Pendant drops bubbles

Provides measuring techniques of contact angle, surface tension, interfacial tension, and bubble pressure. Suitable methods for both static and dynamic inteifacial tension of liquids include du Nous ring, Wilhelmy plate, spinning drop, pendant drop, bubble pressure, and drop volume techniques. Methods for solids include sessile drop, dynamic Wilhelmy, single fiber, and powder contact angle techniques. 

Surface tension of the nonpolarized ITIES was investigated by using the drop-weight [2,3,29], maximum bubble pressure [30] and pendant drop [4] methods. The latter method... 

Several additional points might be noted about the use of the Bashforth-Adams tables to evaluate 7. If interpolation is necessary to arrive at the proper (3 value, then interpolation will also be necessary to determine (x/bl. . This results in some loss of accuracy. With pendant drops or sessile bubbles (i.e., negative /3 values), it is difficult to measure the maximum radius since the curvature is least along the equator of such drops (see Figure 6.15b). The Bashforth-Adams tables have been rearranged to facilitate their use for pendant drops. The interested reader will find tables adapted for pendant drops in the material by Padday (1969). The pendant drop method utilizes an equilibrium drop attached to a support and should not be confused with the drop weight method, which involves drop detachment. 

For foams, it is the surface tension of the foaming solution that is usually of most interest. For this, the most commonly used methods are the du Noiiy ring, Wilhelmy plate, drop weight or volume, pendant drop, and the maximum bubble pressure method. For suspensions it is again usually the surface tension of the continuous phase that is of most interest, with the same methods being used in most cases. Some work has also been done on the surface tension of the overall suspension itself using, for example, the du Noiiy ring and maximum bubble pressure methods (see Section 3.2.4). 

A number of methods are available for the measurement of surface and interfacial tension of liquid systems. Surface tension of liquids is determined by static and dynamic surface tension methods. Static surface tension characterises the surface tension of the liquid in equilibrium and the commonly used measurement methods are Du Notiy ring, Wilhelmy plate, spinning drop and pendant drop. Dynamic surface tension determines the surface tension as a function of time and the bubble pressure method is the most common method used for its determination. 

Surface Tension Measurement. The surface tension of the surfactant solution was determined by means of the Dynamic Contact Angle Tester FIBRO DAT 1100 (FIBRO Systems, Sweden) using the pendant drop method. It was also an output of the ADSA captive bubble contact angle measurements with surfactant solutions. 

Abbreviations CR = capillary rise DM = detachment method (including Wilhelmy plate and Du Nouy ring) DW = drop weight ( stalagmometer ) HD = hanging (pendant) drop MGP = maximum gais bubble pressure SD = sessile drop. 

The parameter fi is positive for oblate figures of revolution, i.e. for the meniscus in a capillary, a sessile drop, and a bubble under a plate, and is negative for prolate figures, i.e. for a pendant drop or an adjacent bubble. Bashforth and Adams (1883) reported their results as tables. For more detailed information, see for example in Adamson (1967). 

Figure 6.2 Liquid surface tension determination by the drop shape method a. A pendant drop is formed by suspending the liquid from the tip of a thin tube. b. A sessile air (or vapor) bubble is formed in a liquid by injecting the gas from the tip of a needle connected to a syringe.

In theory, every surface tension measurement method can be used to determine the interfacial tension between two liquids. However, the accuracy of these methods is reduced when applied to liquid-liquid interfaces, or when one or both of the liquids is viscous. In practice, the maximum bubble pressure and pendant drop methods are the most suitable, giving consistent and reliable values for interfacial tensions, although there is sometimes the... 

In order to measure the surface tension of solutions containing surfactants, the maximum bubble pressure, pendant drop and Wilhelmy plate (immersed at a constant depth) methods are suitable capillary rise, ring, mobile Wilhelmy plate, sessile drop and drop weight methods are not very suitable. These methods are not recommended because surfactants preferably adsorb onto the solid surfaces of capillaries, substrates, rings, or plates used during the measurement. In a liquid-liquid system, if an interfacially active surfactant is present, the freshly created interface is not generally in equilibrium with the two immiscible liquids it separates. This interface will achieve its equilibrium state after the redistribution of solute molecules in both phases. Only then can dynamic methods be applied to measure the interfacial tension of these freshly created interfaces. 

Many methods are available for the measurement of surface and interfacial tensions. Details of these experimental techniques and their limitations are available in several good reviews (75—27). Some methods that are used frequently in foam work are the du Nouy ring, Wilhelmy plate, drop weight or volume, pendant drop, and the maximum bubble-pressure method. In all cases, when solutions, rather than pure liquids, are involved, appreciable changes can take place with time at the surfaces and interfaces. 

Pendant Drop Method A method for determining surface or interfacial tension based on measuring the shape of a droplet hanging from the tip of a capillary (in interfacial tension the droplet may alternatively hang upward from the tip of an inverted capillary). Also termed the hanging drop (or bubble) method. 

As a rule, results from the sessile drop" and maximum bubble pressure" as well as from the pendant drop" methods are preferable to results obtained from other methods for metals with very high melting points. 

Bubble pressure Bubble pressure Bubble pressure Bubble pressure Bubble pressure Sessile drop Dynam. dix weight Sessile drop Sessile drop Pendant drop Drop weight Pendant drop pendant drop Pendant drop... 

Alternatively, interfacial tension can also be measured continuously with area changes, with an expanding or contracting pendant-drop instrument adapted to oscillatory measurements as described by Bhardwaj and Hartland (74). Dilational data are obtained without introducing shear as the interfaee is expanded and contracted. This is also often achieved with an expanding drop volume or bubble-pressure tensiometer (65). Nikolov et al. (170) have recently developed this technique and an instrument to study oiLwater systems. 

The interfacial tension, y, in the Gibbs adsorption equation is used for equilibrium conditions as bitumen components are adsorbed. Measurement techniques available are extensive. Some of these methods are duNouy ring, maximum bubble pressure, drop volume, Wilmhelmy plate, sessile drop, spinning drop, pendant drop, capillary rise, oscillating jet, and capillary ripples. These and many others are referenced extensively by Malhotra and Wasan (153). These authors also showed that there is no correlation between emulsion stability and interfacial tension. The nature of the film dominates stability. Some relationships between interfacial tensions and crude oil properties follow. 

The presently most powerful technique for obtaining the liquid-vapour or liquid-liquid interfacial tension is based on the shape of a drop or bubble. In essence, the shape of a drop or bubble is determined by balance of surface tension and gravity effects. Surface forces tend to make drops spherical whereas gravity tends to elongate a pendant drop or buoyant bubble. Fig. 26 shows the schematic of an experimental set-up (for details see Chen et al. 1998, Loglio et al. 2001). 

Pendant or Sessile Drop Method The surface tension can be easily measured by analyzing the shape of a drop. This is often done by optical means. Assuming that the drop is axially symmetric and in equilibrium (no viscous and inertial effects), the only effective forces are gravity and surface or interfacial forces. In this case, the Young-Laplace equation relates the shape of the droplet to the pressure jump across the interface. Surface tension is, then, measured by fitting the drop shape to the Young-Laplace equation. Either a pendant or a sessile drop can be used for surface tension measurement. The pendant drop approach is often more accurate than the sessile drop approach since it is easier to satisfy the axisymmetric assumption. Similar techniques can be used for measuring surface tension in a bubble. 

For polymers interfacial and surface tensions are more practically obtainable from analysing the shapes of pendant or sessile drops or bubbles, all of which are examples of axisymmetrical drops. Bubbles may be used to obtain surface tensions at liquid/vapour interfaces over a range of temperatures and for vapours other than air. Drops can also be used to obtain vapour/liquid surface tensions but they are particularly suited to determination of liquid/liquid interfacial tensions, for example for polymer/polymer interfaces. All the methods are based on the application of equation (2.2.1). The principles are illustrated in figure 2.4, in which a sessile drop is used as the specific example. Just like for the capillary meniscus, the drop has two principal radii of curvature, R in the plane of the axis of symmetry and / 2 normal to the plane of the paper. At the apex, O, the drop is spherically symmetrical and R = Rz = b and equation (2.2.12) becomes... 

Both sessile and pendant drops are suitable for use in liquid-gas and liquid-liquid systems, provided, of course, that the liquid surrounding the drop is transparent. Changes in drop shape can be followed to determine time-dependent effects on interfadal tension. Both methods have been used to measure low interfacial tensions in liquid-liquid systems, but only the sessile drop works well for tensions below about 0.01 mN/m. Finally, it should be noted that sessile and pendant bubbles of the less dense phase can be employed where the whole apparatus is, in effect, turned upside down (Figure 1.8). 

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