Transport Phenomena in Complex Materials
One main thrust of Professor Venerus’ research, done in collaboration with Professor J. Schieber, is directed towards the measurement of anisotropic heat conduction in polymers subjected to deformation. These measurements are made using a non-invasive, optical technique known as forced Rayleigh Scattering (FRS). Using a novel FRS setup developed in our lab, we have studied several polymeric systems and obtained the first (and only) quantitative measurements of the thermal conductivity tensor. These data include time-dependent measurements of the full thermal conductivity tensor in polymer melts following step shear and cessation of constant shear rate flows. We have also obtained the full thermal conductivity tensor in cross-linked polymers (rubbers) subjected to uniaxial elongation. These data clearly demonstrate that the thermal conductivity is anisotropic in deformed polymers, and that a generalized form of Fourier’s law with a thermal conductivity tensor is required to describe heat transfer in such systems. In conjunction with mechanical and rheo-optical data, these data have also been used to confirm the existence of the stress-thermal rule, which states that the stress and thermal conductivity tensors are linearly related.
We have also applied the FRS technique to study heat transfer in fluids containing nano-particles, or nano-fluids, which possess anomalously high thermal conductivities. Systems of interest include aqueous and organic liquid based nano-fluids containing Au, Al 2O 3 and carbon nano-tube nano-particles. We believe that the sensitive and non-invasive nature of FRS will provide new insights on the poorly understood mechanism that leads to enhanced thermal transport in nano-fluids.
Professor Venerus’ research group has also applied FRS to study mass diffusion in polymers. In a recent project, direct evidence for non-Fickian tracer diffusion has been identified in a polymer melt well above its glass transition temperature. A second project is currently underway in collaboration Professor W. Köhler (Universität Bayrueth) on non-Fickian diffusion in polymer solutions.
A second thrust of Professor Venerus’ research activities is experimental polymer rheology. One aspect of experimental work involves shear flows with novel deformation histories designed to elucidate mechanisms of polymer relaxation and dynamics. Much of this work is carried out in collaboration with Professor J. Schieber, who has developed very successful molecular models for entangled polymers. Experimental work also involves the use of mechanical and optical techniques for rheological study of polymer melts in extensional flow fields. A project currently underway (in collaboration with TA Instruments, Inc.) is the development of a modified lubricated squeezing flow for generating equibiaxial extensional flows in polymer melts. In addition, Professor Venerus has several collaborative projects with Professor H.C. Öttinger (ETH Zürich).
Polymer Foam Processing
Realistic models that capture the complex physics of cellular and micro-cellular polymer foaming processes are necessary for the efficient design and operation of these technologies. Increases in computational capabilities combined with advances in our understanding of transport phenomena in polymeric materials has made the formulation and solution of such models feasible. Here, experimental results from polymer rheology and transport phenomena studies are used to formulate realistic models for polymer foaming processes. To date, we have developed rigorous models for diffusion-induced bubble growth in polymeric liquids. This work will be extended to include thermal effects and bubble-bubble interactions. In addition, we plan to develop an experimental apparatus for studying bubble growth dynamics that will be used to evaluate and refine transport models.
U.S Patent No. 5,916,599 “Apparatus for Generating Generally Uniform Compression in Viscous Liquids,” D.C. Venerus and M. Kompani (1999).
“Continuous lubricated squeezing flow: a novel technique for equibiaxial elongational viscosity measurements on polymer melts,” D.C. Venerus, T.-Y. Shiu, T. Kashyap and J. Hosttetler, J. Rheol., 54, 1-xx (2010).
“Stress relaxation in polymer melts following equibiaxial step strain,” T. Kashyap and D.C. Venerus, Macromol., 43, 5874–5880 (2010).
“Compressible Laminar Flow in a Channel,” D.C. Venerus and D. Bugajsky, Phys. Fluids, 22, 046101 (2010).
“Viscosity measurements on colloidal dispersions (nanofluids) for heat transfer fluids,” D.C. Venerus, J. Buongiorno, et al., Appl. Rheol., 20, 44582 (2010).
“A benchmark study on the thermal conductivity of nanofluids,” J. Buongiorno, D.C. Venerus, et al., J. Appl. Phys., 106, 094312 (2009).
“Nonequilibrium thermodynamics of transport through moving interfaces with application to bubble growth and collapse,” H.C. Öttinger, D. Bedeaux and D.C. Venerus, Phys. Rev. E., 80, 021606 (2009).
“Anisotropic thermal conductivity in cross-linked poly-butadienes subjected to uniaxial elongation,” D.C. Venerus and D.N. Kolev, Macromol., 43, 2594-2598 (2009).
"Characterization of Type I Collagen Gels Modified by Non-Enzymatic Glycosylation," M. Francis-Sedlak, S. Uriel, J.C. Larson, D.C. Venerus, H.P. Greisler, E.M. Brey, Biomat., 30, 1851-1856 (2009).
“Direct comparisons of equibiaxial elongational viscosity measurements from Lubricated Squeezing Flow (LSF) and the MultiAxiale Dehnung (MAD) rheometer,” T. Guadarrama-Medina, T.-Y. Shiu, and D.C. Venerus, Rheol. Acta, 48, 11-17 (2009).
“Lubricated cross-slot flow of a low density polyethylene melt,’ J. Soulages, T. Schweizer, D.C. Venerus, M. Kröger and H.C. Öttinger, J. Non-Newt. Fluid Mech. 154, 52-64 (2008).
“Compliance Effects on the Torsional Flow of a Viscoelastic Fluid,” C.S. Dutcher and D.C. Venerus, J. Non-Newt. Fluid Mech., 150, 154-161 (2008).
“Study of Thermal Transport in nanoparticle suspensions using Forced Rayleigh Scattering,” D.C. Venerus, M. S. Kabahdi, S. Lee and V. Perez-Luna, J. Appl. Phys., 100, 094310 (2006).
“Laminar Capillary Flow of Compressible Viscous Fluids,” D.C. Venerus, J. Fluid Mech., 555, 59-80 (2006).
“Measurements of the Thermal Conductivity Tensor in Poly-isobutylene Melts Following Step Shear,” V. Balasubramanian, K. Bush, S. Smoukov, D.C Venerus and J.D. Schieber, Macromol., 38, 6210-6215 (2005).
“A Critical Evaluation of Step Strain Flows of Entangled Linear Polymer Liquids,” D.C. Venerus, J. Rheol., 49, 277-295 (2005).