Professor, Chemical Engineering and Chemistry (Courtesy) and Material Science and Engineering (Courtesy)
BioProfessor Lynn Walker is currently Professor of Chemical Engineering with courtesy appointments in Chemistry and in Materials Science & Engineering. Prof. Walker received her BS Chemical Engineering degree from the University of New Hampshire in 1990 and her PhD from the University of Delaware in 1995, under the supervision of Norman Wagner. After holding a postdoctoral position at Katholieke Universiteit Leuven, Leuven Belgium, Prof. Walker joined Carnegie Mellon's Department of Chemical Engineering in 1997. Prof. Walker was a Visiting Professor during a sabbatical at Polymer IRC, University of Leeds in 2007.
EducationPh.D., 1995, University of Delaware
B.S., 1990, University of New Hampshire
My research program tackles a series of specific issues in the broader problem of the processing of soft materials. Soft materials are those held together by weak (on order kT) forces and usually present as fluid or gel-like macroscopically. This includes biological systems, consumer products, food products, processing fluids, and pharmaceutical formulations; in fact, processing of most materials involves a fluid phase that must be fully understood to optimize a process and ensure success. Within this class of materials, the nanometer through micrometer structures that control macroscopic behavior need to be controlled and manipulated. The weak forces that lead to these materials being soft are the same ones that allow for facile manipulation via external forces, meaning that these materials are processible; the structure can continually be altered and changed in response to a processing field or stimuli. The field of polymer processing has lead the way on using external fields to control structure and function; the expansion to include soft materials containing self-assembling molecules and colloidal particles is a frontier that will take materials science and engineering into its next era. There is no group of trained scientists more ready to tackle these problems than chemical engineers, especially those trained in colloid and polymer sciences. Current foci and projects in the group are outlined below.
Rheology and Structure of Concentrated Block Polymer Solutions
Block copolymers are a distinct class of molecules that result from covalently bonding together strings of chemically dissimilar materials. Almost any formulation situation (solvent, temperature, additives) will be favorable for one block over the other. This drives self-assembly and the rich phase behavior of these materials and has resulted in new industries and technologies; their discovery was and continues to be transformative. Recent recognition of the added complexity (and potential) of lower molecular weight analogs in solvents brings together the polymer materials and surfactant fields and the potential for new materials development is immense. In these systems, the structures that form are non-ergodic and mobility of molecules within the system provides a dynamical control parameter that is not as accessible in high molecular weight block copolymer melts. The potential for kinetically trapping structures, using weak external fields to align and change phases, and generating novel complex structures is immense and drives this part of our work. One example that has been developed in our group is the mixture of “soft” nanoscale structures formed from block copolymer molecules and “hard” nanoparticles; this adds a level of structural hierarchy to these materials that is, at the least, just as rich as alloying in metals but more likely opens up entire new types of structural phase behavior. Preliminary evidence of quasi-crystalline order in a water swollen block copolymer micelles solution represents the potential richness of behavior in these common material systems. Identifying these behaviors is of immediate interest to industries that process surfactant pastes (to avoid process failures) and of broader interest to the soft material research field as identification of new phase behavior based on moduli and ergodicity of micelles and self-assembled objects. Using our unique expertise in rheology and small-angle scattering techniques, we are attacking the impact of weak shear fields on phase behavior in block copolymer solutions and composites.
Interfaces between two fluids (liquid-liquid and gas-liquid) are controlled by the concentration of adsorbed species. Adsorbed surface-active species control the tension (thermodynamics) and mechanics of these interfaces. Those properties in turn relate directly to pertinent properties like the stability of an emulsion, the deformability of a droplet in a microfluidic device, and the structure of a filament in 3D printing. The rate of adsorption of surface-active species to an interface is a function of bulk concentration, local flow fields, and deformation of the interface. While understanding of equilibrium and even steady state interfaces is at a high level and has allowed for control and design of high interfacial area materials, the lack of understanding of transport and mechanical properties limits the ability to engineer and efficiently process materials with internal interfaces. The ability to control processing will result in more efficient processes and allow for the development of novel materials. One example from our work, through collaboration with the Anna group at CMU, is the demonstration of jamming of nanoparticles on fluid-fluid interfaces to form rigid films at significantly lower levels of interfacial loading than previously observed. Through repeated compressions of a partially coated interface, a rigid film was generated by inducing two-dimensional flocculation on the interface. Using a microtensiometer platform developed at CMU and other capabilities on campus, we are attacking problems associated with control of spontaneous emulsification, impact of electric fields on deformable interfaces, design of demulsifiers for oil recovery and processing. In all cases, the connection between molecular/colloidal properties of surface active agents and processing history on interfaces is quantified.
High Compositional Resolution Characterization of Multicomponent Formulations
In multicomponent fluid formulations, phase and structural spaces are so complex that potentially relevant or problematic structures and phases exist over very small regions of composition space. The current approach of discreet sampling with binary or ternary component mixtures is likely to miss phenomena and structures that negatively impact processing and, in some cases, offer novel material properties. We are developing different classes of microfluidic and millifluidic processes that provide high resolution on the composition axes. Since these technques provide nearly continuous sampling along composition axes, the data is more appropriate for current data mining and ML approaches. Our use of micron through millimeter lengthscales allows the utilization of new structural sensing techniques (optical and small angle scattering) that have not been applied to these sorts of problems. One example from our work, in collaboration with Dow Chemical, was to characterize the impact of an oil-soluble dispersant (surfactant) on the stabilization of carbon black in oils. Our use of millifluidic devices with a series of discreet samples (droplets) provided detailed information on the transition of controlling phenomena with concentration. Previous understanding suggested an on/off stabilization mechanism and our results showed a gradual change from unstable to stable leading to improved mechanistic understanding and guidance for more efficient formulation. Using two classes of devices developed at CMU, we are attacking problems in liquid-liquid phase separation in aerosol droplets, nanoscale structural transitions in colloidal suspensions, viscosity of protein solutions, and details in isotropic-nematic transitions in rod-like nanoparticles.
Awards and Honors
- 2016 Barbara Lazarus Award
- AIChE Fellow, 2016
- 2015 AIChE Woman's Initiatives Committee (WIC) Mentorship Award
- NSF Career Award, 2001
- Kun Li Award for Excellence in Education, Carnegie Mellon University, 2000 and 2003
- DuPont Young Professor, 2000-2001
- NSF Postdoctoral Fellow, Katholieke Universiteit Leuven, Belgium, 1996
- Departmental Teaching Fellowship, University of Delaware, 1994
- Women of Excellence Award, University of Delaware, 1991
- Kirby, S. M.; Anna, S. L.; Walker, L. M. Effect of Surfactant Tail Length and Ionic Strength on the Interfacial Properties of Nanoparticle-Surfactant Complexes. Soft Matter 2017, 14 (1)
- Rajarshi Sengupta; Walker, L. M.; Khair, A. S. The Role of Surface Charge Convection in the Electrohydrodynamics and Breakup of Prolate Drops. J. Fluid Mech. 2017
- Dao, M. M.; Domach, M. M.; Walker, L. M. Impact of Dispersed Particles on the Structure and Shear Alignment of Block Copolymer Soft Solids. J. Rheol. 2017, 61, 237–252
- Bleier, B. J.; Yezer, B. A.; Freireich, B. J.; Anna, S. L.; Walker, L. M. Journal of Colloid and Interface Science Droplet-Based Characterization of Surfactant Efficacy in Colloidal Stabilization of Carbon Black in Nonpolar Solvents. J. Colloid Interface Sci. 2017, 493, 265–274
- Kirby, S. M.; Zhang, X.; Russo, P. S.; Anna, S. L.; Walker, L. M. Formation of a Rigid Hydrophobin Film and Disruption by an Anionic Surfactant at an Air/Water Interface. Langmuir 2016, 32 (22), 5542–5551
- Lanauze, J. A.; Walker, L. M.; Khair, A. S. Relaxation or Breakup of a Low-Conductivity Drop upon Removal of a Uniform Dc Electric Field. Phys. Rev. Fluids 2016, 1 (3), 33902
- Cheng, V. A.; Walker, L. M. Transport of Nanoparticulate Material in Self-Assembled Block Copolymer Micelle Solutions and Crystals. Faraday Discuss. 2016, 0, 1–20
- Reichert, M. D.; Alvarez, N. J.; Brooks, C. F.; Grillet, A. M.; Mondy, L. A.; Anna, S. L.; Walker, L. M. The Importance of Experimental Design on Measurement of Dynamic Interfacial Tension and Interfacial Rheology in Diffusion-Limited Surfactant Systems. Colloids Surfaces A-Physicochemical Eng. Asp. 2015, 467, 135–142
- Reichert, M. D.; Walker, L. M. Coalescence Behavior of Oil Droplets Coated in Irreversibly-Adsorbed Surfactant Layers. J. Colloid Interface Sci. 2015, 449, 480–487
- Lanauze, J. A.; Walker, L. M.; Khair, A. S. Nonlinear Electrohydrodynamics of Slightly Deformed Oblate Drops. J. Fluid Mech. 2015, 774, 245–266
- Kirby, S. M.; Anna, S. L.; Walker, L. M. Sequential Adsorption of an Irreversibly Adsorbed Nonionic Surfactant and an Anionic Surfactant at an Oil/Aqueous Interface. Langmuir 2015, 31 (14), 4063–4071