The Ewoldt Research Group 
Research See publications for the most uptodate representation of research activity. Engineers will increasingly deal with soft, viscoelastic, and complex materials, including biological and nonNewtonian fluids. Such complex fluids are both unavoidable and opportunistic for novel functionality. The wideranging applications of complex fluids, from energy to biomedicine to robotics, motivate fundamental studies of fluid mechanics and rheology, which is the core of our research. In common terms, this is the study of how materials squish, ooze, stretch, flow, and deform. Many biological systems involve a complex combination of viscous (fluid) and elastic (solid) material behavior for their function, including the locomotion of snails, the locomotion of ulcercausing bacteria, and the predatory defense mechanism used by hagfish. New engineered functionality is now being achieved by deliberately using viscoelastic materials in engineered systems; wallclimbing robots and tunable magnetic fluid adhesion are examples. Future developments will be enabled by a better understanding and description of soft materials and complex fluids. Our research spans from engineering applications of nonlinear viscoelastic materials to the theoretical foundations for describing and characterizing these ubiquitous but technologically underutilized materials. Biological Fluids and Systems Engineered Fluids and Systems General Rheology Tools
 
Hagfish slime: A volumeexpanding



Microalgae suspensionsThe photosynthetic cells of algae produce an oily goo that can be converted to biofuels. Microalgae are considered one of the most promising feedstocks for biofuels, but challenges remain in the economical scaleup, including costeffective growth chambers and efficient harvesting of oil from the cells. Algae processing is influenced by the viscosity and rheological properties of microalgae suspensions. This motivates a fundamental scientific question: how do actively swimming particles change suspension viscosity differently than passive particles? With undergraduate student Lucas Caretta, and in collaboration with PhD student Anwar Chengala (Saint Anthony Falls Laboratory) and Prof. Jian Sheng (University of Minnesota), we are using a rotational rheometer to experimentally visualize and measure the flow properties for motile and nonmotile suspensions of unicellular green algae (Dunaliella primolecta, a biflagellated ``puller''). The low viscosity biological samples require careful experimental protocols to avoid settling and flowinduced migration, and to minimize precision error. With these protocols in place we can distinguish the intrinsic viscosity of the suspensions, allowing us to put the results in the context of more traditional theories (e.g. Einstein 1906) related to the intrinsic viscosity of passive suspensions, and alo compare with recently proposed diluteregime theories which predict that ``pullers'' should have a higher viscosity than nonmotile suspensions. References:



Bioinspired snaillike locomotion (retired)Snails can climb walls and ceilings because they excrete and crawl upon a fluid with nonlinear rheology: a yield stress fluid. The remnants of this adhesive fluid can be seen as the trail left behind a crawling animal. The slime acts like a solid glue at rest, but flows as a fluid when an adequate stress is applied (a stress exceeding the apparent yield stress). When the stress is removed, the slime quickly resolidifies. Such a material is known as a yield stress fluid. By exploiting this reversible solidtoliquid transition, a snail can keep part of its foot stuck to the wall while another part moves forward. The picture on the lowerright shows the bottom of a Leopard Slug, Limax maximus, during locomotion. Robosnail and slime simulants References:

Inverted locomotion of snails


Ulcercausing bacteria locomotion (retired)In collaboration with Dr. Jon Celli and the Biological Physics research group at Boston University, rheological measurements were used to reveal the pHdependent solgel transition of mucin, the principal glycoprotein component of mucus. We have found that mucin solutions exhibit nearly critical gel behavior near pH4. These rheological results enabled an additional study to identify the manner in which the ulcercausing gastric pathogen Heliobacter pylori moves through the viscoelastic mucus gel that coats the stomach. Our study shows that the common perception of the helical bacterium moving in a corkscrew like manner through a viscoelastic gel is wrong. Instead, H. pylori produces the enzyme urease, which catalyzes hydrolysis of urea to yield ammonia thus elevating the pH of its environment. The elevated pH reduces mucin viscoelasticity into a sol phase, allowing free swimming.



Yield stress fluidsWe use the term ``yield stress fluid'' pragmatically to refer to any material or model which exhibits a dramatic and reversible change in viscosity (orders of magnitude) over a small range of applied stress (Barnes and Walters 1985; Barnes 1999), i.e. a material that is ``solid'' at low stress and ``fluid'' at larger stress. Examples include whipped cream, peanut butter, toothpaste, and hair gel. Yield stress fluid behavior is ubiquitous in food and personal care products, since it inhibits settling,provides functional use,and appeals to sensory perception.Many of the projects listed on this page involve yield stress fluids, including (snails), (bacteria), (magnetorheological fluids), and (large amplitude oscillatory shear  laos).



Tunable adhesion with magnetorheological yield stress fluidsWe have demonstrated experimentally that fieldresponsive magnetorheological fluids can adhere to nonmagnetic substrates. The tunable adhesive performance is measured experimentally with pulloff tests (a.k.a. probetack experiments) in which the external magnetic field and fluid geometry are varied. The peak adhesive force is predicted by a lubrication model which treats the adhesive as a yield stress fluid with inhomogeneous yield stress (caused by the inhomogeneous magnetic field strength). The peak adhesive force, the 'work of adhesion' and the mode of failure are all controlled by the fieldresponsive nature of the magnetorheological fluid forming the adhesive layer. Adhesive locomotion with a magnetorheological fluid could result in a robotic snail with virtually no trail left behind. Reference: 


Gel networksGluten dough Hydrogels Triblock copolymers References:



Adhesion and interfaces (retired)Immiscible polymer interfaces Soft material adhesion References:



Describing nonlinear viscoelasticity: LAOSViscosity and elastic modulus are meaningful ways to describe the mechanical behavior of fluids and solids, respectively. But for nonlinear viscoelastic materials the description is more complicated. We have developed a framework, or ontology, for interpreting nonlinear material behavior using Large Amplitude Oscillatory Shear (LAOS) deformation. For many systems the common practice of reporting only "viscoelastic moduli" as calculated by commercial rheometers (typically the first harmonic Fourier coefficients G1' , G1") is insufficient and/or misleading in describing the nonlinear phenomena. Although the higher Fourier harmonics of the material response capture the mathematical structure, they lack a clear physical interpretation. Part of our framework gives a physical interpretation to the thirdorder Fourier coefficients. We build on the earlier geometrical interpretation of Cho et al. (2005) which decomposes a nonlinear stress response into elastic and viscous stress contributions using symmetry arguments. We then use Chebyshev polynomials (closely related to the Fourier decomposition) as orthonormal basis functions to further decompose these stresses into harmonic components having physical interpretations. We also introduce new measures for reporting the firstorder (linear) viscoelastic moduli. These measures give deeper physical insight than reporting only the first harmonic Fourier coefficients G1' , G1", and reduce to the linear viscoelastic framework of G', G" at small strains. Software References



Inertioelastic ringing (retired)The finite rotational inertia of a stresscontrolled rheometer introduces experimental artifacts during creep tests that are often misunderstood. We have reviewed these artifacts and summarized the experimental techniques available to extract viscoelastic material parameters from this oftdiscarded data. Reference



