Speaker:
Mehrnoosh Afshang (Purdue University)
Title:
Rheological Characterization of Microfibrillated Cellulose During Dynamic Phase Transitions
Authors:
Caggioni, M. (Procter & Gamble Co), Lindberg,S.(Procter & Gamble Co), Kelly M. Schultz (Purdue University)
Abstract:
Microfibrillated cellulose (MFC) has potential applications as a rheological modifier to change flow characteristics of a product, including changing the viscosity or enabling phase transitions in response to external stimuli. Our work focuses on MFC from wood, with the goal of piloting this work prior to extending it to repurpose paper waste into new consumer products. To enhance MFC versatility, we modify its surface chemistry, enabling its use across a broader range of formulations. We then use techniques including multiple particle tracking microrheology (MPT) and bulk rheology to characterize structural evolution and rheological properties during phase transitions and after equilibration. For this work, the thermoresponsive polymer Jeffamine polyetheramine M2005, which has a lower critical solution temperature (LCST) of 16°C, is grafted onto the surface of MFC. Thermoresponsive MFC is characterized with MPT and bulk rheology to measure dynamic temperature-dependent rheology during sol-gel and gel-sol phase transitions. MPT measures the Brownian motion of fluorescent probe particles and relates this motion to material rheological properties. Using MPT, we measure the evolving material rheology and structure and identify the state of the material in response to temperature changes. At lower temperatures, the system is in a sol state. Increasing temperature leads to thermoresponsive MFC gelation and network formation. During network formation, the critical transition temperature, Tc, and critical relaxation exponent, n are quantified using time-cure superposition. Using bulk rheology, we vary fiber concentration to change fiber interactions and temperature. We define the interaction concentration of MFC fibers, c∗∗, as the concentration when the material shifts from the semi-dilute to concentrated regime. Below the c∗∗, this material changes from a viscous fluid to a structure with a higher elastic than viscous component as temperature increases. Above c∗∗, we measure elastic behavior for all temperatures due to initial associated fibers and an increase in elastic moduli above the LCST of the grafted polymer. Using both characterization techniques, we measure thermoresponsive MFC undergoes a phase transition at a temperature close to the LCST of the grafted thermoresponsive polymer. This indicates that the transition temperature of the thermoresponsive MFC can be tuned by changing the identity of the polymer grafted to the fiber. Together, this information can be used in future design of consumer products that require structure and properties over a specified temperature range.
Speaker:
Sreeram Rajesh (University of California, Santa Barbara)
Title:
Capillary Dynamics of Complex Fluids: and Their Relevance to Macroscopic flows of Cohesive Particles
Authors:
Sreeram Rajesh, Alban Sauret (University of California, Santa Barbara and University of Maryland, College Park)
Abstract:
The flow of cohesive particulate materials is relevant in environmental and industrial applications, ranging from soil erosion and the transport of microplastic particles to the industrial handling of powders. Cohesion leads to the transport of aggregates rather than individual grains, resulting in complex behaviors that are often difficult to predict or control. The cohesion between particles in the size range of a few microns to a few millimeters is typically governed by the formation of capillary liquid bridges, which exert an attractive force between the particles. These liquid bridges often consist of heterogeneous materials, such as particles and polymers dissolved in water-based solvents, resulting in a complex fluid. While the dynamics of capillary bridges and the forces they exert are well understood for Newtonian fluids, the axial forces arising in a liquid bridge of complex fluids remain elusive. In this talk, I will present my investigations into the capillary flow dynamics of such liquid bridges, including the dynamics of the transition from a Newtonian viscosity-dominated thinning regime to a viscoelastic regime through experiments. Using axial force measurements between two particles, I will report how the capillary bridge force evolves throughout the thinning process. Finally, I will also discuss preliminary results that connect these microscale properties to macroscale particulate flows, and in particular, how complex fluids such as polymer solutions modify the cohesive yield stress of particle-polymer mixtures. These findings provide initial insights to aid in predicting the macroscopic flow behavior of particulate systems made cohesive by polymer solution bridges.