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Hagfishes defend themselves from gill-breathing predators by producing large volumes of fibrous slime when attacked. The slime's effectiveness comes from its ability to clog predators' gills, but the mechanisms by which hagfish slime clogs are uncertain, especially given its remarkably dilute concentration of solids. We quantified the clogging performance of hagfish slime over a range of concentrations, measured the contributions of its mucous and thread components, and measured the effect of turbulent mixing on clogging. To assess the porous structure of hagfish slime, we used a custom device to measure its Darcy permeability. We show that hagfish slime clogs at extremely dilute concentrations like those found in native hagfish slime and displays clogging performance that is superior to three thickening agents. We report an extremely low Darcy permeability for hagfish slime, and an effective pore size of 10–300 nm. We also show that the mucous and thread components play distinct yet crucial roles, with mucus being responsible for effective clogging and low permeability and the threads imparting mechanical strength and retaining clogging function over time. Our results provide new insights into the mechanisms by which hagfish slime clogs gills and may inspire the development of ultra-soft materials with novel properties. 

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.

 

Taking a small step away from Newtonian fluid behavior creates an explosion in the range of possibilities. Non-Newtonian fluid properties can achieve diverse flow objectives, but the complexity introduces challenges. We survey useful rheological complexity along with organizing principles and design methods as we consider the following questions: How can non-Newtonian properties be useful? What properties are needed? How can we get those properties? 

A new type of base-triggered self-amplifying degradable polyurethane is reported that degrades under mild conditions, with the release of increasing amounts of amine product leading to self-amplified degradation. The polymer incorporates a base-sensitive Fmoc-derivative into every repeating unit to enable highly sensitive amine amplified degradation. A sigmoidal degradation curve for the linear polymer was observed consistent with a self-amplifying degradation mechanism. An analogous cross-linked polyurethane gel was prepared and also found to undergo amplified breakdown. In this case, a trace amount of localized base initiates the degradation, which in turn propagates through the material in an amplified manner. The results demonstrate the potential utility of these new generation polyurethanes in enhanced disposability and as stimuli responsive materials. 

Fabricating complex structures on micro‐ and mesoscales is a critical aspect in the design of advanced sensors and soft electronics. However, soft lithographic methods offer an important approach to fabricating such structures, the progress in the field of additive manufacturing (e.g., 3D printing) offers methods of fabrication with much more material complexity. The rheological complexity of the printing material, however, often dictates the limitations of printing. In particular, the challenges involved in synthesizing printing materials that can enable shape retention at smaller scales (<100 μm), yet be conductive, limits many applications of 3D printing to soft microelectronics. Herein, a printing‐centered approach using a novel particle‐free conductive emulsion ink is presented. This approach separates the printing and polymerization of a conductive monomer (pyrrole) and renders a novel ink that is used to print filaments with heretofore impossible to realize 3D feature dimensions and build structures with high shape retention. The printability of the ink is evaluated, and post‐treatment properties assessed. Multidirectional strain sensors are printed using the emulsion ink to illustrate an exemplary application in soft electronics.

 

Increasing redox‐active species concentrations can improve viability for organic redox flow batteries by enabling higher energy densities, but the required concentrated solutions can become viscous and less conductive, leading to inefficient electrochemical cycling and low material utilization at higher current densities. To better understand these tradeoffs in a model system, we study a highly soluble and stable redox‐active couple,N‐(2‐(2‐methoxyethoxy)ethyl)phenothiazine (MEEPT), and its bis(trifluoromethanesulfonyl)imide radical cation salt (MEEPT‐TFSI). We measure the physicochemical properties of electrolytes containing 0.2–1 M active species and connect these to symmetric cell cycling behavior, achieving robust cycling performance. Specifically, for a 1 M electrolyte concentration, we demonstrate 94% materials utilization, 89% capacity retention, and 99.8% average coulombic efficiency over 435 h (100 full cycles). This demonstration helps to establish potential for high‐performing, concentrated nonaqueous electrolytes and highlights possible failure modes in such systems.

 

The ability to manufacture highly intricate designs is one of the key advantages of 3D printing. Achieving high dimensional accuracy requires precise, often time‐consuming calibration of the process parameters. Computerized feedback control systems for 3D printing enable sensing and real‐time adaptation and optimization of these parameters at every stage of the print, but multiple challenges remain with sensor embedment and measurement accuracy. In contrast to these active control approaches, here, the authors harness frontal polymerization (FP) to rapidly cure extruded filament in tandem with the printing process. A temperature gradient present along the filament, which is dependent on the printing parameters, can impose control over this exothermic reaction. Experiments and theory reveal a self‐regulative mechanism between filament temperature and cure kinetics that allows the frontal cure speed to autonomously match the print speed. This self‐regulative printing process rapidly adapts to changes in print speed and environmental conditions to produce complex, high‐fidelity structures and freestanding architectures spanning up to 100 mm, greatly expanding the capabilities of direct ink writing (DIW).