Profiles of the precursor of spreading drops of siloxane oil on glass, fused silica, and mica

Abstract

J. Phys. Chem.of Figure 12 corresponds to gzz, g, , , and gYy, respectively, in the above equations. (Note that in circumstances that the 0, ground state becomes a12b22a12a~b12; 2Bl, then one has to reverse the x and y axis designation.) For the case of the 2A2 ground state one calculates 6 = 1.23 eV. The larger crystal field parameter 6 = 1.23 eV for [ C~~C r ] + o~-/ N a~~-y compared to 6 = 0.424 eV for 02-/NaM-Y is n o t e~o r t h y .~~,~' ,~~ For matrix-isolated alkali-metal superoxide M 0 2 species, 6 is relatively insensitive to the nature of the cation for Na, K, Rb, and Cs, as well as the matrix material for N2, Ar, and Kr.39 The enhanced 6 value for the superoxide anion in [Cp2Cr]+0y/Na56-Y appears to be due to the presence of the Cp2Cr+ cation. The effect could be steric and/or electrostatic in origin, although the mechanism of the enhancement a t present remains obscure. The coexistence of Cp2Cr+ in the proximity of the Na+.-02-ion pair might induce a change in the electronic ground state from the normal 2A2 to 2Bl as alluded to earlier. One should also consider the possibility that the oxygens of the 02-are inequivalent, with the 0-0 bond making an angle to the interior zeolite surface36 rather than being oriented parallel to it (see "0-labeling studiesM). Further work will be required to clarify this interesting point.From an examination of the data of Table IV, one learns that 0,-dynamical effects (EPR line broadening) are only observed for crystal field interactions having 6 I 0.50 eV.36 One deduces from this that the 6 = 1.23 eV value for superoxide in [Cp2Cr]+02-/NaS6-Y is consistent with the lack of any obvious temperature-dependent 0,-EPR line-shape effects and supports the rigid-limit, intrazeolite tight ion-pair description, Na+-02-depicted in Figure 16. This paper describes the use of a microscopic imaging ellipsometer to determine both the macroscopic spreading parameters and the profile of the microscopic precursor of spreading drops of siloxane oil on glass, fused silica, and freshly cleaved mica surfaces. The glass surfaces were subjected to a series of acid etches, which altered their spreading dynamics. Many of the features of the precursor follow de Gennes' predictions, but particularly the profile shows consistently different features. Experimental values for the van der Waals interaction parameter agree with theoretical estimates confirming the importance of this interaction in determining the profile shape, and lower bounds for the positive spreading coefficient S lying between 1 and 2 erg/cm2 for the three surfaces have been determined.de Gennes' has recently revived interest in the microscopic shapes of drops spreading on solids. If the spreading parameter S = usv -(usL + uLv) is negative, a drop will spread to a finite limiting size and contact angle. If however S I 0, the drop spreads

CFD simulation and validation of self-cleaning on solar panel surfaces with superhydrophilic coating

“…The first reported observation of an 'invisible' film spreading ahead of the edge of a macroscopic drop stems from the pioneering work by (Hardy 1919 and1936), Hardy was unable to detect its presence directly at that time. Numerous studies have subsequently confirmed the existence of precursor films using ellipsometry Beaglehole (1989); Bascom et al (1964), interference microscopy patterns Bascom et al (1964), and polarized reflection microscopy (Ausserre et al 1986). Simultaneous observations of the moving droplet and of the fringe pattern by the laser ellipsometry (Ueno and Watanabe 2005) reveal the existence of the precursor film, which is ahead the moving contact line and traveled with varying its profile.…”

“…The typical thickness of the precursor film moving in front of macroscopic body of fluid is in the range of 500-3000 Å (Bascom et al 1964;Beaglehole 1989). For a fixed volume of water droplet, according to Tanner's laws (Tanner 1979), the spreading of the macroscopic part of the droplet is rather slow; it would take a very long time to completely spread a macroscopic drop (Voinov 1976;Tanner 1979).…”

Influence of Chemical Interactions on the Macroscopic Spreading of a Maleic Anhydride Copolymer Melt

“…Systematic deviations of the expected spreading behavior of silicon oils were detected and found to be dependent on the chemical compositions of glass surfaces. [11] In the literature, both accelerations [12,13] and delaying [14] of the macroscopic spreading process were reported. Recently, Shanahan and Carre [15] found an anomalous spreading behavior of tricresyl phosphate (TCP) on glass and polymer surfaces in comparison with silicon oil and poly(dimethylsiloxane).…”

PDMS spreading morphological patterns on substrates of different hydrophilicity in air vacuum and water

“…This force pulls out of the drop a film whose thickness results from a balance between the large capillary term and large disjoining pressure [1], and the extent of the film can be macroscopic. The spreading and thin-film dynamics of the precursor film have been the subject of a number of theoretical [2,3] and experimental [4][5][6][7][8][9][10] papers. The profile of the macroscopic droplet is measured using interference microscopy [11] but thickness of the precursor film is experimentally measured using ellipsometry [5,8].…”