We report an in situ observation of water condensation and evaporation on lotus leaf surfaces inside an environmental scanning electron microscope. The real-time observation shows, at the micrometer length scale, how water drops grow to large contact angles during water condensation, and decrease in size and contact angle during the evaporation phase of the experiment. To rationalize the observations, we propose a geometric model for liquid drops on rough surfaces when the size of the drop and surface roughness scale are comparable. This model suggests that when drop size and surface roughness are of the same magnitude, such as micrometer size water drops on lotus leaves, well-known equations for wetting on rough surfaces may not be applicable.

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Pulsed field gradient (PFG) NMR at high field was utilized to directly observe a transition between two different diffusion regimes in a Nafion 117 membrane loaded with water and acetone. Although water selfdiffusivity at small water loadings was observed to be diffusion timeindependent in the limit of small and large diffusion times, it showed a significant decrease with increasing diffusion time at intermediate times corresponding to root mean square displacements on the order of several microns. Under our experimental conditions, no self-diffusivity dependence on diffusion time was found for water at large water loadings and for acetone at all studied acetone loadings. The diffusion time-dependent self-diffusivity at small water concentration is explained by the existence of finite domains of interconnected water channels with sizes in the range of several microns that form in Nafion in the presence of acetone. The domain sizes and permeance of transport barriers separating adjacent domains are estimated based on the measured PFG NMR data. At large water concentrations, the water channels form a fully interconnected network, resulting in time-independent self-diffusivity. The absence of such a percolation-like transition with increasing molecular concentration for acetone is attributed to a difference in the regions available for water and acetone diffusion in Nafion. The diffusion data are correlated with and supported by structural data obtained using small-angle X-ray and neutron scattering techniques. These techniques reveal distinct water channels with radial dimensions in the nanometer range increasing upon water addition, while acetone appears to be in an interfacial perfluoroether region, reducing the size of the radial channel dimension.

A novel synthesis technique has been developed that yields monodisperse Pt particles in electrostatically stabilized suspensions without the use of structure directing organic surfactants. The approach uses stannous chloride as both reducing and stabilizing agent to form multifaceted Pt single crystal nanoparticles and clusters of less than 20 atoms. These particles may be assembled into layered electrode structures having well-controlled Pt loadings without precipitation onto organic supports or sintering to remove organic residues, both of which are known to yield particle aggregation and the formation of nonregular structures. Consequently, the particles may be used for fundamental investigations on the effect of platinum dispersion on catalytic activity never previously possible. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) of these particles provides the first direct evidence that peak oxygen reduction reaction (ORR) activity with increased catalyst dispersion is associated with the crystal to cluster transition and a change in reaction mechanism as reflected by the change in the Tafel slope from 120 mV/decade for the crystals to 220 mV/decade for the clusters at high current density. ORR mass activities obtained at 0.9 V versus reversible hydrogen electrode (RHE) from rotating disk electrode (RDE) experiments in perchloric acid were found to systematically vary from a minimum of about 18 A/g for the atomic clusters, to about 48 A/g for the single crystals, to a peak activity of 74 A/g for transitional structures (twice the value measured on commercial catalyst). Furthermore, the peak electrochemically active area (ECA) obtained from proton underpotential deposition is found to occur well within the atomic cluster regime.

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