Dr. Subramanian Ramakrishnan seminar, PPG Lecture
Dr. Subramanian Ramakrishnan, the 3M Distinguished Professor of Chemical Engineering and Biomedical Engineering; Department Graduate Director (CBE); Director, CREST CoMand, at Florida A&M University-Florida State University, will deliver the PPG Lecture, "Universal Scaling of Quench-Dependent Dynamics in Intermediate Concentration Colloidal Gels" on April 18.
This seminar is generously sponsored by PPG.
The ability to pattern multiple materials, on micrometer length scales, in three dimensions is critical for several technological applications including composites, microfluidics, photonics, and tissue engineering. The aim of the recently established NSF CREST Center at Florida A&M University is to promote additive manufacturing of novel device structures with an effort towards ab-initio fundamental understanding of material-property relationships that govern the working forces behind high-rate processing applications, and thus low-cost energy products. It is our goal to develop a materials-to-manufacturing framework for research in 3D printing of structures and devices for a wide variety of applications. In this talk, I will give a brief overview of the center first and the different projects. This will be followed by our recent work on colloidal gels where we study the structure, dynamics and aging and rheology of colloidal gels with time varying attraction. We have examined the formation and dissolution of gels composed of intermediate volume fraction nanoparticles with temperature-dependent short-range attraction using smallangle xray scattering (SAXS), x-ray photon correlation spectroscopy (XPCS), and rheology to obtain nanoscale and macroscale sensitivity to structure and dynamics. Gel formation after temperature quenches to the vicinity of the rheologically-determined gel temperature Tgel was characterized via the slow-down of dynamics observed in the intensity autocorrelation functions (g2) as a function of quench depth (ΔT = T -Tgel), wave vector, and formation time (tf). We find self-similarity in the slow-down of dynamics that maps the wave-vector-dependent dynamics at a particular ΔT and tf to that at other ΔTs and tfs via an effective scaling temperature, Ts. A single Ts applies to a broad range of ΔT and tf but does depend on the particle size. The rate of dynamical evolution implied by the scaling is a far stronger function of ΔT than the attraction strength between colloids. We interpret this strong temperature dependence as collective particle motion possibly arising from rearrangements of energetically-favored locally structures observed in confocal microscopy and via simulations.
S. Ramakrishnan is a professor of chemical and Biomedical Engineering at Florida A&M – Florida State University (FAMU-FSU) College of Engineering in Tallahassee, Florida. After earning a B. Tech degree in chemical engineering from Indian Institute of Technology (Chennai) he proceeded to earn MS (1998) and PhD (2001) in Chemical Engineering from the University of Illinois at Urbana-Champaign. He conducted post-doctoral work at Princeton University from 2001-2002 and at the Nanoscience Center at University of Illinois from 2002-2005. In August 2005 he joined the Chemical engineering department in Tallahassee and was promoted to full professor in August 2018. During 2016-2017 he was a visiting associate professor at Harvard University on a NSF funded sabbatical.
Research in his group focuses on understanding the physics, chemistry and processing of complex fluids (colloids, proteins, polymers and other “soft materials”) with an aim of producing useful materials for engineering applications. In the process fundamental questions that arise in assembling them into useful structures will also be answered. He is currently the director of the interdisciplinary NSF funded center on additive manufacturing with three thrust areas (1) Nanostructured lightweight magnetic materials for shielding/sensing applications. (2) Nanostructured materials for energy applications and (3) Nanostructured Materials for Biological Applications. The center also involves collaborative projects with Harvard University (MRSEC), MIT, Florida State University (National High Magnetic Field Laboratory), Air Force Research Laboratory, Army Research Labs and Argonne National Laboratory.