Dynamics of Fattening and Thinning 2D Sessile Droplets

We investigate the dynamics of a droplet on a planar substrate as the droplet volume changes dynamically due to liquid being pumped in or out through a pore. We adopt a diffuse-interface formulation which is appropriately modified to account for a localized inflow-outflow boundary condition (the pore) at the bottom of the droplet, hence allowing to dynamically control its volume, as the droplet moves on a flat substrate with a periodic chemical pattern. We find that the droplet undergoes a stick-slip motion as the volume is increased (fattening droplet) which can be monitored by tracking the droplet contact points. If we then switch over to outflow conditions (thinning droplet), the droplet follows a different path (i.e., the distance of the droplet midpoint from the pore location evolves differently), giving rise to a hysteretic behavior. By means of geometrical arguments, we are able to theoretically construct the full bifurcation diagram of the droplet equilibria (positions and droplet shapes) as the droplet volume is changed, finding excellent agreement with time-dependent computations of our diffuse-interface model.

Non-Fickian water vapor sorption dynamics by Nafion membranes

Dynamics of water absorption from a saturated vapor and water desorption into dry air for Nafion 1100 EW ionomers have been measured for film thicknesses between 51 and 606 microm and at temperatures ranging from 30 to 90 degrees C. Water absorption and desorption exhibit two distinct non-Fickian characteristics: (1) desorption is 10 times faster than absorption and (2) the normalized mass change does not collapse to a single master curve when plotted against time normalized by membrane thickness squared, t/l2, for either absorption or desorption. Water desorption data were fit well by a model in which diffusion is rapid and interfacial mass transport resistance is the rate-limiting process for water loss. Water absorption is described by a two-stage process. At early times, interfacial mass transport controls water absorption, and at longer times, water absorption is controlled by the dynamics of polymer swelling and relaxation.

An instrument for environmental control of vapor pressure and temperature for tensile creep and other mechanical property measurements

An instrument for measuring the creep response of a material maintained under a controlled environment of temperature and vapor pressure is described. The temperature range of the instrument is 20-250 degrees C while the range of vapor pressure is 0-1 atm. Data are presented for tests conducted on this instrument with Nafion, a perfluorinated ionomer developed by DuPont and used as a membrane in polymer exchange membrane fuel cells, over a range of temperature and water vapor pressure. The data are useful for predicting long-term creep behavior of the material in the fuel cell environment as well as providing insight to molecular level interactions in the material as a function of temperature and hydration. Measurements including dynamic and equilibrium strain due to water uptake as well as elastic modulus are described. The main features of the instrument are presented along with experimental methodology and analysis of results. The adaptation of the instrument to other mechanical tests is briefly described.

The Effect of Polymer Substrate Surface Basicity on the Adsorption of a Cationic Polyacrylamide

The adsorption of a cationic polyacrylamide onto polytetrafluoroethylene, bisphenol-A based epoxy resins, and acrylate substrates was characterized with X-ray photoelectron spectroscopy (XPS) and dynamic contact angle measurement in water. Surface acidity-basicity was quantified via dynamic contact angle measurement in water, ethylene glycol, and diiodomethane. The epoxy resins exhibited intermediate surface basicity and adsorbed the most polyelectrolyte, suggesting that specific substrate-adsorbate acid-base interactions govern adsorption of the cationic polyacrylamide onto surfaces of low to intermediate basicity. In general polar interactions appear to dominate the adsorption process. Advancing contact angle measurements have yielded quantitative fractional surface coverage information for polymer-on-polymer adsorption comparable to that obtained via XPS. Copyright 1999 Academic Press.

Cationic Polyacrylamide Adsorption on Epoxy Surfaces

The adsorption of a cationic polyacrylamide (acrylamide/beta-methacryloxyethyltrimethylammonium methyl sulfate copolymer) onto an imidazole catalyzed bisphenol-A based epoxy resin surface was examined by surface wetting. The work of adhesion of diodomethane, ethylene glycol and water was measured on microscopically smooth epoxy surfaces using a Wilhemy plate method, in both advancing and receding modes. The cationic polyacrylamide was adsorbed from aqueous solutions as a function of the pH of the solution. In addition to the cationic quaternary amine groups along the polymer chain, there were carboxylate groups from amide hydrolysis, which had a pKa of 10.73. The surface wetting of neither diodomethane nor ethylene glycol was sensitive to the adsorbed polyacrylamide. Wetting of the surface by water was very sensitive to the presence of adsorbed polyacrylamide. Polyacrylamides in solutions below pH 9 adsorbed onto the epoxy surface resulting in increased water adhesion to the surface. When the polyacrylamide solution was greater than pH 9 carboxylate groups ionized and the anions inhibited adsorption of the polyacrylamide. The water wetting measurements were shown to correlate with the adsorption of Pd/Sn colloidal particles used for electroless deposition of copper circuit lines on printed circuit boards.

Considerations in building a low-noise reflection absorption infrared spectrometer

The design and performance of several dispersive spectrometers for reflection absorption infrared spectroscopy are presented. Sources of noise and their control are discussed in detail. Individual component selection for the spectrometers is discussed with particular attention paid to optimizing sensitivity for surface measurements. These efforts culminated in an ellipsometric spectrometer with a sensitivity of <0.01% absorption and capable of following transients with a temporal resolution of 10 ms.