Surface drop method

Pour plate Requires no pre-drying of the agar surface Will detect lower concentrations than surface spread/surface drop methods Very small colonies of strict aerobes at the base of the agar may be missed Colonies of different species within the agar appear similar —so it is difficult to detect contaminants... 

Surface spread and surface drop methods Surface spread often gives larger colonies than pour plates—thus they are easier to count Easier to identify contaminants by appearance of the colonies Agar surface requires pre- drying to absorb sample Possibility of confluent growth, particularly with moulds, masking individual colonies... 

The following values for the surface tension of a 10 Af solution of sodium oleate at 25°C are reported by various authors (a) by the capillary rise method, y - 43 mN/m (b) by the drop weight method, 7 = 50 mN/m and (c) by the sessile drop method, 7 = 40 mN/m. Explain how these discrepancies might arise. Which value should be the most reliable and why ... 

Recently, Samec et al. [38] have investigated the same system by the video-image pendant drop method. Surface tension data from the two studies are compared in Fig. 2, where the potential scale from the study [36] was shifted so that the positions of the electrocapillary maxima coincide. The systematic difference in the surface tension data of ca. 3%, cf. the dotted line in Fig. 2, was ascribed to the inaccurate determination of the drop volume, which was calculated from the shape of the drop image and used further in the evaluation of the surface tension [38]. A point of interest is the inner-layer potential difference A (pj, which can be evaluated relative to the zero-charge potential difference A cpp c by using Eq. 

The sessile drop method has several drawbacks. Several days elapse between each displacement, and total test times exceeding one month are not uncommon. It can be difficult to determine that the interface has actually advanced across the face of the crystal. Displacement frequency and distance are variable and dependent upon the operator. Tests are conducted on pure mineral surfaces, usually quartz, which does not adequately model the heterogeneous rock surfaces in reservoirs. There is a need for a simple technique that gives reproducible data and can be used to characterize various mineral surfaces. The dynamic Wilhelmy plate technique has such a potential. This paper discusses the dynamic Wilhelmy plate apparatus used to study wetting properties of liquid/liquid/solid systems important to the oil industry. 

When a colour layer of an artwork is analysed, a drop (5 10 pi) of this solution is localised on the sample surface. This method of enzymatic digestion can be, in principle, applied to all types of samples that occur in restoration practice fragments, cross-sections, microtome slices, etc. The samples are digested in closed microtubes to prevent evaporation of the solution. In the case of the cross-sections and microtome slices, it is essential to ensure the wetness of sample surface for the whole time of digestion. 

Lewis, as already mentioned, used a solution of sodium glycocholate and determined the adsorption of the salt by a surface of paraffin oil. The interfacial tension solution—paraffin oil this was measured for a number of concentrations by the drop method just discussed, and the [Pg.42]

The sources of error indicated above were avoided in a series of experiments carried out by Donnan and Barker, which in principle resemble those made by Lewis, so that only a brief reference to them is necessary. The dissolved substance was nonylic acid, and a drop method. The results could be reproduced with very great accuracy, i.e., to a fraction of one drop in 300—500 drops. Adsorption was produced at a surface air-liquid, air being passed through the solution in bubbles of known size and number, so that the total active surface could be calculated. The bubbles, on reaching the surface, burst, hence the excess of solute carried by them remained in the surface very effective precautions were used to prevent diffusion backwards from this portion into... 

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). 

Optical devices are sometimes employed for the measurement of contact angle, wherein the operator must attempt to establish the tangent to the contact angle of a drop of mercury resting on a plane surface. This method has never proven sufficiently accurate because of its inherent subjectivity. Different experimenters will inevitably measure substantially different contact angles and even the same person will observe different angles on the same material on different occasions. 

In order to calculate polymer/filler interaction, or more exactly the reversible work of adhesion characterizing it, the surface tension of the polymer must also be known. This quantity is usually determined by contact angle measurements or occasionally the pendant drop method is used. The former method is based on the Young, Dupre and Eowkes equations (Eqs. 21,8, and 10), but the result is influenced by the surface quality of the substrate. Moreover, the surface (structure, orientation, density) of polymers usually differs from the bulk, which might bias the results. Accuracy of the technique maybe increased by using two or more liquids for the measurements. The use of the pendant drop method is limited due to technical problems (long time to reach equilibrium, stability of the polymer, evaluation problems etc.). Occasionally IGC is also used for the characterization of polymers [30]. 

The pendant drop method has been used to measure the interfacial tension at the surface between mercury and cyclohexane solutions of stearic acid at 30 and 50°C. 

Weaver, F. Epoxy Adhesive Surface Energies Via The Pendant Drop Method, Air Force Materials Laboratory Report, AFWAL-TR-82-4179 (1982)... 

Sharma, R. R. 1963. Determination of surface tension of milk by the drop method and the ring method. Ind. J. Dairy Sci. 16, 101-108. 

The liquid media that are most germane to studies on interactions between cells and/or biopolymers are blood plasma and serum. A closer investigation into the surface tensions of blood plasma and serum thus seems essential. As early as 1913 the surface tension of human blood serum at 37°C was reported as 45.4 dyn/cm (measured by the falling-drop method) (8). More recently, Lewin (using platinum ring torsiome-try) found values of 47.8 and 50.5 dyn/cm at 37°C and 20°C re-... 

Important techniques to measure the surface tension of liquids are the sessile drop method, the pendant or sessile bubble method, the Du-Notiy ring tensiometer, and the Wilhelmy-plate method. 

To apply the sitting or pendant drop method to measure the surface tension of a liquid the drop must be large enough so that gravitation plays a significant role. Why ... 

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