MRS Fall 2011 Meeting & Exhibit
November 28 – December 2, 2011 – Boston, MA
Varanasi Group members Adam Paxson, Alex Slocum, Chris Love, Yuehua Cui, Ghazal Azimi and Professor Kripa Varanasi will present their research at the Hynes Convention Center here in Boston for the MRS Fall Meeting & Exhibit.
1) Adam: Tues. 29th, 9:00am-9:15pm: Superhydrophobic Hierarchical polymer Surfaces via Single-Step Solvent -Induced Crystallization. Y. Cui, A. T. Paxson, K. M. Smyth and K. Varanasi
Superhydrophobic surfaces are widely present in nature and have inspired a significant body of research due to their wide range of applications including self-cleaning, drag reduction, and droplet impact resistance. Although synthetic superhydrophobic surfaces are now prevalent, their means of manufacture often are expensive and time-consuming, or require exotic materials and tools. Here we report on a rapid, single-step method using simple materials to produce large-area superhydrophobic surfaces via acetone-induced phase transformation of polycarbonate. Crystallization of the polymer leads to the formation of a hierarchical structure composed of microporous spherulites covered with nano-fibrils, and results in superhydrophobic wetting behavior. We systematically investigate the influence of the solvent treatment time on the surface morphology and wettability of the polycarbonate and outline the mechanism of structure formation. The resulting surfaces exhibit high contact angles, low contact angle hysteresis, and complete dewetting during droplet impact. Theoretical analysis of the wetting and anti-wetting pressures show that the nano-scale morphology is critical for achieving droplet impact resistance. This simple phase transformation approach could be more broadly applied to other solvent-polymer systems for fabricating large-area hierarchical surface textures.
2) Yuehua: Tues. 29th, 8:00pm-11pm: Preparation of Hierarchical Structures by Anodization of Patterned Metal Microparticles. Y. Cui, A. T. Paxson, R. Dhiman and K. Varanasi
Anodization of metals, especially Al, has inspired a significant body of research due to the resulting well-ordered nanostructures and their applications in photonic structures, superhydrophobic/philic surfaces, catalysis, and fuel cells. Most studies focus on the formation of textured surfaces with a single length scale, for example hexagonal arrays of nanopores on a flat surface. Although a few anodization studies have been performed on of microstructured substrates, the fabrication of hierarchical structures by anodization has not been extensively studied. Here we develop a new low-cost process for producing hierarchical structures across large areas. A monolayer of metal microparticles is coated on a metal substrate using a polymer adhesive layer, and then anodized to produce nanopores. The dependence of the nanopore dimensions and spacing on electrolyte composition, anodization voltage, and substrate geometry was systematically studied, and we develop a design space for obtaining desired hierarchical structures. We further investigate the wetting properties of these surfaces in such scenarios as drop impact and shedding, condensation, and boiling. We find that the addition of a microscale roughness to the conventional single length scale nanostructures results in superior droplet impact and shedding properties.
3) Ghazal: Tues. 29th, 8:00pm-11pm: Nanoengineered Surfaces and Coating Technologies for Scale Remediation. G. Azimi, Y. Cui, J. D. Smith and K. Varanasi
In this work, durable coatings and innovative surfaces with significantly improved scale nucleation and adhesion properties were engineered and fabricated. To this end, a fundamental understating of the surface chemistry effect on the scale adhesion strength was established, which then made it possible to design the surfaces with reduced scale-surface interactions by manipulating both the surface chemistry and morphology (heterogeneity and texture). A number of self-assembling organic silane coatings with various surface energies were deposited on glass substrates to investigate the effect of passive material coatings on the scale nucleation and adhesion. Subsequently, a number of innovative nanoengineered surfaces with a systematic variation in surface energy were developed. The new coatings and surfaces were tested in-situ to examine the adhesion and growth of mineral scales of sodium chloride and calcium sulfate. The results showed significant improvement in “scale-phobicity” of the surfaces; about 10-fold reduction in both growth rate and adhesion strength between bare steel and coated surfaces was observed. The advancing and receding contact angles of three liquids (water, ethylene glycol, and diiodomethane) on the substrates were measured and used to quantify the surface energies utilizing the van Oss-Chaudhury-Good analysis. The results of this work would provide a pathway to explore and exploit durable nanoengineered designs with controlled scale nucleation and adhesion properties. Despite the perseverance of the scaling problems, no previous studies has been undertaken to investigate practical solutions for scale prevention or mitigation by reducing the adhesion forces between scale and the surface. Successful deployment of the developed surfaces and coatings would reduce costs involved in scale inhibition and remediation and would improve process reliability by preventing catastrophic failures in various industrial sectors.
4) Chris: Fri. 2nd, 9:15am-9:30am: Hierarchical Nanowire Structures by Thermal Oxidation of Metals. C. Love, J. D. Smith, Y. Cui and K. Varanasi
Thermal oxidation of metals is a simple and scalable process to form metal oxide nanowires, which are useful in a variety of applications including gas-sensors, nanoelectronics, energy harvesting, and photonics. We are particularly interested in copper oxide for its applications as a hybrid wetting surface, oxygen carrier in chemical looping combustion, and good catalytic properties. Prior work has demonstrated the formation of copper oxide nanowires on foils, grids, and wires. We report the single-step formation of a copper oxide hierarchical structure by thermal oxidation of copper powder in ambient air. The hierarchical structure includes hollow spherical particles with nanowires protruding outward from the surface. The resulting nanowire coverage is found to depend on initial particle size, thereby making the hierarchical features tunable. Systematic thermogravimetric analysis (TGA) and in-situ x-ray diffraction (XRD) studies of thermal oxidation of particles of different sizes provide insights into the size-dependent process and evolution of the various phases of copper and copper oxide with time. We propose a mechanism of hollow particle formation based on the Kirkendall effect. The unique tunability of hierarchical features and hollow interior can be applied in a variety of areas including thermal management and catalysis.
5) Alex: Fri. 2nd, 10:45am-11:00am: Nano-Structured Contoured Surface Coatings for Orthopaedic Implants. A. Slocum, A. Paxson1 and Kripa K. Varanasi
The application of porous coatings to the surfaces of orthopaedic implants can result in drastic improvements in the degree of integration of the implant into the surrounding tissue, and also the subsequent overall success of the procedure. In some implants, for example the current bio-mimetic arrangement of metal-on-polymer joints used in Total Knee Arthroplasty (TKA), loading conditions result in fracture of the soft polymer bearing surface into nm to micron-sized particles. The body’s natural immune response to diffusion of these particles throughout the synovial fluid, referred to as “particle disease”, results in periprosthetic osteolysis; bone surrounding the implant is resorbed into the body. This, in addition to stress shielding more commonly seen in the stems of Total Hip Arthroplasty devices, can result in significant bone resorption and subsequent catastrophic failure of the implant. In order to counteract these processes, porous oxide coatings fabricated directly into the surface of implants can be modeled as open-cell foams, and their modulus contoured using nano-fabrication techniques to match that of the adjacent trabecular bone. Additionally, the resulting “elastically averaged” implant/bone interface should increase the degree to which osteoblasts integrate into the implant suggesting formation of a more structurally sound joint. Results of nano-indentation tests on porous coatings will be compared to moduli predicted using a theoretical foam modulus, as well as a proposed method of modeling the coating/implant interface.
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