Pendant drop technique

The automated pendant drop technique has been used as a film balance to study the surface tension of insoluble monolayers [75] (see Chapter IV). A motor-driven syringe allows changes in drop volume to study surface tension as a function of surface areas as in conventional film balance measurements. This approach is useful for materials available in limited quantities and it can be extended to study monolayers at liquid-liquid interfaces [76],... 

The integrated DLS device provides an example of a measurement tool tailored to nano-scale structure determination in fluids, e.g., polymers induced to form specific assemblies in selective solvents. There is, however, a critical need to understand the behavior of polymers and other interfacial modifiers at the interface of immiscible fluids, such as surfactants in oil-water mixtures. Typical measurement methods used to determine the interfacial tension in such mixtures tend to be time-consuming and had been described as a major barrier to systematic surveys of variable space in libraries of interfacial modifiers. Critical information relating to the behavior of such mixtures, for example, in the effective removal of soil from clothing, would be available simply by measuring interfacial tension (ILT ) for immiscible solutions with different droplet sizes, a variable not accessible by drop-volume or pendant drop techniques [107]. 

AMBWANI, D.S. and FORT, T., Pendant drop technique for measuring liquid boundary tensions , in reference 9 11, 93-119 (1979)... 

The pendant drop technique measures the shape of a liquid drop suspended from the tip of a capillary needle. The drop is optically observed and the surface tension is calculated from the shape of the drop. This method is not as precise as the force measurement method because it depends on the eye of the operator or the sophistication of detection hardware and analysis software. 

The measurements of the interfacial tension were made with the pendant drop technique using the selected plane method as described by Andreas et al. [8], This method was chosen because it is simple to use in high pressure systems. 

D.S. Ambwani, T. Fort, Pendant Drop Technique for Measuring Liquid Boundary Tensions, in Surface and Colloid Sci Vol. II, R.J. Good and R.R. Stromberg Eds., Plenum (1979), chapter 3. 

Interfacial Tensions. Initially interfacial tensions were measured by the pendant drop technique. Later studies with an EOR spinning drop interfacial tensiometer gave much information on middle phase films, as shown in the photographs made with a camera mounted on our tensiometer. 

Interfacial agents, such as block copolymers, are known to reduce the Interfaclal tension and hence are expected to Increase the degree of dispersion in blends. The measurement of Interfacial tension for polymer systems is not easy. Most measurements have been made by the pendant drop technique. Measurements of Interfacial thickness are also difficult. They have been made using electron microscopy and, mostly in the case of block copolymers, by x-ray and neutron scattering. Recent results using neutron reflection suggest that this will be a useful technique in the future. 

Figure 2.4-1 Tandem variable-volume view cell tensiometer for measuring the interfacial tension by the pendant drop technique TC = temperature controller PG = pressure gauge.

Typical experiments to study the adsorption of a surfactant at a water/oil interface consists in the measurement of the interfacial tension with time. An example is shown in Fig. 4.39. In this experiment, a drop of an aqueous surfactant solution is first formed in a cell filled with pure hexane. The volume ratio Q=10 of the volume of the water drop (containing the surfactant) to that of the hexane bulk. The dynamic interfacial tension y(t) is then monitored by the pendant drop technique. The time dependence of y for four initial concentrations of C13DMPO in aqueous solution is shown in Fig. 4.41. 

Tensions at the n-hexadecane—water interface were measured at short times 30 min) by drop-volume and pendant-drop techniques 2), and at longer times using a Wilhelmy-plate torsion balance (2) Under conditions for which protein concentration, aqueous phase volume and surface area were similar to those existing in the surface viscometer, all the pure proteins gave a steady-state tension within 5—10 h. 

Another simple technique involves a hquid which is slowly extruded vertically downward through a capillary into another immiscible fluid of lower density (the submerged pendant drop technique). The heavier ejeeted fluid forms a sphere at the nozzle tip, and grows until the drop is no longer supported by surface tension. The drop then falls and stretches the ligament which connects the drop to the nozzle [Matta, 1984]. 

Cho et al. (2000) studied the segregation dynamics of block copolymers to the interface of an immiscible polymer blend and compared experimental results to the predictions of various theories for a poly(styrene-b-dimethylsiloxane) [P(S-b-DMS) M = 13,000] symmetric diblock copolymer system added to a molten blend of the corresponding immiscible homopolymers. They used the pendant drop technique at intermediate times and compared their results to the predictions of diffusion-limited segregation models proposed by Budkowski, Losch, and Klein (BLK) and by Semenov that have been modified to treat interfacial tension data. The apparent block copolymer diffusion coefficients obtained from the two analyses fall in the range of 10 -10 cm /s, in agreement with the estimated self-diffusion coefficient of the PDMS homopolymer matrix. 

Fig. 4.15 Interfacial tension coefficient as a function of diblock copolymer in the minor phase. System matrix, polybutadiene dispersed phase (pendant drop technique), polystyrene with styrene-b-butadiene diblock copolymer (Data Anastasiadis et al. 1988). Dotted and solid lines were computed from Eqs. 4.29 and 4.30, respectively...

Figure 6.2 A schematic diagram of the pendant-drop technique for measuring surface energies of liquids.

Stauffer CE (1965) The measurement of surface tension by the pendant drop technique. J Phys Chem 69 1933-1938... 

Other techniques are of interest but will not be covered here. For example, emphasis is put on the pros and cons of the pendant drop technique by Ambwani and Fort [63]. The pendant drop method has been very laborious traditionally. A fas standard instrumentation using computer image analysis has been described by Hansen and Rodsrud [64]. The characterization of monolayers can be carried out by this technique. For example, Li et al. described the required equipment and the application of the pendant drop technique to measure the surface tension of a insoluble monolayer covered onto a water drop surface [65]. 

Figure 4-23 Pendant drop technique for interfacial tension measurement.

The interfacial tension at the interface between two polymers is an expression of different energetics of bulk materials. It reflects differences in thermodynamics, which are related to the x parameter, as shown by Eq. (3.4). The experimental evaluation of the interfacial tension with polymeric melts is extremely difficult due to problems associated with sample preparation and equilibration [77, 78]. Several techniques have been proposed for the measurement. The most commonly used techniques include the pendant drop method, the embedded fiber retraction technique, and the breaking thread method. Classical equilibrium interfacial tension experiments like the pendant drop technique are very difficult to apply to high polymers because of their high melt viscosities (10 -10 Pas). There are many practical problems associated with the pendant drop technique ... 

It is claimed that the pendant drop technique and the drop weight technique give very accurate values which agree within 4 % [63A11]. Surface energy data on the liquid vapour interface which were obtained by both techniques are given in section 4.4.7.2 in Table 3. 

Most interfacial tension measurements have been made by the pendant drop technique. A drop of a polymer melt is formed in another polymer melt. The shape is recorded photographically and the interfacial tension found from drop dimensions, i,e. maximum width and width at a height equal to the maximum width, using tabulated ratios found from numerical solutions of the equations of Bashforth and Adams. The interfacial tension can also be found by a full analysis of the shape of the drop. The method is preferred over alternatives since equilibration is not a serious problem but accurate density measurements are required. Tabulated values of interfacial tensions are available in the literature and range from about ImNm" for polymers similar in polarity such as polyethylene/polypropylene to llmNm" for dissimilar polymers such as polyethylene poly-(methyl methacrylate). 

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