We present a new method based on B-spline snakes (active contours) for measuring high-accuracy contact angles. In this approach, we avoid making physical assumptions by defining the contour of the drop as a versatile B-spline curve. When useful, we extend this curve by mirror symmetry so that we can take advantage of the reflection of the drop onto the substrate to detect the position of the contact points. To keep a wide range of applicability, we refrain from discretizing the contour of the drop, and we choose to optimize an advanced image-energy term to drive the evolution of the curve. This term has directional gradient and region-based components; additionally, another term-an internal energy-is responsible for the snake elasticity and constrains the parameterization of the spline. While preserving precision at the contact points, we limit the computational complexity by constraining a non-uniform repartition of the control points. The elasticity property of the snake links the local nature of the contact angle to the global contour of the drop. A global knowledge of the drop contour allows us to use the reflection of the drop on the substrate to automatically and precisely detect a line of contact points (vertical position and tilt). We apply cubic-spline interpolation over the image of the drop; then, the evolution procedure takes part in this continuous domain to avoid the inaccuracies introduced by pixelization and discretization.We have programmed our method as a Java software and we make it freely available [A.F. Stalder, DropSnake, Biomedical Imaging Group, EPFL, [ON LINE] visited 2005. http://bigwww.epfl.ch/demo/dropanalysis]. Our experiments result in good accuracy thanks to our high-quality image-interpolation model, while they show applicability to a variety of images thanks to our advanced image-energy term.

The surface properties of aluminum, such as chemical composition, roughness, friction, adhesion, and wear, can play an important role in the performance of micro-/nano-electromechanical systems, e.g., digital micromirror devices. Aluminum substrates chemically reacted with octadecylphosphonic acid (ODP/Al), decylphosphonic acid (DP/Al), and octylphosphonic acid (OP/Al) have been investigated and characterized by X-ray photoelectron spectroscopy (XPS), contact angle measurements, and atomic force microscopy (AFM). XPS analysis confirmed the presence of alkylphosphonate molecules on ODP/Al, DP/Al, and OP/Al. No phosphonates were found on bare Al as a control. The sessile drop static contact angle of pure water on ODP/Al and DP/Al was typically more than 115 degrees and on OP/Al typically less than 105 degrees indicating that all phosphonic acid reacted Al samples were highly hydrophobic. The root-mean-square surface roughness for ODP/Al, DP/Al, OP/Al, and bare Al was less than 15 nm as determined by AFM. The surface energy for ODP/Al and DP/Al was determined to be approximately 21 and 22 mJ/m2, respectively, by the Zisman plot method, compared to 25 mJ/m2 for OP/Al. ODP/Al and OP/Al were studied by friction force microscopy, a derivative of AFM, to better understand their micro-/nano-tribological properties. ODP/Al gave the lowest coefficient of friction values while bare Al gave the highest. The adhesion forces for ODP/Al and OP/Al were comparable.

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In micro/nanoelectromechanical systems ͑MEMS/NEMS͒, surface-dominated forces, such as stiction/adhesion and friction, play an important role because of the large surface-area-to-volume ratio. In order to control these forces and wear properties, optimal lubricant systems have been extensively investigated. Perfluoroalkyl self-assembled monolayers ͑SAMs͒ are considered to be a strong candidate since the fluorinated carbon backbones are expected to show lower adhesion and friction. In this paper, surface properties of perfluoroalkylsilane SAMs are investigated and compared with those of standard alkylsilane SAMs. The SAMs are deposited on silicon with a native oxide layer and silica substrates by a vapor deposition process. Surface properties, such as surface energy, water contact angle, roughness, adhesive and friction forces, and wear resistance, are evaluated. An atomic force microscope ͑AFM͒ is used for evaluations of the micro/nanotribological properties. The influence of humidity, temperature, and sliding velocity is also examined. In addition, the tribological mechanisms of the SAMs on molecular scale are discussed based on the AFM observations to aid the design and selection of proper lubricants for MEMS/NEMS.

Uniformly dispersed deposition of various nanoparticles ͑gold and silver of 20-50 nm͒ and nanowires ͑ZnO and VO x ͒ on different surfaces ͑Si, GaAs, mica, and steel͒ was obtained by boiling colloid drops on preheated substrates. Our results suggest a deposition mechanism based on the formation of microdrops induced by boiling. Indeed, microdrops produced by an ink-jet printing system resulted in uniformly dispersed deposits when the substrate was heated to high temperatures. We demonstrate that boil-deposited gold nanoparticles could be used for the catalytic growth of ZnO nanowires as well as for the manipulation of individual particles by means of an atomic force microscope.