https://hdl.handle.net/1721.1/33950 2026-04-11T13:21:56Z https://hdl.handle.net/1721.1/37326 Tuning Gastropod Locomotion: Modeling The Influence Of Mucus Rheology on the Cost of Crawling Lauga, Eric; Hosoi, A.E. Common gastropods such as snails crawl on a solid substrate by propagating muscular waves of shear stress on a viscoelastic mucus. Producing the mucus accounts for the largest component in the gastropod’s energy budget, more than twenty times the amount of mechanical work used in crawling. Using a simple mechanical model, we show that the shear-thinnning properties of the mucus favor a decrease in the amount of mucus necessary for crawling, thereby decreasing the overall energetic cost of locomotion. Submitted to Phys. Fluids. 2007-05-04T18:15:06Z https://hdl.handle.net/1721.1/37325 Experimental Investigations of Elastic Tail Propulsion At Low Reynolds Number Yu, Tony S.; Lauga, Eric; Hosoi, A.E. A simple way to generate propulsion at low Reynolds number is to periodically oscillate a passive flexible filament. Here we present a macroscopic experimental investigation of such a propulsive mechanism. A robotic swimmer is constructed and both tail shape and propulsive force are measured. Filament characteristics and the actuation are varied and resulting data are quantitatively compared with existing linear and nonlinear theories. Accepted for publication in Phys. Fluids. 2007-05-04T18:14:22Z https://hdl.handle.net/1721.1/35772 The Flexure-based Microgap Rheometer (FMR) Clasen, Christian; Gearing, Brian P.; McKinley, Gareth H. We describe the design and construction of a new microrheometer designed to facilitate the viscometric study of complex fluids with very small sample volumes (1-10 μl)and gaps of micrometer dimensions. The Flexure-based Microgap Rheometer (FMR) is a shear-rate-controlled device capable of measuring the shear stress in a plane Couette configuration with directly-controlled gaps between 1 μm and 200 μm. White light interferometry and a three-point nanopositioning stage using piezo-stepping motors are used to control the parallelism of the upper and lower shearing surfaces which are constructed from glass optical flats. A compound flexure system is used to hold the fluid sample testing unit between a drive spring connected to an ‘inchworm’ motor and an independent sensor spring. Displacements in the sensing flexure are detected using an inductive proximity sensor. Ready optical access to the transparent shearing surfaces enables monitoring of the structural evolution in the gap with a long working-distance video-microscope. This configuration then allows us to determine the microgap-dependent flow behavior of complex fluids over 5 decades of shear rate. We demonstrate the capability of the FMR by characterizing the complex stress and gap dependent flow behavior of a typical microstructured food product (mayonnaise) over the range of gaps from 8 to 100 μm and stresses from 10 to 1500 Pa. We correlate the gap-dependent rheological response to the microstructure of the emulsion and changes induced in the material by prolonged shearing. Submitted to J. Rheol. 2007-01-23T12:07:15Z https://hdl.handle.net/1721.1/35771 How Dilute are Dilute Solutions in Extensional Flows? Clasen, Christian; Plog, J.P.; Kulicke, W.-M.; Owens, M.; Macosko, C.; Scriven, L.E.; Verani, M.; McKinley, Gareth H. We investigate the concentration-dependence of the characteristic relaxation time of dilute polymer solutions in transient uniaxial elongational flow. A series of monodisperse polystyrene solutions of five different molecular weights (1.8×10^6 ≤ M ≤ 8.3×10^6 g/mol) with concentrations spanning five orders of magnitude were dissolved in two solvents of differing solvent quality (diethyl phthalate and oligomeric styrene). Optical measurements of the rate of filament thinning and the time to break-up in each fluid are used to determine the characteristic relaxation time. A lower sensitivity limit for the measurements was determined experimentally and confirmed by comparison to numerical calculations. Above this sensitivity limit we show that the effective relaxation time of moderately dilute solutions (0.01 ≤ c/c* ≤ 1) in transient extensional flow rises substantially above the fitted value of the relaxation time extracted from small amplitude oscillatory shear flow and above the Zimm relaxation time computed from kinetic theory and intrinsic viscosity measurements. This effective relaxation time exhibits a power-law scaling with the reduced concentration (c/c*) and the magnitude of the exponent varies with the thermodynamic quality of the solvent. This scaling appears to be roughly consistent to that predicted when the dynamics of the partially elongated and overlapping polymer chains are described within the framework of blob theories for semi-dilute solutions. Submitted to J. Rheol. 2007-01-23T12:05:57Z https://hdl.handle.net/1721.1/35770 Linear to Non-linear Rheology of Wheat Flour Dough Ng, Trevor S.K.; McKinley, Gareth H.; Padmanabhan, Mahesh We provide an overview of transient extensional rheometry techniques for wheat flour doughs in which the deformation and material response is well defined. The behavior of a range of model doughs was explored with a Filament Stretching Extensional Rheometer (FISER). The measurements were also compared to data obtained with a new wind-up extensional rheometer; the SER universal testing platform. A simple empirical constitutive equation, which allows characterization of the experimental results with a small number of parameters, is presented to describe the resulting measurements. To characterize the relaxation modulus of the doughs, small amplitude oscillatory tests were performed on samples that have been shear-mixed in a mixograph for varying lengths of time. The linear viscoelastic properties were found to exhibit a broad power-law dependence on the imposed oscillatory frequency that is very reminiscent of that exhibited by a critical gel. The critical gel model of Winter-Chambon [1, 2] was used as the basis for constructing a non-linear constitutive equation for the material stress by combining the relaxation modulus for the critical gel with a Lodge rubber-like liquid form for the kinematics. Transient uniaxial extensional data recorded from the FISER and SER instruments were then compared to the predictions of the constitutive equation. The model captures the initial power-law response and subsequent strain-hardening; however additional physics is required to describe the rheological phenomena at very large Hencky strains, including finite extensibility effects and filament rupture in extensional flows. Submitted to Appl. Rheol. 2007-01-23T12:04:39Z https://hdl.handle.net/1721.1/35769 Preferential Association of Segment Blocks in Polyurethane Nanocomposites James-Korley, LaShanda T.; Liff, Shawna M.; Kumar, Nitin; McKinley, Gareth H.; Hammond, Paula T. Controlling the level of dispersion of silicate layers in polymer matrices through intermolecular interactions and exploiting these interactions to enhance thermomechanical behavior are key challenges in the field of polymer nanocomposites. In this investigation, unmodified Laponite platelets are dispersed in a segmented polyurethane containing polar, hydrophilic soft segments and a hydrophobic hard segment using a novel solvent exchange method and compared to polyurethane nanocomposites containing more hydrophobic hard and soft domains. It was determined that the silicate layers were preferentially, but not exclusively, attracted to the hydrophilic, polar soft domains. An apparent micro-phase segregated morphology was observed in transmission electron microscopy for this system, revealing regions of exfoliation and intercalation. According to polarizing optical microscopy, strain-induced alignment is inhibited for this polyurethane nanocomposite, which is reflected in dramatic reductions in tensile strength and ultimate extensibility. In comparison, the Laponite discs appear to be preferentially, but not exclusively, embedded to the hard domains in the segmented polyurethanes containing more hydrophobic hard and soft domains. Exfoliation of the clay platelets leads to enhanced modulus and toughness without a reduction in extensibility. This study provides clues for exploiting silicate-polymer interactions to tune material properties without chemical modification. Submitted to Macromolecules 2007-01-23T12:03:41Z https://hdl.handle.net/1721.1/35768 Elongational Viscosity of Monodisperse and Bidisperse Polystyrene Melts Nielsen, Jens K.; Rasmussen, Henrik K.; Hassager, Ole; McKinley, Gareth H. The startup and steady uniaxial elongational viscosity have been measured for two monodisperse polystyrene melts with molecular weights of 52 kg/mole and 103 kg/mole, and for three bidisperse polystyrene melts. The monodisperse melts show a maximum in the steady elongational viscosity vs. the elongational rate, ε, of about two times 3η_0 whereas the bidisperse melts have a maximum of up to a factor of seven times the Trouton limit of 3η_0. The Wiest model which incorporates anisotropic drag and finite extensibility may be used to interpret the results in molecular terms. Submitted to J. Rheol. 2007-01-23T11:56:00Z https://hdl.handle.net/1721.1/35767 Inductively Heated Shape Memory Polymer for the Magnetic Actuation of Medical Devices Buckley, Patrick R.; McKinley, Gareth H.; Wilson, Thomas S.; Small IV, Ward; Benett, William J.; Bearinger, Jane P.; McElfresh, Michael W.; Maitland, Duncan J. Presently there is interest in making medical devices such as expandable stents and intravascular microactuators from shape memory polymer (SMP). One of the key challenges in realizing SMP medical devices is the implementation of a safe and effective method of thermally actuating various device geometries in vivo. A novel scheme of actuation by Curie-thermoregulated inductive heating is presented. Prototype medical devices made from SMP loaded with Nickel Zinc ferrite ferromagnetic particles were actuated in air by applying an alternating magnetic field to induce heating. Dynamic mechanical thermal analysis was performed on both the particle-loaded and neat SMP materials to assess the impact of the ferrite particles on the mechanical properties of the samples. Calorimetry was used to quantify the rate of heat generation as a function of particle size and volumetric loading of ferrite particles in the SMP. These tests demonstrated the feasibility of SMP actuation by inductive heating. Rapid and uniform heating was achieved in complex device geometries and particle loading up to 10% volume content did not interfere with the shape recovery of the SMP. Submitted to IEEE Trans. Biomed. Eng. 2007-01-23T11:53:32Z https://hdl.handle.net/1721.1/35766 The Beads-on-String Structure of Viscoelastic Threads Clasen, Christian; Eggers, Jens; Fontelos, Marco A.; Li, Jie; McKinley, Gareth H. By adding minute concentrations of a high molecular weight polymer, liquid jets or bridges collapsing under the action of surface tension develop a characteristic shape of uniform threads connecting spherical uid drops. In this paper, high-precision measurements of this beads-on-string structure are combined with a theoretical analysis of the limiting case of large polymer relaxation times and high polymer extensibilities, for which the evolution can be divided into two distinct regimes. For times smaller than the polymer relaxation time, over which the beads-on-string structure develops, we give a simplfied local description, which still retains the essential physics of the problem. At times much larger than the relaxation time, we show that the solution consists of exponentially thinning threads connecting almost spherical drops. Both experiment and theoretical analysis of a one-dimensional model equation reveal a self-similar structure of the corner where a thread is attached to the neighbouring drops. Submitted to J. Fluid Mech. 2007-01-23T11:51:44Z https://hdl.handle.net/1721.1/35765 Drop Formation and Breakup of Low Viscosity Elastic Fluids: Effects of Molecular Weight and Concentration Tirtaatmadja, Viyada; McKinley, Gareth H.; Cooper-White, Justin J. The dynamics of drop formation and pinch-off have been investigated for a series of low viscosity elastic fluids possessing similar shear viscosities, but differing substantially in elastic properties. On initial approach to the pinch region, the viscoelastic fluids all exhibit the same global necking behaviour that is observed for a Newtonian fluid of equivalent shear viscosity. For these low viscosity dilute polymer solutions, inertial and capillary forces form the dominant balance in this potential flow regime, with the viscous force being negligible. The approach to the pinch point, which corresponds to the point of rupture for a Newtonian fluid, is extremely rapid in such solutions, with the sudden increase in curvature producing very large extension rates at this location. In this region the polymer molecules are significantly extended, causing a localised increase in the elastic stresses, which grow to balance the capillary pressure. This prevents the necked fluid from breaking off, as would occur in the equivalent Newtonian fluid. Alternatively, a cylindrical filament forms in which elastic stresses and capillary pressure balance, and the radius decreases exponentially with time. A (0+1)-dimensional FENE dumbbell theory incorporating inertial, capillary and elastic stresses is able to capture the basic features of the experimental observations. Before the critical ‘pinch time’ tp , an inertial-capillary balance leads to the expected 2/3-power scaling of the minimum radius with time, Rmin ∼ (tp − t)^2/3. However, the diverging deformation rate results in large molecular deformations and rapid crossover to an elasto-capillary balance for times t > tp. In this region the filament radius decreases exponentially with time Rmin ~exp[(tp - t) / λ1], where λ1 is the characteristic time constant of the polymer molecules. Measurements of the relaxation times of PEO solutions of varying concentrations and molecular weights obtained from high speed imaging of the rate of change of filament radius are significantly higher than the relaxation times estimated from Rouse-Zimm theory, even though the solutions are within the dilute concentration region as determined using intrinsic viscosity measurements. The effective relaxation times exhibit the expected scaling with molecular weight but with an additional dependence on the concentration of the polymer in solution. This is consistent with the expectation that the polymer molecules are in fact highly extended during the approach to the pinch region (i.e. prior to the elasto-capillary filament thinning regime) and subsequently as the filament is formed they are further extended by filament stretching at a constant rate until full extension of the polymer coil is achieved. In this highly-extended state, inter-molecular interactions become significant producing relaxation times far above theoretical predictions for dilute polymer solutions under equilibrium conditions. Submitted to Phys. Fluids 2007-01-23T11:42:01Z