation spectroscopy, multivariate self-modeling curve resolution (SMCR), and kernel analysis, to elucidate the fine details of the complex reaction process. Generalized 2D correlation analysis of time-resolved Raman spectra readily identified the characteristic Raman scattering bands for the monomers and copolymer. Cross-peaks appearing in 2D Raman correlation spectra showed that the decrease in the spectral intensity of Raman bands assignable to 1,3-butadiene occurs before the band intensity changes for styrene or SBR copolymer. The positions of asynchronous cross-peaks were used to identify a spectral region with the most distinct pattern of intensity variations, which in turn could be used as the starting point for the alternating least squares iteration of the SMCR analysis. SMCR analysis of the time-resolved Raman spectra generated a set of estimated pure component spectra and concentration profiles of styrene, 1,3-butadiene, and SBR copolymer without requiring independently measured calibration data. The estimated concentration profiles of monomers and copolymer indicated that the reaction of 1,3-butadiene started before the consumption of styrene and production of SBR copolymer. Kernel analysis of the estimated concentration profiles provided a succinct measure of the similarity and dissimilarity of the concentration changes of monomers and copolymer.

Formation of Liquid-Crystalline Structures in the Bile Salt–Chitosan System and Triggered Release from Lamellar Phase Bile Salt–Chitosan Capsules

Nanostructured capsules comprised of the anionic bile salt, sodium taurodeoxycholate (STDC), and the biocompatible cationic polymer, chitosan, were prepared to assess their potential as novel tailored release nanomaterials. For comparison, a previously studied system, sodium dodecyl sulfate (SDS), and polydiallyldimethylammonium chloride (polyDADMAC) was also investigated. Crossed-polarizing light microscopy (CPLM) and small-angle X-ray scattering (SAXS) identified the presence of lamellar and hexagonal phase at the surfactant-polymer interface of the respective systems. The hydrophobic and electrostatic interactions between the oppositely charged components were studied by varying temperature and salt concentration, respectively, and were found to influence the liquid-crystalline nanostructure formed. The hexagonal phase persisted at high temperatures, however the lamellar phase structure was lost above ca. 45 °C. Both mesophases were found to dissociate upon addition of 4% NaCl solution. The rate of release of the model hydrophilic drug, Rhodamine B (RhB), from the lamellar phase significantly increased in response to changes in the solution conditions studied, suggesting that modulating the drug release from these bile salt-chitosan capsules is readily achieved. In contrast, release from the hexagonal phase capsules had no appreciable response to the stimuli applied. These findings provide a platform for these oppositely charged surfactant and polymer systems to function as stimuli-responsive or sustained-release drug delivery systems.

Getting in shape: molten wax drop deformation and solidification at an immiscible liquid interface.

The controlled production of non-spherical shaped particles is important for many applications such as food processing, consumer goods, adsorbents, drug delivery, and optical sensing. In this paper, we investigated the deformation and simultaneous solidification of millimeter size molten wax drops as they impacted an immiscible liquid interface of higher density. By varying initial temperature and viscoelasticity of the molten drop, drop size, impact velocity, viscosity and temperature of the bath fluid, and the interfacial tension between the molten wax and bath fluid, spherical molten wax drops impinged on a cooling water bath and were arrested into non-spherical solidified particles in the form of ellipsoid, mushroom, disc, and flake-like shapes. We constructed cursory phase diagrams for the various particle shapes generated over a range of Weber, Capillary, Reynolds, and Stefan numbers, governed by the interfacial, inertial, viscous, and thermal effects. We solved a simplified heat transfer problem to estimate the time required to initiate the solidification at the interface of a spherical molten wax droplet and cooling aqueous bath after impact. By correlating this time with the molten wax drop deformation history captured from high speed imaging experiments, we elucidate the delicate balance of interfacial, inertial, viscous, and thermal forces that determine the final morphology of wax particles.

Impact of shear on the structure and rheological properties of a hydrogenated castor oil colloidal gel during dynamic phase transitions

Rheological modifiers are subject to processing steps, including mixing and dilution, that can have permanent structural effects. This work investigates rheological changes of a fibrous colloid, hydrogenated castor oil (HCO), when sheared during sample preparation. HCO is a polydisperse system that undergoes phase transitions in response to osmotic pressure gradients. Two HCO materials are characterized during phase transitions, a nonsheared 4 wt. % gel and a presheared 0.125 wt. % solution. Material properties are measured using multiple particle tracking (MPT) microrheology, μ2rheology, the combination of microfluidics and MPT, and bulk rheology. MPT quantitatively determines the critical relaxation exponent, n, which is constant for a material. MPT determines n is dependent on the starting HCO material, indicating that preshear has changed the structure. μ2rheology identifies consistent equilibrium states during consecutive phase transitions. Bulk rheology determines that the nonsheared gel does not completely degrade into a sol, indicated by no G′ and G″ crossover. The presheared material has a crossover indicating a sol-gel transition. The phases of HCO are identified by comparison of rheological data to previous work by Wilkins et al. [Langmuir 25, 8951–8959 (2009)], who determined the structure of a similar colloidal fiber system using confocal microscopy. The equilibrium moduli at the completion of both experiments are similar and indicate that the scaffold is in a transitional phase. These three techniques give consistent measurements of the rheological properties, and indicate the structure of the scaffold by comparison to previous works. During degradation, nonsheared HCO gels change from entangled networks to a transitional state with fiber entanglement. During gelation, presheared HCO solutions transition from bundles in solution to an associated network of bundles with few entanglements. These measurements confirm that shear history can permanently change rheological properties, affecting the scaffolds applications.Rheological modifiers are subject to processing steps, including mixing and dilution, that can have permanent structural effects. This work investigates rheological changes of a fibrous colloid, hydrogenated castor oil (HCO), when sheared during sample preparation. HCO is a polydisperse system that undergoes phase transitions in response to osmotic pressure gradients. Two HCO materials are characterized during phase transitions, a nonsheared 4 wt. % gel and a presheared 0.125 wt. % solution. Material properties are measured using multiple particle tracking (MPT) microrheology, μ2rheology, the combination of microfluidics and MPT, and bulk rheology. MPT quantitatively determines the critical relaxation exponent, n, which is constant for a material. MPT determines n is dependent on the starting HCO material, indicating that preshear has changed the structure. μ2rheology identifies consistent equilibrium states during consecutive phase transitions. Bulk rheology determines that the nonsheared gel does not co...

Combining Microfluidics and Microrheology to Determine Rheological Properties of Soft Matter during Repeated Phase Transitions

The microstructure of soft matter directly impacts macroscopic rheological properties and can be changed by factors including colloidal rearrangement during previous phase changes and applied shear. To determine the extent of these changes, we have developed a microfluidic device that enables repeated phase transitions induced by exchange of the surrounding fluid and microrheological characterization while limiting shear on the sample. This technique is µ2rheology, the combination of microfluidics and microrheology. The microfluidic device is a two-layer design with symmetric inlet streams entering a sample chamber that traps the gel sample in place during fluid exchange. Suction can be applied far away from the sample chamber to pull fluids into the sample chamber. Material rheological properties are characterized using multiple particle tracking microrheology (MPT). In MPT, fluorescent probe particles are embedded into the material and the Brownian motion of the probes is recorded using video microscopy. The movement of the particles is tracked and the mean-squared displacement (MSD) is calculated. The MSD is related to macroscopic rheological properties, using the Generalized Stokes-Einstein Relation. The phase of the material is identified by comparison to the critical relaxation exponent, determined using time-cure superposition. Measurements of a fibrous colloidal gel illustrate the utility of the technique. This gel has a delicate structure that can be irreversibly changed when shear is applied. µ2rheology data shows that the material repeatedly equilibrates to the same rheological properties after each phase transition, indicating that phase transitions do not play a role in microstructural changes. To determine the role of shear, samples can be sheared prior to injection into our microfluidic device. µ2rheology is a widely applicable technique for the characterization of soft matter enabling the determination of rheological properties of delicate microstructures in a single sample during phase transitions in response to repeated changes in the surrounding environmental conditions.