DOI: 10.1021/la103772h 18581 Langmuir 2010, 26(23), 18581–18584 Published on Web 11/12/2010 pubs.acs.org/Langmuir © 2010 American Chemical Society Dynamic Study of Nanodroplet Nucleation and Growth on Self-Supported Nanothick Liquid Films Z. Barkay* Wolfson Applied Materials Research Center, Tel-Aviv University, Tel-Aviv 69978, Israel Received September 20, 2010. Revised Manuscript Received November 1, 2010 The dynamics of water condensation on self-supported thin films was studied at the nanoscale using transmitted electrons in an environmental scanning electron microscope. The initial stages of nucleation and growth over nanothick water films have been investigated. Irregularities at the water-film boundaries constituted nucleation sites for asymmetric dropwise and filmwise condensation. Nanodroplet growth was associated with center of mass movement, and the dynamic growth power law dependence was explored for the nanoscale. 1. Introduction The investigation of wetting properties of surfaces at nanoscale spatial resolution and high temporal resolution is an emerging field from both theoretical and practical aspects. The driving force stems from fundamental theories of nucleation and growth as well as from nanofluidic technological requirements 1,2 in biotechnol- ogy and materials sciences. Device miniaturization would require the understanding of the physical phenomena associated with the nanoscale and, in particular, the role of boundary conditions. The development of innovative experimental methods for the nano- scale and in situ dynamic nanofluidic characterization is an intrinsic part of future progress. As previously 3 indicated, the current two main imaging meth- ods for wettability study at high spatial resolution are atomic force microscopy (AFM) and environmental scanning electron microscopy (ESEM). The AFM method provides a wettability study of nanoscale droplet sizes over solid surfaces being limited to time resolution of a few minutes. A wettability study by ESEM 4-8 is usually restricted to micrometer-sized droplets over bulk surfaces with time resolution of 1 s. In situ condensation and evaporation experiments in ESEM on smooth or textured bulk surfaces provide static contact angles, as well as retarding and advancing angles by analysis of reflected secondary electrons due to electron-specimen interaction. Alternative imaging in ESEM by transmitted electrons has been recently used 9 for wettability study on self-supported nanothick liquid films. The method is based on wet scanning transmission electron microscope (wet-STEM) 10,11 detection in ESEM. The calculation of contact angles for droplets was derived 9 by fitting Monte Carlo simulation results for trans- mitted electrons with the measured signal using calibration additive particles. The current study provides direct imaging of droplet growth on a fluid self-supported interface (instead of common 4-8 fluid-solid interface) with typical lateral resolution of 10 nm and temporal resolution of 1 s. The dynamics of water condensation using wet- STEM in ESEM demonstrates for the first time both dropwise and filmwise growth on a fluid interface. The droplet growth results show a distribution in power law values and are compared with available results 12,13 for the micrometer and sub-millimeter- scale drops. The possible role of irregularities around the self- supported nanothick water films is discussed in relation to the asymmetric droplet nucleation, droplet growth radius, and center of mass movement. 2. Experimental Details Investigation was carried out using wet-STEM detector in the FEI Quanta 200 field emission gun (FEG) ESEM. The FEI wet- mode STEM detector is a solid-state two-segment device attached underneath the sample grid holder assembly. The samples were holey carbon grids immersed in distilled water without any additives in order to rule out any external effect on the results. Prior to the ESEM pump down process, the samples were cooled down to 2 °C by a Peltier cooling wet-STEM stage. The tempera- ture of the Peltier cooling stage is maintained by a thermoelectric module, which is externally water-cooled for removal of excess heat. A microprocessor-controlled board provides accurate and stable automatic temperature control of the stage temperature. A resistive temperature device (RTD) in the stage measures the sam- ple temperature. The temperature measurement accuracy is (0.5° as determined by the standard error limits of the RTD and the accuracy of the temperature measurement module. 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