W.R. Ashurst

Auburn University, Auburn, Alabama, United States

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Publications (38)77.87 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: It is well known that the environment in which micromechanical systems operate significantly affects their performance. It is, therefore, important to characterize micromachine behavior in environments where the humidity, pressure, and chemical composition of the ambient can be precisely controlled. Achieving such a level of environmental control presents significant challenges in view of the required instrumentation. To that end, a custom micromachine characterization system is built that allows for full environmental control (pressure, humidity, and gas composition) while retaining full micromachine characterization techniques (long working distance interferometry, electrical probe connectivity, actuation scripting capability). The system also includes an effective in situ surface cleaning mechanism. As an example of the system's utility, a microcantilever crack healing experiment is conducted and surface adhesion energy measurements are tracked over time after a step change in humidity is applied.
    The Review of scientific instruments 07/2013; 84(7):075006. · 1.52 Impact Factor
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    ABSTRACT: In this study, we have developed and characterized two previously unstudied alkoxysilane surface chemistries for use with superparamagnetic iron oxide (SPIO) nanoparticles as a magnetic resonance imaging contrast agent. We modified superparamagnetic iron oxide nanoparticles (SPIO) using aminopropyl triethoxysilane and two analogous alkoxysilanes, aminopropyl dimethylethoxysilane and aminopropyl methyldiethoxysilane, to compare a mono- and dialkoxysilane, respectively, to a more commonly used trialkoxysilane as two new SPIO surface chemistries capable of forming ultrathin functional surface coatings. The ligand densities of the mono- and dialkoxysilane-modified SPIO produced in this study are consistent with near monolayers of ligands on the SPIO surface. We studied the chemical stability of the mono-, di-, and trialkoxysilane-modified SPIO in neutral and acidic media to evaluate the viability of these surface chemistries for use in long-term intracellular applications. The mono- and dialkoxysilane-modified SPIO demonstrate comparable chemical stability to the trialkoxysilane-modified SPIO, indicating that the mono- and dialkoxysilane are both viable new SPIO surface chemistries for future applications requiring minimally thick alkoxysilane surface coatings.
    Journal of Materials Research 07/2012; 27(14):1846-1852. · 1.82 Impact Factor
  • N Ansari, W R Ashurst
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    ABSTRACT: Since the advent of microelectromechanical systems (MEMS) technology, friction and wear are considered as key factors that determine the lifetime and reliability of MEMS devices that contain contacting interfaces. However, to date, our knowledge of the mechanisms that govern friction and wear in MEMS is insufficient. Therefore, systematically investigating friction and wear at MEMS scale is critical for the commercial success of many potential MEMS devices. Specifically, since many emerging MEMS devices contain more sidewall interfaces, which are topographically and chemically different from in-plane interfaces, studying the friction and wear characteristics of MEMS sidewall surfaces is important. The microinstruments that have been used to date to investigate the friction and wear characteristics of MEMS sidewall surfaces possess several limitations induced either by their design or the structural film used to fabricate them. Therefore, in this paper, we report on a single-crystal-silicon-based microinstrument to study the frictional and wear behavior of MEMS sidewalls, which not only addresses some of the limitations of other microinstruments but is also easy to fabricate. The design, modeling and fabrication of the microinstrument are described in this paper. Additionally, the coefficients of static and dynamic friction of octadecyltrichlorosilane-coated sidewall surfaces as well as sidewall surfaces with only native oxide on them are also reported in this paper.
    Journal of Micromechanics and Microengineering 01/2012; 22(2):025008. · 1.79 Impact Factor
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    ABSTRACT: Dodecanethiol-stabilized gold nanoparticles (AuNPs) were deposited via a gas-expanded liquid (GXL) technique utilizing CO(2)-expanded hexane onto substrates of different surface energy. The different surface energies were achieved by coating silicon (100) substrates with various organic self-assembled monolayers (SAMs). Following the deposition of AuNP films, the films were characterized to determine the effect of substrate surface energy on nanoparticle film deposition and growth. Interestingly, the critical surface tension of a given substrate does not directly describe nanoparticle film morphology. However, the results in this study indicate a shift between layer-by-layer and island film growth based on the critical surface tension of the capping ligand. Additionally, the fraction of surface area covered by the AuNP film decreases as the oleophobic nature of the surfaces increases. On the basis of this information, the potential exists to engineer nanoparticle films with desired morphologies and characteristics.
    Langmuir 10/2011; 27(2):651-5. · 4.38 Impact Factor
  • N. Ansari, K. M. Hurst, W. R. Ashurst
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    ABSTRACT: Commercialization of a whole spectrum of useful MEMS is still hindered by surface phenomena that dominate at the micron scale. Altering the roughness and surface chemistry of MEMS surfaces by depositing nanoparticles on them is being considered by the MEMS community as a useful strategy to address tribological issues. Although, gold nanoparticle monolayer is reported to reduce adhesion in MEMS, determining its surface coverage still remains a challenge [1]. A technique to determine the surface coverage of deposited gold nanoparticles is needed, so that its effect on the tribology of MEMS surfaces can be studied.
    05/2011: pages 67-68;
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    ABSTRACT: Self-assembled monolayer (SAM) films of p-aminophenyl trimethoxysilane (APhTS) and 3-mercaptopropyl trimethoxysilane (MPTS) were used to immobilize gold nanoparticles (AuNPs) on silicon substrates and silicon-based microdevices, which created robust nanoparticle coatings that reduced microstructure adhesion. The terminal groups of APhTS and MPTS have both been previously shown to strongly interact and/or bind with metals and metallic nanoparticles. Scanning electron microscopy (SEM) analysis indicated that APhTS and MPTS monolayers improved the adhesion of gold nanoparticles deposited on silicon substrates and microstructures. SEM analysis also showed that the gold nanoparticle/organic monolayer (AuNP/APhTS or AuNP/MPTS) films were more robust than non-immobilized AuNP coatings toward both cantilever beam mechanical contact and water erosion testing. The combination of the rough, lower-energy surfaces of AuNP/APhTS and AuNP/MPTS films also effectively reduced the adhesion exhibited between microstructured surfaces by nearly two orders of magnitude as measured by the apparent work of adhesion. Smooth native oxide-coated Si(100) in-plane surfaces typically have an adhesion energy in excess of 30 mJ/m<sup>2</sup> while AuNP/APhTS and AuNP/MPTS coatings reduced the adhesion energy to 0.655 and 1.66 mJ/m<sup>2</sup>, respectively.
    Journal of Microelectromechanical Systems 05/2011; · 2.13 Impact Factor
  • N. Ansari, W. R. Ashurst
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    ABSTRACT: Tribology remains an active area of research for the Micro-electromechanical Systems (MEMS) community. At the micron scale, which is the scale relevant to commercial MEMS, a number of factors namely roughness, apparent contact area, surface topography, surface chemistry, etc. are known to have a significant impact on the tribological properties. Historically, researchers have found it difficult to study the effects of these factors individually. We report on a test platform designed and fabricated using a single mask scheme within a relatively smooth SOI wafer. The test platform includes several different micromechanisms on the same chip so that a systematic investigation of the factors that influence tribology in MEMS can be carried out. The test platform is therefore an ideal stage for testing and comparing the various strategies that can be used to address the tribological issues that are presently plaguing the MEMS community.
    03/2011: pages 85-95;
  • N. Ansari, K. M. Hurst, W. R. Ashurst
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    ABSTRACT: Since the advent of Micro-electromechanical systems (MEMS) technology, researchers have used surface texturing as one of the approaches to alleviate unintentional adhesion in MEMS. However, the conventional methods used for surface texturing are reported to reduce apparent in-plane adhesion only by a factor of 20. Further, the test surfaces used to-date are inherently rough, as a result of which, the effects of surface texturing could not be studied independently. We report on a novel method of texturing inherently smooth Si(100) surfaces by depositing dodecanethiol capped gold nanoparticles using a gas-expanded liquid technique. The dodecanethiol capping ligands are removed by exposing the treated surfaces to UV-Ozone atmosphere for an hour and the textured surfaces thus obtained are characterized by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The textured Si(100) surfaces exhibit a significant reduction in apparent in-plane work of adhesion, which is determined using the cantilever beam array (CBA) technique, compared to untextured smooth Si(100) surfaces having only native oxide on them.
    03/2011: pages 181-187;
  • N. Ansari, W. R. Ashurst
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    ABSTRACT: This paper reports on a MEMS device that can be used to study sidewall stiction. Details on the design, fabrication, modeling and testing of the device are reported. The device is easy to fabricate and overcomes some of the limitations of other devices. The apparent adhesion energy of OTS SAM coated sidewalls is approximately 38 muJ/m2.
    Applied Surface Science 01/2011; 257(24):10917-10925. · 2.54 Impact Factor
  • N Ansari, W R Ashurst
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    ABSTRACT: Digital phase-shifting interferometry (PSI), a technique widely used in optical testing, requires interferograms collected at optical phase differences separated by a definite phase step. The five-frame interferogram-collecting sequence suggested by Hariharan et al. [Appl. Opt. 26, 2504 (1987)] is extremely effective in significantly reducing the errors in height profiles derived using PSI that are caused by phase-step errors. In this Letter, we report on a class of five-frame sequence that, owing to its mathematical equivalence with the one suggested by Hariharan et al. and its ease of execution, is more commonly used but is much less effective in reducing the height profile errors caused by phase-step errors.
    Optics Letters 01/2011; 36(2):214-6. · 3.39 Impact Factor
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    ABSTRACT: In an effort to improve the reliability of microelectromechanical systems (MEMS), silica thin films deposited by chemical vapor deposition were used to encapsulate gold nanoparticle coatings. These composite coatings were shown to provide extremely durable films that significantly reduce the adhesion energy of silicon-based microcantilever beams. The results discussed suggest that encapsulating nanoparticle films with a durable silica thin film may lead to improved MEMS reliability.
    Journal of Microelectromechanical Systems 01/2011; 20(5):1065-1067. · 2.13 Impact Factor
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    ABSTRACT: This paper reports on a novel gold nanoparticle (AuNP) coating, which is deposited on micro-electromechanical systems (MEMS) surfaces using a gas-expanded liquid technique and characterized using atomic force microscopy (AFM), scanning electron microscopy (SEM) and several microinstruments. Surface coverage of the AuNP coating, which is determined using a resonating microinstrument; and the tribological properties including the work of adhesion, the coefficient of static friction and the rupture strength of the AuNP coating, which are determined using specific microinstruments, are reported in the paper. effect of nanoparticle (NP) based surface texturing, reported only moderate reduction in stiction between textured in- plane surfaces, we postulated that the effectiveness of the NP coating was reduced by the inherent roughness of the polysilicon test surfaces used in those investigations. Therefore, we investigated the effect of AuNP coating on stiction between relatively smooth single crystal silicon in- plane surfaces. Our investigations revealed that the effectiveness of AuNP based surface texturing not only depends on the inherent roughness of the test surfaces but also depends strongly on the surface coverage of the coating. In this paper, we report the reduced stiction between relatively smooth single crystal silicon in-plane MEMS surfaces textured using the AuNP coating with the most effective surface coverage. Further, the effect of the same AuNP coating on stiction as well as friction between sidewall surfaces is reported. Surface coverage and rupture strength of the studied AuNP coating are also reported in the paper.
    Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS) 01/2011;
  • A. Poda, A. Anderson, W.R. Ashurst
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    ABSTRACT: A MVD silica layer that consists of a highly hydrated surface favorable for organosilane surface reaction is investigated. The MVD silica layer lacks free surface silanol groups while supporting a more extensive adsorbed water layer as compared to oxidized Si(1 0 0). Octadecyltrichlorosilane monolayers (OTS) deposited on the MVD silica layer are found to follow the same mechanisms of growth and exhibit properties comparable to those formed on oxidized Si(1 0 0) surfaces. The growth process of octadecylsiloxane films is investigated as a function of immersion time and temperature by utilizing ATR-FTIR, ellipsometry, contact angle analysis, and AFM. The MVD silica layer is shown to support an ordered interfacial water structure that is more tightly bound due to a higher degree of hydrogen bonding associated with the hydroxylated surface. The importance of interfacial water on the OTS film formation process is highlighted and the role of free OH groups on the adsorption mechanism is diminished. It is shown that OTS films can be formed on a highly hydrated surface comparable to those formed on oxidized Si(1 0 0) surfaces.
    Applied Surface Science 01/2010; 256(22):6805-6813. · 2.54 Impact Factor
  • A Anderson, W R Ashurst
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    ABSTRACT: Given the large surface area-to-volume ratios commonly encountered in microfluidics applications, the ability to engineer the chemical properties of surfaces encountered in these applications is critically important. However, as various polymers are rapidly replacing glass and silicon as the chosen materials for microfluidics devices, the ability to easily modify the surface chemistry has been diminished by the relatively inert nature of some commonly employed polymer surfaces, such as poly(methyl methacrylate) (PMMA), polystyrene, and polydimethylsiloxane (PDMS). This paper describes the low-temperature, vapor-phase deposition of robust silica layers to PMMA, polystyrene, and PDMS surfaces, which enables the functionalization of these surfaces by standard organosilane chemistries. Attenuated total reflection infrared spectroscopy, contact angle goniometry, ellipsometry, and atomic force microscopy are used to characterize the silica layers that form on these surfaces. Aqueous immersion experiments indicate that the silica layer has excellent stability in aqueous environments, which is a prerequisite for microfluidics applications, but for PMMA surfaces, low adhesion of the silica layer to the underlying substrate is problematic. For PDMS substrates, the presence of the silica layer helps to slow the process of hydrophobic recovery, which is an additional advantage.
    Langmuir 09/2009; 25(19):11541-8. · 4.38 Impact Factor
  • A Anderson, W R Ashurst
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    ABSTRACT: Evidence suggests that in order for a surface to support an extensive water structure, it must possess sufficient Lewis cites so that water-surface interactions are favored over water-water interactions. In this paper we use ATR-FTIR to comparatively study, as a function of relative humidity, the water structure that exists on three surfaces: silicon, PEG-modified silicon, and a highly hydroxylated silica film which is formed from the room temperature, vapor phase hydrolysis of tetrachlorosilane. Results indicate that the PEG-modified silicon surface supports a water structure nearly 2.5 times as extensive as that which exists on unmodified silicon surfaces, which is an expected result in light of previous molecular dynamics simulations that indicate extensive hydrogen bonding between PEG monolayers and water molecules. The silica layer supports a water structure that is nearly an order of magnitude more extensive than that which exists on clean silicon surfaces and approximately 3.5 times more extensive than is adsorbed on PEG-modified silicon surfaces at similar relative humidities. Furthermore, the water layer on the silica surface exists mostly in an "ice-like" structure which is also more strongly hydrogen bonded than that which exists on clean silicon and PEG-modified silicon surfaces.
    Langmuir 08/2009; 25(19):11549-54. · 4.38 Impact Factor
  • K M Hurst, C B Roberts, W R Ashurst
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    ABSTRACT: A gas-expanded liquid-based nanoparticle deposition technique was integrated with a critical point drying process to modify the surface of polysilicon microstructures in order to reduce the adhesion that ordinarily occurs due to dominant interfacial surface forces. Dodecanethiol-capped gold nanoparticles (AuNPs) were deposited onto arrays of cantilever beams using gas-expanded liquid technology in an effort to increase the surface roughness, thereby reducing the real contact surface area as well as changing the chemical constituents of the contacting areas. Both AuNP-coated and uncoated (native oxide surface) arrays were actuated electrostatically in order to determine the work of adhesion. The results of this study indicate that while cantilever beams with only their native oxide exhibit apparent adhesion energies of about 700 +/- 100 microJ m(-2), cantilever beam arrays coated with AuNPs exhibit an apparent adhesion energy of about 8 microJ m(-2) or less. These results indicate that metallic nanoparticle coatings can be successfully applied to micromachines and provide a drastic reduction in apparent adhesion energy.
    Nanotechnology 06/2009; 20(18):185303. · 3.84 Impact Factor
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    ABSTRACT: A new method for calculating the apparent work of adhesion of microfabricated cantilever beams is presented and validated. This technique, based on popular beam mechanics, directly utilizes interferometrically produced deflection experimental data. The data are analyzed based on an energy minimization approach along the lines of a previous methodology. However, the new technique differs in that it does not rely on a priori knowledge of the shape of cantilever beams to evaluate work of adhesion. In order to validate the new method, both synthetic and empirical cantilever beam data were examined. The results show that apparent work of adhesion values calculated using this method agree well with values determined using a technique previously developed and widely accepted.
    Tribology Letters 01/2009; 35(1):9-15. · 2.15 Impact Factor
  • A Anderson, W Robert Ashurst
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    ABSTRACT: Surface modification reactions by organosilicon compounds have demonstrated great success in a wide variety of applications. However, they are of limited usefulness in that they only proceed appreciably on surfaces that have an abundance of reactive hydroxyl groups, thus preventing their application to some materials of technological relevance, such as plastics and polymers. A process capable of depositing a surface rich in reactive hydroxyl groups onto a wide variety of substrates could potentially enable the extension of organosilane surface modification reactions to new materials, but conventional processes for depositing oxide layers require temperatures that are too high for most polymers and plastics. It has been shown that silica layers can be deposited from the vapor-phase hydrolysis of tetrachlorosilane at room temperature, but little if any work has been done to characterize the resulting films. In this work, ellipsometry, atomic force microscopy, and Fourier transform infrared spectroscopy are employed to study the characteristics of films formed from this process. Interestingly, very different film morphologies can be obtained by changing key processing parameters. Furthermore, isotopic exchange experiments and dehydration studies show that the surfaces of the silica films obtained by this method are composed entirely of hydrogen-bonded silanol groups and do not exhibit any freely vibrating surface silanol groups, a result that is in contrast with conventionally prepared silica materials. Still, this layer has been shown to behave very similarly to conventional silica materials with respect to surface reactions. Finally, infrared spectral data and contact angle data demonstrate that this method can be employed to deposit silica layers onto poly(methyl methacrylate) and polystyrene surfaces.
    Langmuir 08/2008; 24(15):7947-54. · 4.38 Impact Factor
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    ABSTRACT: A thermodynamic model was developed for the size-selective fractionation of ligand-stabilized nanoparticles by a CO2 gas-expanded liquid precipitation process. The tunable solvent strength of gas-expanded liquids, via CO2 pressurization, results in an effective method to fractionate nanoparticles, based on the size-dependent dispersibility of the particles. Specifically, the thermodynamic model is used to estimate the size of dodecanethiol-capped Ag nanoparticles that can be dispersed at a given CO2 pressure by equating the total interparticle interaction energy to the Boltzmann threshold stabilization energy (−3/2kBT). The ligand−solvent interaction is found to have the greatest impact on the total interaction energy. This model illustrates that the entire length of the ligand is not accessible to the solvent, and three phenomenological model variations were developed to vary the ligand−solvent interaction.
    Industrial & Engineering Chemistry Research - IND ENG CHEM RES. 01/2008; 47(3).
  • A. Anderson, W. R. Ashurst
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    ABSTRACT: Previously, a “seed” layer has been demonstrated to improve the thermal and immersion stability of organosilane films deposited on titanium nitride and aluminum substrates through the creation of a highly hydroxylated surface favorable for organosilane surface reaction. In order to determine if a similar silica seed layer can improve the stability of similar monolayers deposited on surfaces that are favorable for organosilane surface modification in their native state, the thermal and aqueous immersion stability of various alkylsilane molecular films on clean silicon oxide surfaces and on silicon oxide surfaces treated with the silica seed layer have been investigated. Contact angle, ellipsometry and atomic force microscopy are employed to monitor samples treated with various alkylsilane monolayers as they are subjected to elevated temperatures and aqueous immersion. Nearly identical trends in contact angle and ellipsometric data demonstrate that the silica seed layer does not improve the thermal and aqueous immersion stability of alkylsilane films on silicon oxide surfaces. Furthermore, this study demonstrates the importance of collecting both contact angle and ellipsometric data during immersion experiments, as reliance on contact angle data alone may lead to faulty conclusions regarding film stability.
    Thin Solid Films 01/2008; 516(21):7538-7546. · 1.87 Impact Factor

Publication Stats

596 Citations
77.87 Total Impact Points

Institutions

  • 2008–2013
    • Auburn University
      • Department of Chemical Engineering
      Auburn, Alabama, United States
  • 2001–2004
    • University of California, Berkeley
      • Department of Chemical and Biomolecular Engineering
      Berkeley, CA, United States
  • 2002
    • CSU Mentor
      Long Beach, California, United States