Stanford Advisors


All Publications


  • Insights into the Mechanical Properties of Ultrathin Perfluoropolyether-Silane Coatings. Langmuir : the ACS journal of surfaces and colloids Zhao, O., Collinson, D. W., Ohshita, S., Naito, M., Nakano, N., Tortissier, G., Nomura, T., Dauskardt, R. H. 2022

    Abstract

    Ultrathin perfluoropolyether-silane (PFPE-silane) films offer excellent functionality as antifingerprint coatings for display touchscreens due to their oleophobic, hydrophobic, and good adhesion properties. During smartphone use, PFPE-silane coatings undergo many abrasion cycles which limit the coating lifetime, so a better understanding of how to optimize the film structure for improved mechanical durability is desired. However, the hydrophobic and ultrathin (1-10 nm) nature of PFPE-silane films renders them very difficult to experimentally characterize. In this study, the cohesive fracture energy and elastic modulus, which are directly correlated with hardness and better wear resistance of 3.5 nm-thick PFPE-silane films were, respectively, measured by double cantilever beam testing and atomic force microscopy indentation. Both the cohesive fracture energy and modulus are shown to be highly dependent on the underlying film structure. Both values increase with optimal substrate conditions and a higher number of silane groups in the PFPE-silane precursor. The higher cohesive fracture energy and modulus values are suggested to be the result of the changes in the film chemistry and structure, leading to higher cross-linking density. Therefore, future work on optimizing PFPE-silane film wear resistance should focus on pathways to improve the cross-linking density. Subcritical fracture testing in humid environments reveals that humidity negatively affects the fracture properties of PFPE-silane films.

    View details for DOI 10.1021/acs.langmuir.2c00625

    View details for PubMedID 35543410

  • Direct evidence of interfacial crystallization preventing weld formation during fused filament fabrication of poly(ether ether ketone) ADDITIVE MANUFACTURING Collinson, D. W., von Windheim, N., Gall, K., Brinson, L. 2022; 51
  • Best practices and recommendations for accurate nanomechanical characterization of heterogeneous polymer systems with atomic force microscopy PROGRESS IN POLYMER SCIENCE Collinson, D. W., Sheridan, R. J., Palmeri, M. J., Brinson, L. 2021; 119
  • The influence of porosity, crystallinity and interlayer adhesion on the tensile strength of 3D printed polylactic acid (PLA) RAPID PROTOTYPING JOURNAL von Windheim, N., Collinson, D. W., Lau, T., Brinson, L., Gall, K. 2021
  • Distribution of rubber particles in the weld zone of fused filament fabricated acrylonitrile butadiene styrene and the impact on weld strength ADDITIVE MANUFACTURING Collinson, D. W., Kolluru, P. V., Windheim, N., Brinson, L. 2021; 41
  • Tapered Polymer Whiskers to Enable Three-Dimensional Tactile Feature Extraction SOFT ROBOTICS Collinson, D. W., Emnett, H. M., Ning, J., Hartmann, M. Z., Brinson, L. 2021; 8 (1): 44-58

    Abstract

    Many mammals use their vibrissae (whiskers) to tactually explore their surrounding environment. Vibrissae are thin tapered structures that transmit mechanical signals to a wealth of mechanical receptors (sensors) located in a follicle at each vibrissal base. A recent study has shown that-provided that the whisker is tapered-three mechanical signals at the base are sufficient to determine the three-dimensional location at which a whisker made contact with an object. However, creating biomimetic tapered whiskers has proved challenging from both materials and manufacturing standpoints. This study develops and characterizes an artificial whisker for use as part of a sensory input device that is a biomimic of the biological rat whisker neurosensory system. A novel manufacturing process termed surface conforming fiber drawing (SCFD) is developed to produce artificial whiskers that meet the requirements to be a successful mechanical and geometric mimic of the biological rat vibrissae. Testing the sensory capabilities of the artificial whisker shows improved performance over previous nontapered filaments. SCFD-manufactured tapered whiskers demonstrate the ability to predict contact point locations with a median distance error of 0.47 cm.

    View details for DOI 10.1089/soro.2019.0055

    View details for Web of Science ID 000539637100001

    View details for PubMedID 32513071

  • Deconvolution of Stress Interaction Effects from Atomic Force Spectroscopy Data across Polymer-Particle Interfaces MACROMOLECULES Collinson, D. W., Eaton, M. D., Shull, K. R., Brinson, L. 2019; 52 (22): 8940-8955
  • Temperature effects on the nanoindentation characterization of stiffness gradients in confined polymers SOFT MATTER Song, J., Kahraman, R., Collinson, D. W., Xia, W., Brinson, L., Keten, S. 2019; 15 (3): 359-370

    Abstract

    The stiffening of polymers near inorganic fillers plays an important role in strengthening polymer nanocomposites, and recent advances in metrology have allowed us to sample such effects using local mechanical measurement techniques such as nanoindentation and atomic force microscopy. A general understanding of temperature and confinement effects on the measured stiffness gradient length-scale ξint is lacking however, which convolutes molecular interpretation of local property measurements. Using coarse-grained molecular dynamics and finite element nanoindentation simulations, we show that the measured ξint increases with temperature in highly confined polymer systems, a dependence which acts in the opposite direction in systems with low confinement. These disparate trends are closely related to the polymer's viscoelastic state and the resulting changes in incompressibility and dissipative ability as the polymer transitions from glassy to rubbery. At high temperatures above the glass transition temperature, a geometrically confined system restricts the viscous dissipation of the applied load by the increasingly incompressible polymer. The indentation causes a dramatic build-up of hydrostatic pressure near the confining surface, which contributes to an enlarged measurement of ξint. By contrast, a less-confined system allows the pressure to dissipate via intermolecular motion, thus lowering the measured ξint with increased temperature above the glass transition temperature. These findings suggest that the well-established thin film-nancomposite analogy for polymer mobility near interfaces can be convoluted when measuring local mechanical properties, as the viscoelastic state and geometric confinement of the polymer can affect the nanomechanical response during indentation purely from continuum effects.

    View details for DOI 10.1039/c8sm01539b

    View details for Web of Science ID 000457278300017

    View details for PubMedID 30421764

  • AFM-based Dynamic Scanning Indentation (DSI) Method for Fast, High-resolution Spatial Mapping of Local Viscoelastic Properties in Soft Materials MACROMOLECULES Kolluru, P. V., Eaton, M. D., Collinson, D. W., Cheng, X., Delgado, D. E., Shull, K. R., Brinson, L. 2018; 51 (21): 8964-8978
  • Open-source micro-tensile testers via additive manufacturing for the mechanical characterization of thin films and papers PLOS ONE Nandy, K., Collinson, D. W., Scheftic, C. M., Brinson, L. 2018; 13 (5): e0197999

    Abstract

    The cost of specialized scientific equipment can be high and with limited funding resources, researchers and students are often unable to access or purchase the ideal equipment for their projects. In the fields of materials science and mechanical engineering, fundamental equipment such as tensile testing devices can cost tens to hundreds of thousands of dollars. While a research lab often has access to a large-scale testing machine suitable for conventional samples, loading devices for meso- and micro-scale samples for in-situ testing with the myriad of microscopy tools are often hard to source and cost prohibitive. Open-source software has allowed for great strides in the reduction of costs associated with software development and open-source hardware and additive manufacturing have the potential to similarly reduce the costs of scientific equipment and increase the accessibility of scientific research. To investigate the feasibility of open-source hardware, a micro-tensile tester was designed with a freely accessible computer-aided design package and manufactured with a desktop 3D-printer and off-the-shelf components. To our knowledge this is one of the first demonstrations of a tensile tester with additively manufactured components for scientific research. The capabilities of the tensile tester were demonstrated by investigating the mechanical properties of Graphene Oxide (GO) paper and thin films. A 3D printed tensile tester was successfully used in conjunction with an atomic force microscope to provide one of the first quantitative measurements of GO thin film buckling under compression. The tensile tester was also used in conjunction with an atomic force microscope to observe the change in surface topology of a GO paper in response to increasing tensile strain. No significant change in surface topology was observed in contrast to prior hypotheses from the literature. Based on this result obtained with the new open source tensile stage we propose an alternative hypothesis we term 'superlamellae consolidation' to explain the initial deformation of GO paper. The additively manufactured tensile tester tested represents cost savings of >99% compared to commercial solutions in its class and offers simple customization. However, continued development is needed for the tensile tester presented here to approach the technical specifications achievable with commercial solutions.

    View details for DOI 10.1371/journal.pone.0197999

    View details for Web of Science ID 000433521800038

    View details for PubMedID 29813103

    View details for PubMedCentralID PMC5973562