(4bf) Soft Materials Mechanics: Instabilities, Hierarchies, and Anisotropies

My main scientific interest lies in understanding fundamental mechanisms governing deformation and failure in soft materials, e.g., hydrogels, elastomers, and carbon nanotubes, for the advancement of socially impacting technologies such as soft robotics, biomaterials, and low energy devices. To this end, I have acquired experience in multiple areas of expertise including small-scale mechanics of materials, finite element mechanical modeling, polymer science, and mean-field theory. I am currently a post-doctoral researcher at the University of Massachusetts–Amherst working under Prof. Alfred Crosby.  Previously, I completed my Ph.D. in Chemical Engineering with Prof. Julia Greer in 2011 and M.S. with Prof. Zhen-Gang Wang in 2006 at the California Institute of Technology. During this time, I gave numerous technical talks and posters and afterward was recognized with two thesis awards: the Demetriades-Tsafka-Kokkalis Prize for best thesis in “Nanotechnology and Related Fields” at Caltech and the 2013 international Quadrant Thesis Award presented in Zurich, Switzerland. I obtained my B.S. in Chemical Engineering in 2004 from Oklahoma State University. Given both my research and professional experience, I believe I would make a good colleague and enthusiastic addition to any department looking to start or expand their participation in soft materials research.

Research Background

Postdoctoral Research

I play a role in several projects in the Crosby lab at UMass-Amherst, with my main focus on two areas: understanding and utilizing a soft materials characterization technique developed in that lab called Cavitation Rheology, and using nanoparticle assembly to enhance performance in organic photovoltaic active layers. In the former, I am combining analytical theory, finite element calculation, and a simple experimental technique to determine the instability mechanism, either cavitation or fracture, for rapid void growth at the tip of an embedded, pressurized needle. This knowledge is then utilized for the localized characterization of soft materials, e.g., hydrogels and biomaterials as well as for probing length scale dependent properties ranging from microns to millimeters depending on the size of needle used. In addition to providing spatial resolution for investigation of graded materials, this localized, size dependent mechanical probe presents a promising method to evaluate hierarchical materials. In the latter project, I am exploring processing methods by which to pattern 2-D nanoscopic assemblies of functionalized quantum dots. The aim of these assemblies is to provide a flexible platform, e.g., controllable geometry and readily interchangeable component materials, from which to study the complex charge transfer between photoactive organic materials while providing a percolated, but still pliant, pathway for enhanced charge extraction. These projects have given me expertise in mechanical instabilities in soft materials, particularly polymers, a potential characterization technique for hierarchical material properties, and the fabrication of mesoscopic polymer-like composite structures.

Graduate Research

My past studies have ranged from the mechanical characterization of vertically aligned carbon nanotubes (VACNTs) to an analytical theory for charged macromolecule solutions.  While often associated with a high tensile modulus and toughness, VACNTs are as soft as many elastomers when responding collectively in as-grown structures. Additionally, VACNTs possess many of the excellent electrical and thermal properties of their carbon nanotube constituents, making them potential candidates for applications requiring multiple, extreme functionalities and properties such as energy dampers in space. These foam-like materials undergo a distinctive layer-by-layer, localized folding response when compressed uniaxially. For my thesis research, I both observed this response in situ and reproduced key elements of it numerically using the combination of finite element calculations and a proposed constitutive model that included a material instability. In my earliest studies, I explored a mechanism for the condensation of charged macromolecule solutions, e.g., proteins, which have been observed to undergo a two-step nucleation mechanism that including small intermediate clusters before crystalizing. I utilized thermodynamics, specifically density functional theory, to develop an analytical prediction for metastable cluster intermediates as a function of ion concentration and relative dielectric constant. This work gave me experience in applying fundamental thermodynamic principles to generate and numerically solve complicated sets of equations. These projects have given me expertise in an atypical soft-material, VACNTS, numerical calculation with finite elements, the development of constitutive relations, and theoretical thermodynamics.

Proposed Research

As an independent researcher, my aim is to leverage my experimental and numerical expertise in large strain, large displacement, and instabilities in soft materials to open and explore additional subfields of study. In particular, I am interested in two fundamental areas:

1. Systems that achieve large deformation using small strain. What are the necessary geometric and material attributes for such systems? How can the response timescale of such systems be tuned? Answers to these questions will enhance design practice in soft robotics and explore paths leading to the adoption of phyto-inspired, low energy input movement.  I aim to elucidate these answers using phyto-inspired, artificial cellular materials with well-controlled geometries and common, non-toxic solid-fluid elements. Plant tissues are known to undergo nastic movements on timescales ranging from milliseconds to hours using only a narrow range of materials properties, varying their microstructure, and employing instabilities, diffusion, and phase change as driving forces.
2. Materials that are both conformal/connective, while being low stress/non-damaging. What intensive and inhomogeneous properties lead to the optimization of this type of behavior? Can such materials be made in a manipulable and processable way? One challenge for which soft materials are ideally suited is conformal adoption of a high aspect ratio from an initial 2-D geometry. An example of an application for this behavior is at an artificial interface with neurons. Current contacts achieve only short lifetimes before cells become inactive, which is thought to be due to an unfavorable mechanical environment. I will approach the transmittance of stress upon large deformations through the study of materials with controlled spatial and material anisotropy, e.g., graded elastomers, graded cellular solids, VACNTs, and CNT/polymer composites. Hierarchical materials in which conformability can vary with length scale will also be fabricated using nanoparticle assembly techniques to create flexible, mesoscale network super-structures.

Teaching and Professional Activities

In addition to my scholarly interests, I have spent time broadening my professional experience through teaching assignments, lecture organization, and outreach to under-represented demographics. I was a teaching assistant twice each for two graduate level courses at Caltech, statistical thermodynamics and polymer physics. As a result of this experience and in conjunction with my research expertise, I can teach courses in several subject areas including thermodynamics, polymer physics, and mechanics of materials. As a graduate student, I organized and hosted seven speakers for the Materials Science Department departmental lecture series as well as serving on the Everhart Lecture Series Committee which yearly selects three outstanding graduate student researcher/communicators. At both Caltech and UMass-Amherst, I have served as a mentor for women-in-science through a mentoring program and as a participant on two panels discussing success in graduate school.