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Films of silicone oils (or alkanes) deposited on low-energy surfaces (grafted silicon wafers) are metastatic We nucleate a dry spot at the center of the wafer and observe the rate of growth V^dR/dt of the dry patch of radius R. We find that Fis (a) independent of R, (b) independent of the film thickness, (c) inversely proportional to the oil viscosity, and (d) very sensitive to the value of 0 e , the equilibrium contact angle between oil and wafer. At small 9 e , V~ 6e. All these features are in agreement with a recent hydrodynamic theory.PACS numbers: 68.15.+e, 47.20.-k, 62.15.-hi The spontaneous spreading of nonvolatile liquids on ideal wettable solid surfaces is relatively well understood. 1 " 4 In the present experiments, we study for the first time the inverse process, where the solid is not wettable but a liquid film (of typical thickness 20 fim) has been forced onto the surface. For these macroscopic films, the long-range van der Waals forces are negligible and the plot of the free energy F versus the thickness e in a horizontal geometry is controlled by weak gravitational forces ( Fig. 1). All films below a certain thickness e c (« 1 mm) are metastable. 5,6 They evolve by nucleation and growth of dry patches. These dewetting processes (which can often be seen qualitatively on a windshield or FIG. 1. Free energy per unit area F vs thickness e in the case of a solid wetted by a liquid film of thickness e « 10-1000 fim. F is controlled by interfacial and gravitational energies. /so, /si, and y are the solid-vapor, solid-liquid, and liquid-vapor interfacial tensions, respectively. The film is metastable at e 0 and evolves via nucleation and growth of a dry spot. in a bathtub) are of importance in many engineering problems: In some cases (e.g., greenhouses) the aim is to maintain such a film and avoid droplet formation, whereas in other cases (e.g., windscreens) the film must be eliminated rapidly (to avoid frost formation).On the fundamental side, there is an interesting similarity between Fig. 1, which shows the possibility of an equilibrium between a dry and a wet solid, and phaseseparation processes. In our case, however, the dynamics is controlled by macroscopic hydrodynamic flows. 7 Moreover, this work has a usefulness beyond the drying mechanisms: It relates to other hydrodynamic problems such as the draining and formation of dark spots in soap films. 8 We have used two different liquid-solid pairs to produce nonwettable surfaces: (a) alkanes on aliphatic chains and (b) silicone oils on perfluorinated chains.In both cases, the chains were grafted onto the SiC>2 surface of a silicon wafer 9 using octadecyltrichlorosilane in case (a) and heptadecafluoro l,l,2,2,tetrahydrodecyltrichlorosilane in case (b). Following Zisman, 10 contact-angle measurements against a series of alkanes gave critical surface tensions y c (a) =21 ±2 mNm" 1 and y f (b) = 15±2 mNm -1 , respectively. The alkanes used in our dewetting experiments (purchased from Merk) have surface tensions y ranging from 21....

The deposition of organic monolayers containing quaternary ammonium groups has been shown by many authors to confer biocidal properties on a large variety of solid surfaces. In a search for the controlling factors, the authors have grafted quaternized poly(vinylpyridine) chains on glass surfaces by two different methods and varied the charge density within the organic layer between 10 12 and 10 16 positive charges per cm 2 . The measurements show that this parameter has a large influence on the killing efficiency. Bacterial death occurs in less than 10 min in the quiescent state above a threshold value. The value is smaller for bacteria in the growth state. It also depends on the bacterial type. An electrostatic mechanism based on the exchange of counterions between the functionalized cationic surface and the bacterial membrane is proposed and appears consistent with the results.