Sessile drop measurements

This type of difficulty associated with measurements using chemical ly iI I-defined substrates was also observed during sessile drop measurements carried out on Athabasca bitumen in D20 T311. Values in the range of 15-20 mN/m were obtained for measurements with several drops of bitumen, while interfacial tensions for other pure aqueous and oleic systems were accurate to 0.5 mN/m. 

Values are mean values with the standard deviation shown in parentheses n=7 in each case, a ESCA measurements on silicon substrates, b Sessile drop measurements on glass slides, c Ellipsometry measurements on silicon substrates, d First receding contact angle. 

In their experiments the authors worked with gold-coated silica spheres, with self-assembled monolayers of mixtures of undecanethiols and co-hydroxy undecane thiols, to vaiy the contact angle between 20 and 100°. The results were compared with sessile drop measurements on the same system. A systematic difference up to 20° for the receding angles) between the two sets of results was observed, but since these differences depend on the nature of the solid surface it was not possible to pinpoint the origin definitely. A remarkable feature was that in the AFM method, hysteresis was very small, if not absent. Anyhow, this appears to be a promising technique. 

Data for EVOH and SAN at 220°C and at 160°C for EVAc the experimental data, y and dyIdT, result from sessile drop measurements. Compositions are given as defined in Section 8.3. 

Table 2. Sensitivity of the Sessile Drop Measurement 1. High Interfacial Tension Measurements...

Surface science. Surface characterization studies included those for base polyurethane segmented block copolymers and base polymer modified by <2 wt% U-P[AB], Surface analytical techniques included tapping mode atomic force microscopy (TM-AFM), X-ray photoelectron spectroscopy (XPS), wetting behavior by dynamic contact angle (DCA) analysis and sessile drop measurements, and attenuated total reflectance infrared spectroscopy (ATR-IR). Contact antimicrobial behavior of U-P[AB] containing hydantoin and aikylammonium B side chains was determined by spray-on and sandwich tests previously described in detail elsewhere [11, 22, 38]. 

Figure 21.4. Drop of water in sessile drop measurement of contact angle [37]. (Reproduced from Wang CY. Two-phase flow and transport. In Vielstieh W, Lamm A, Gasteiger HA, editors. Handbook of fuel cells - fundamentals, technology and applications, volume 3. Chichester, England Wiley, 2003 337-47. Copyright John Wiley Sons Limited.)...

Given the importance of contact angles for capillary flow calculations, the relevant measurements on PTL materials have been the focus of intense efforts. The initial approaches implemented sessile drop measurements of contact angles on GDL surfaces. However, these measurements do not provide adequate quantification of the wetting and transport characteristics within the porous structures. 

Figure 4.5 shows water droplets on a 510 x 3600 screen during Sessile Drop measurements. Measured contact angles for the 325 x 2300 screen using acetone, methanol, IPA,... 

FIGURE 4.5 Water Droplets on a 510 x 3600 Screen Sample during Sessile Drop Measurements. 

The most recent technique for measuring the contact angle is the axisym-metric drop shape analysis profile (ADSA-P) method [1329,1871]. The experimental setup for sessile drop measurements is shown in Figure 10.105. A monochrome video camera is mounted on a microscope the video signal... 

Fig. 10.105. Experimental setup for pendant drop and sessile drop measurements [1329]. (Reproduced with permission from Academic Press, Orlando, 1992.)...

The usual experimental situation is that of a sessile drop and, as with the pendant drop, it is necessary to determine a shape parameter and some absolute length. Thus /3 may be determined by profile fitting, and Ze measured, where Ze is the distance from the plane at = 90 to the apex. If the drop rests with... 

Usually one varies the head of mercury or applied gas pressure so as to bring the meniscus to a fixed reference point [118], Grahame and co-workers [119], Hansen and co-workers [120] (see also Ref. 121), and Hills and Payne [122] have given more or less elaborate descriptions of the capillary electrometer apparatus. Nowadays, the capillary electrometer is customarily used in conjunction with capacitance measurements (see below). Vos and Vos [111] describe the use of sessile drop profiles (Section II-7B) for interfacial tension measurements, thus avoiding an assumption as to the solution-Hg-glass contact angle. 

The most commonly used techniques for contact angle measurements are the sessile drop method and the Wil-helmy plate method. Results obtained from these two methods are in good agreement. 

To investigate the influence of swelhng of the substrate by the contacting liquid, the contact angle 6 of sessile drops of tricresylphosphate, TCP (drop volume 2 p,L, viscosity t = 70 cP, surface tension = 40.9 mN m ), has been measured as a function of time after deposition, t, on flat, smooth, horizontal surfaces of soft and rigid solids at 20°C. The method of measurement of contact angle is the same as in Section Ill.A. 

Analytical Techniques. Sessile drop contact angles were measured with a NRL C.A. Goniometer (Rame -Hart, Inc.) using triply distilled water. The contact angles reported are averages of 2-8 identically treated samples with at least three measurements taken on each sample. ESCA spectra were obtained on a Kratos ES-300 X-ray Photoelectron Spectrometer under the control of a DS-300 Data System. Peak area measurements and band resolutions were performed with a DuPont 310 Curve Resolver. 

The four-to-six day duration of the dynamic Wilhelmy tests (wherein equilibrium actually occurred after one day) were much shorter than the times generally required for the sessile drop test. The conventional contact angle measurements on oil from the fields mentioned above required up to 48 days (12). 

The measurement of contact angles for a sessile drop or bubble resting on or against a plane solid surface can be measured by direct microscopic examination. 

One of the most common ways to characterize the hydrophobicity (or hydrophilicity) of a material is through measurement of the contact angle, which is the angle between the liquid-gas interface and the solid surface measured at the triple point at which all three phases interconnect. The two most popular techniques to measure contact angles for diffusion layers are the sessile drop method and the capillary rise method (or Wihelmy method) [9,192]. 

For the DMFC, Zhang et al. [127] used the sessile drop method to study the wettabilities of liquid methanol solutions on the surface of the anode DLs and MPLs. They were able to observe that the contact angles of the materials were the smallest with low PTFE content. In addition, the effect of Nafion ionomer content on the MPL (to increase hydrophilicity see Section 4.3.2) was also shown through the contact angle measurements (i.e., smaller contact angles compared to MPLs with PTFE). 

Lim, C. Wang, C. Y. Measurement of contact angles of liquid water in PEM fuel cell gas diffusion layer (GDL) by sessile drop and capillary rise methods. Penn State University Electrochemical Engine Center (ECEC) Technical Report no. 2001 03, Perm State University State College, PA, 2001. 

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