Gels and hydrogels have attracted a great attention for potential applications in tissue engineering, drug delivery, actuators, and soft robots. There has been a significant progress to engineer hydrogels from both synthetic and natural precursors to be as tough as a solid and as stretchable as a rubbery material while maintaining high water/solvent content. Despite considerable advances in rationally designing hydrogels, our understanding of their complex nonlinear mechanical deformation behavior is incomplete. This is partially due to the difficulty in conducting mechanical characterization on slippery, soft and swollen gels. Thus, it is required to develop new experimental techniques in order to better characterize them. Further, analyzing the experimental observations and link it with the molecular networks is an important factor. With this perspective, in this dissertation, nonlinear mechanical properties of different gel like materials have been investigated. We chose different gels with varied molecular structure, from molecular gel to self-assembled copolymer gels with flexible chains, to semiflexible polysaccharide based polymers. By developing suitable experimental protocols, strain-stiffening behavior of these materials, similar to that observed in biological materials, have been captured. Chain flexibility is a dominant factor in mechanical behavior of gels. For example, gels with flexible chains dilate orthogonal to an external shear load, whereas gels with semilexible chains contract similar to biological gel-like materials. In order to investigate the failure mechanism in our gels, cavitation rheology technique was also applied. We found that cavitation phenomenon in gels is related to the molecular architecture of the gels. The present work provides a better understanding of the deformation behavior of soft gels when subjected to a large load.

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Thesis advisor: Seth C. Kruckenberg / In the earth’s lithosphere there exists both homogeneous and heterogeneous deformation on a variety of scales. The lower crust specifically plays a critical role in lithospheric deformation; however, the lower crust does not deform homogenously but rather heterogeneously in space and time. One of the best avenues for addressing heterogeneous lower crustal deformation is through an integrated study of shear zones. While many studies have identified factors such as strain rate and temperature as key actors in lower crustal strain localization, more studies are needed to characterize the dominant grain-scale mechanisms that accommodate the development of lower crustal shear zones. The primary aim of this research is to investigate the dominant mechanisms that lead to strain localization in the lower crust. The Capricorn Ridge Shear Zone (CRSZ), Central Australia, is an ideal location for study because it is a lower crustal shear zone that contains discrete zones of strain localization, primarily adjacent to major lithological boundaries. Previous studies conclude that competency contrast caused strain to localize at the lithologic boundaries of the CRSZ, a hypothesis that is tested in this study. Using microstructural, textural, and rheologic analysis, as well as field-based mapping and grain size piezometry, this study finds that differential stresses in Capricorn Ridge range from 17-27 MPa for quartz, 31-42 MPa for plagioclase, and 2.8-7.6 MPa for enstatite. Monophase aggregate strain rates range from 1.6 x 10-15 to 1.7 x 10-14 s-1 for quartz, 4.5 x 10-15 to 3.3 x 10-14 s-1 for plagioclase, and 6.0 x 10-20 to 1.2 x 10-18 s-1 for enstatite; corresponding effective viscosities 0.3-1.7 x 1021 Pa.s, 0.3-1.5 x 1021 Pa.s, and 0.2-1.8 x 1025 Pa.s for quartz, plagioclase, and enstatite, respectively. Data across the CRSZ show that while strain rate (viscosity) in monophase aggregates of quartz and plagioclase are generally similar across the shear zone, they do decrease at lithologic boundaries. In contrast to a previous study’s finding that competency contrast caused strain to localize at these boundaries, both quartz and plagioclase appear to record strain accumulation through grain size reduction. However, the observations made in previous studies are not negated by this study, as it is possible that grain size reduction in the mylonite zones near the boundaries caused strain to accumulate over time and therefore produce the observed pattern of increasing fabric intensity with proximity to the lithologic boundaries. / Thesis (MS) — Boston College, 2021. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

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ABSTRACT Effects of Filtration Temperature and Heat Treatment on Composition and Rheological Properties of Whole Milk Ultrafiltration Retentates John William Montella For the first part of my thesis, the effects of filtration temperature and heat treatment on the compositional properties of whole milk Ultrafiltration retentate (UF) were studied. Ultrafiltration is primarily run at temperatures in the range of 50-55°C but more and more plants are starting to filter at refrigeration temperatures. In the ultrafiltration of milk, filtration temperature can affect the composition of the retentate by affecting the chemistry of milk components. The application of a pasteurization step can also affect the chemistry of milk components. There were two filtration temperatures used: 10°C and 50°C. The effect of stage in the filtration process in which the pasteurization step is applied (before UF vs. after UF) is also studied. The heat treatment used was a batch pasteurization treatment of 63°C for 30 minutes. The milk was concentrated to a Volume Concentration Ratio (VCR) of 3X through a 10,000 Molecular Weight Cut Off polysulfone membrane. Compositional analysis was performed on permeate and retentate. According to my results, there were significant treatment effects on the retention of true protein (both casein and whey protein nitrogen), total protein, non-casein nitrogen, minerals (including Ca) and pH of the retentate. The chemistry of the milk components were considered as possible reasons for these differences. The week of processing did not affect the results. For the second part, the effect of composition of the retentate on their viscosity and flow properties was observed. Rheological properties are very important in process design and for consumer acceptability. Flow and viscosity data was collected using a dynamic stress rheometer. Three analytical temperatures were used during the rheological measurements: 10°C, 40°C, and 70°C. A shear rate of 500 s-1 was used for viscosity analysis. Flow properties were also observed using the same three temperatures. According to the results, all the retentate displayed shear thinning behavior and this behavior became more pronounced as the testing temperature increased. As the shear rate increased, there was a shear thickening effect that became more pronounced as temperature increased. There was a significant effect of treatment on the viscosities of the retentate. Compositional differences in the retentate are possible contributors to observed results. The week of processing had no effect on the results. For the final part, the effect of filtration temperature and heat treatment on rennet coagulation time of retentate was observed. A 22μl aliquot of chymosin was added to 100 ml of retentate heated to 30°C prior to analysis. Rennet coagulation time was monitored using a dynamic stress rheometer. The rennet coagulation time was recorded as the time at which the G’ value reached 1 Pa. There was a significant effect of filtration temperature and heat treatment on the rennet coagulation time of the retentate. Compositional differences are all possible contributors to these differences. From the observations from all three studies, the following conclusions can be made: (1) There were significant differences observed with respect to filtration temperature and heat treatments on chemical composition of the retentate; (2) The retentate displayed a shear thinning behavior and the chemical composition of the retentate could be a contributing factor as well as the sample testing temperature. There was also a significant treatment effect on the viscosity of the retentate; and (3) Significant differences in rennet coagulation times were observed, possibly due to compositional differences of retentate. Processing week did not have a significant effect on my results.

The relationship between cells and their environment is one of dynamic reciprocity, whereby cells can influence their surrounding and the surroundings can influence the cells. One example of this relationship arises from the effect of the mechanical properties of an environment on a cell and of a cell on its environment. Inspired by this relationship, we investigate 1) the local environment of biological materials, both native and synthetic, and 2) the forces that cell sheets exert on surfaces. We do this by developing techniques that focus on local mechanical properties and experimental strategies that provide insight into intercellular mechanics. We first focus on determining local mechanical properties of hydrogel materials by developing the Cavitation Rheology technique. This process involves inducing a cavitation event at the tip of a syringe needle. We develop theory to show that the critical pressure to cavitate can be directly related to the modulus of the material (Chapter 2). This allows us to experimentally determine the mechanical properties at arbitrary locations throughout a material scaffold over a range of length scales defined by the needle radius (Chapter 3). We then demonstrate that we can viturally elminate the energy contribution from the creation of new surface area to the critical pressure by cavitating with a media of lower surface energy (Chapter 4). In chapter 5, we show that Cavitation Rheology can be used on native biological tissues and we go on to demonstrate the importance of measuring the mechanical properties in vivo. We then focus on understanding the force development of cells as they grow to confluency on a dynamic substrate (Chapter 6). We demonstrate the method of living microlenses to measure the collective strains cell sheets attain by growing cells on a thin polystyrene film supported by a surface of microwells. The cells cause the film to buckle and the resultant buckling can be directly related to the strain. We use this technique to study the strains exerted by various cell types and to determine the importance of the cell-cell junctions on the strain development.

Free-standing ultra-thin films and micro to nanoscale fibers offer a unique geometry in which to study the dynamics of thin film stability and polymer chain dynamics. By melting these films and investigating the subsequent processes of hole formation and growth, and fiber thinning and breakup, many interesting phenomena can be explored, including the nucleation of holes, shear-thinning during hole formation, finite-extensibility of capillary thinning viscoelastic fibers, and confinement effects on entanglement of polymer chains. Free-standing films in the melt are unstable and rupture due to instabilities. The mechanism of membrane failure and hole nucleation is modeled using an energy barrier approach which is shown to capture the dependence of hole nucleation on thickness. The formed holes grow exponentially and are found to grow under a shear thinning, nonlinear viscoelastic, high shear strain regime. These holes impinge upon each other to form suspended fibers. The fibers thin according to a model for the elasto-capillary thinning of the suspended viscoelastic fluid filaments. Monitoring fiber thinning allows for the acquisition of rheological properties as well as the transient, apparent extensional viscosity giving insight into strain hardening and eventual steady-state extensional viscosity. The decay and breakup of these fibers and their interconnected branched structure indicates the effects of confinement on chain entanglement in ultra-thin films. A transition below a critical film thickness, comparable to the dimensions of a polymer chain, shows drastically reduced interchain entanglements and a remarkably faster breakup of suspended fibers. The processes of fiber formation from the melting of ultra-thin films are explored in high detail and produce a new technique for the investigation of rheological and material properties, confinement effects, and the dynamics of thin films and polymer chains.

Microemulsions have important applications in various industries, including enhanced oil recovery, reactions, separations, drug delivery, cosmetics and foods. We investigated two different kinds of water-in-oil microemulsion systems, AOT (bis(2-ethylhexyl) sulfosuccinate) microemulsions with various counterions and perfluorocarbon-based microemulsion gels with triblock copolymers. In the AOT systems, we investigated the viscosity and interdroplet interactions in Ca(AOT)2, Mg(AOT)2 and KAOT microemulsions, and compared our results with the commonly-studied NaAOT/water/decane system. We attribute the differences in behavior to different hydration characteristics of the counterions, and we believe that the results are consistent with a previously proposed charge fluctuation model. Perfluorocarbons (PFCs) are of interest in a variety of biomedical applications as oxygen carriers. We have used triblock copolymer Pluronic® F127 to modify the rheology of PFC-based microemulsions, we have been able to form thermoreversible PFOB (perfluorooctyl bromide)-based gels, and have investigated the phase stability, rheology, microstructure, interactions, and gelation mechanism using scattering, rheometry, and microscopy. Finally, we attempted to use these data to understand the relationship between rheology and structure in soft attractive colloids.