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There is no universal model for thixotropy, and comparing thixotropic effects between different fluids is a subtle yet challenging problem. We introduce a generalized (model-insensitive) framework for comparing thixotropic properties based on thixotropic spectra. A superposition of exponential stress modes distributed over thixotropic time scales is used to quantify buildup and breakdown times and mode strengths in response to step-change input. This mathematical framework is tested with several experimental step-shear rate data on colloidal suspensions. Low-dimensional metrics based on moments of the distribution reveal characteristic average thixotropic properties, which are visualized on Ashby-style diagrams. This method outlines a framework for describing thixotropy across a diverse range of microstructures, supporting scientific studies as well as material selection for engineering design applications. [ABSTRACT FROM AUTHOR]

We describe the structure and outcomes of a course project for do-it-yourself (DIY) rheometry. Although the project was created in response to the shelter-in-place orders of the COVID-19 pandemic, the student learning outcomes were so positive that we have continued implementing the project even when students have access to laboratory rheometers. Students select an interesting complex fluid, collect qualitative visual evidence of key rheological phenomena, and then produce their own readily available flows that they quantitatively analyze to infer rheological properties, such as yield stress, extensional viscosity, or shear viscosity. We provide an example rubric, present example student project outcomes, and discuss learning outcomes that are achieved with DIY measurements. [ABSTRACT FROM AUTHOR]

Abstract: 3D printing is a rapidly growing field that requires the development of yield‐stress fluids that can be used in postprinting transformation processes. There is a limited number of yield‐stress fluids currently available with the desired rheological properties for building structures with small filaments (≤l00 µm) with high shape‐retention. A printing‐centric approach for 3D printing particle‐free silicone oil‐in‐water emulsions with a polymer additive, poly(ethylene oxide) is presented. This particular material structure and formulation is used to build 3D structure and to pattern at filament diameters below that of any other known material in this class. Increasing the molecular weight of poly(ethylene oxide) drastically increases the extensibility of the material without significantly affecting shear flow properties (shear yield stress and linear viscoelastic moduli). Higher extensibility of the emulsion correlates to the ability of filaments to span relatively large gaps (greater than 6 mm) when extruded at large tip diameters (330 µm) and the ability to extrude filaments at high print rates (20 mm s−1). 3D printed structures with these extensible particle‐free emulsions undergo postprinting transformation, which converts them into elastomers. These elastomers can buckle and recover from extreme compressive strain with no permanent deformation, a characteristic not native to the emulsion. [ABSTRACT FROM AUTHOR]

Yield-stress fluids, including gels and pastes, are effectively fluid at high stress and solid at low stress. In liquid-solid impacts, the fluid motion can be halted by the yield stress at different points of the event, and these fluids can therefore stick and accumulate where they impact, motivating several applications of these rheologically complex materials. Here, we use high-speed imaging to experimentally study liquid-solid impact of yield-stress fluids on pre-coated horizontal surfaces. With a pre-coating of the same material, we can observe large long-lifetime ejection sheets with redirected momentum which extend away from the impact location. Under critical splash conditions, sheet breakup occurs and ejected droplets can be nonspherical and threadlike due to the inability of capillary stresses to deform material above a certain lengthscale. By varying the droplet size, impact velocity, surface coating thickness, and rheological material properties, we develop appropriate dimensionless parameters and present a low-dimensional regime map of impact behaviors. [ABSTRACT FROM AUTHOR]

Viscoplastic fluids, also known as yield-stress fluids, can stick and accumulate where they impact. Here we experimentally study conditions for open surfaces to be impermeable to impacting yield-stress fluid drops, and the dynamic conditions for these drops to permeate and coat internal aspects of a complex topography. We experimentally study drops of model yield stress fluids (Carbopol microgel particles in water) impacting open solid meshes (rigid surfaces with small, evenly spaced openings). High speed video reveals dynamics across a range of behavior, from 0% to 100% transmittance, by varying drop size, impact velocity, mesh geometry, and rheological material properties. When inertial stresses are sufficiently high compared to the yield stress, a drop can pass through a mesh, breaking into smaller fluid particles with varying shapes, sizes, and velocities in the process. In contrast, when inertial stresses are sufficiently low compared to the yield stress, a drop can stick to the mesh as though it were a solid surface, inhibited from passing through the holes by the yield stress. Layers of multiple meshes are also examined, demonstrating a range of behaviors and the ability to coat internal aspects of complex topography. Dimensional analysis is performed to characterize material transmittance and velocity of transmitted droplets as a function of dimensionless input parameters. Behavior is found to be well governed by the ratio of inertial stress to the sum of yield and viscous stresses. [ABSTRACT FROM AUTHOR]

[Display omitted] We present an integrated experimental and theoretical study of the dynamics and rheology of self-crosslinked, slightly charged, temperature responsive soft poly(N -isopropylacrylamide) (pNIPAM) microgels over a wide range of concentration and temperature spanning the sharp change in particle size and intermolecular interactions across the lower critical solution temperature (LCST). Dramatic, non-monotonic changes in viscoelasticity are observed as a function of temperature, with distinct concentration dependence in the dense fluid, glassy, and soft-jammed regimes. Motivated by our experimental observations, we formulate a minimalistic model for the size dependence of a single microgel particle and the change of the interparticle interaction from purely repulsive to attractive upon heating. Using microscopic equilibrium and time-dependent statistical mechanical theories, theoretical predictions are quantitatively compared with experimental measurements of the shear modulus. Good agreement is found for the nonmonotonic temperature behavior that originates as a consequence of the competition between reduced microgel packing fraction and increasing interparticle attractions. Testable predictions are made for nonlinear rheological properties such as the yield stress and strain. To our knowledge, this is the first attempt to quantitatively understand in a unified manner the viscoelasticity of dense, temperature-responsive microgel suspensions spanning a wide range of temperatures and concentrations. [ABSTRACT FROM AUTHOR]

Ensuring a high degree of dimensional accuracy of the printed part is critical in the development of additive manufacturing techniques. The advent of computational tools to simulate additive manufacturing has provided a robust way to predict part quality during the entire process. In this paper, a multiphysics model is introduced for the novel additive manufacturing technique of frontal polymerization-based direct ink writing, which enables rapid printing of complex free-form thermoset polymeric structures with excellent strength and stiffness. In direct ink writing, the fluid should possess suitable yield stress behavior to be successfully extruded and cured, and swelling of the fluid upon extrusion impacts the dimension of the printed filament. This model describes the thermo-chemo-rheological behavior of the printing gel during extrusion and polymerization, based on experimental measurements of the nonlinear viscoelastic and cure kinetics behavior. This model is applied to the FP-based printing process by considering an Eulerian domain that follows the motion of the printer head. Simulation results are presented for the vertical printing of both cylindrical filaments and annular tubes. The effect of process parameters on the dimensional accuracy of the printed part and the position of the polymerization front downstream from the printer head are investigated under steady and unsteady print speeds. Measurements from printing experiments validate the accuracy of the model. [ABSTRACT FROM AUTHOR]

• Categorizes "design with" versus "design of" rheologically-complex materials. • Considers "How many ways to get a yield-stress fluid?" • Considers "Can we predict properties from formulation and structure?" • Considers "How to transform a yield-stress fluid to be more useful?" • Future research directions are suggested to build the toolbox of design. We review progress in designing and transforming multi-functional yield-stress fluids and give a perspective on the current state of knowledge that supports each step in the design process. We focus mainly on the rheological properties that make yield-stress fluids so useful and the trade-offs which need to be considered when working with these materials. Thinking in terms of "design with" and "design of" yield-stress fluids motivates how we can organize our scientific understanding of this field. "Design with" involves identification of rheological property requirements independent of the chemical formulation, e.g. for 3D direct-write printing which needs to accommodate a wide range of chemistry and material structures. "Design of" includes microstructural considerations: conceptual models relating formulation to properties, quantitative models of formulation-structure-property relations, and chemical transformation strategies for converting effective yield-stress fluids to be more useful solid engineering materials. Future research directions are suggested at the intersection of chemistry, soft-matter physics, and material science in the context of our desire to design useful rheologically-complex functional materials. [ABSTRACT FROM AUTHOR]