Water Droplet Sliding

The results of the water droplet velocity measurements are summarized in Table 4.4 along with contact-angle values for the samples Ml through M6. As expected, the measured 

These results are only partially consistent with the contact-angle measurements, where the highest contact angle value of 173° was obtained for the sample M5. Generally, it is expected that higher drop velocities should be obtained for more hydrophobic surfaces. However, in the present set of experiments, this was not the case because  

The deviation ((5) values predicted for a water droplet of 2.8 mm ( 0.2 mm) diameter placed on various aerogels are given in the same table above. As the contact angle value increased from 159° to 173°, the 5 value decreased from 0.05 to 0.024. To conclude, the 

Liquid Marbies Superhydrophobic Aerogei-Coated Water Droplets 

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Thin films of polyethylene, poly(ethylene-co-5-norbomen-2-yl acetate), and poly-(cthylene-co-5-norbomen-2-ol) were prepared on glass slides and contact angle measurements of water droplets determined. Testing results are provided in Table 2. 

Van Dam and Le Clerc have observed, using a CCD camera with a microscope objective, the impact of water droplets on glass slides, which had been thoroughly cleaned and treated to produce advancing contact angles of either 15°, 35° or 70°. By using inkjet... 

Yatsuzuka, K. Mizuno, Y. Asano, K. Electrification phenomena of pure water droplets dripping and sliding on a polymer surface. J. Electrostat. 1994, 32, 157-171. 

The procedure for preparing holey carbon films (carbon film containing small holes of 100 nm diam.) is somewhat more complex. In this case the microscope slide is initially dipped into a 0.25% solution of formvar in ethylene dichloride and then held over a source of water vapor for sufficient time for an array of droplets to collect on the formvar surface. Excess formvar is drained off the slide and allowed to dry. During this stage the water droplets evaporate leaving behind a wide size range of bubbles in the formvar film. Carbon is evaporated onto this surface following the procedure described above so that the eventual result is the formation of a holey carbon film. 

Miwa, M., Nakajima, A., Fujishima, A., Hashimoto, K., and Watanabe, T., Effects of the surface roughness on sliding angles of water droplets on superhydrophobic surfaces, Langmuir, 16, 5754, 2000. 

Figure 7. Anisotropic surface based on electrospun fibers, (a) Optical micrograph of a glass slide covered by a low density of aligned PAN electrospun fibers, (b) Photos of water droplet on the uncoated surface taken from direction 1 and 2 as indicated in (a). The bright regions are the wetted areas, (c) Photos of the droplet on the surface coated with PPFEMA taken from direction 1 and 2.

Figure 8. A digital image of water droplets on a colorful superhydrophobic surface composed of electrospun fibers on a glass slide. The picture was taken from a nearly normal direction with a white light source beneath the slide.

Silica films were also prepared on glass microscope slides following the procedure described in the experimental section similar contact angle results to those shown in Fig. 2 were observed. The water droplet shapes on the as prepared silica films with different TFPS/TEOS ratios are shown in Fig. 4. Clearly, the film results follow exactly the same trend as does the silica monolith. SEM images of the surface morphologies of hydrophobic silica films for these TFPS/TEOS ratios... 

Furthermore, the measurements of sliding angles show that although the modified samples are superhydrophobic, a small water droplet (5 pi) on their surface cannot roll off when the samples are tilted to any angle, even upside down, i.e., it is firmly pinned on such surface. Generally, the three-phase (air-liquid-solid) contact line plays an important role in sliding behavior of water droplets [37-39], and a... 

The sliding-pressure Venturi scrubber developed by Siemens/KWU and described by Eckardt (1988) allows operation at the pressure level prevailing inside the containment down to an overpressure of about 0.1 MPa, without need of a throttle valve in the venting line the water droplets generated in the liquid phase of the scrubber during operation of the system are retained by a steel fiber filter located inside the scrubber tank. The scrubber can be designed in a manner to reduce water losses due to evaporation caused by the decay heat of the absorbed radionuclides to an insignificant level. 

Yoshimitsu Z, Nakajima A, Watanabe T, Hashimoto K. Effects of surface structure on the hydrophobicity and sliding behavior of water droplets. Langmuir 2002 18(15) 5818-22. 

Sections are transferred onto a conductive ITO glass slide on top of a water droplet. 

Yoshimitsu, Z., Nakajima, A., Watanabe, T., and Hashimoto, K. 2002. Effects of Surface Structure on the Hydrophobicity and Sliding Behavior of Water Droplets. Langmuir 18 5818. 

Tiller et al. (2001, 2002) have covalently attached poly(4-vinyl-N-alkylpyridinium bromide) to glass slides and, thus, created a surface which supposedly kills airborne bacteria on contact. The authors do not provide evidence about the mechanism of action. It is quite probable that traces of the biocide are dissolved into the water droplets in which the test organisms have been sprayed onto the surface. Killing rates up to 99% are reported, which cannot, however, be considered as sufficient, as they include only two log steps. 

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