Measurements of foam stability

Many other test methods and standards are available. Ross-Miles testing is another important test though it is often criticized for not being representative of real life foaming. In this test, a dilute solution of surfactant is dropped from a fixed height into a pool of the same dilute solution and the foam volume developed is measured. 

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The common understanding of foam stability usually refers to the ability of a foam to maintain its main parameters constant with time, i.e. bubble size, liquid content (expansion ratio) and total foam volume. Foam lifetime is most often used as the simplest measure of foam stability. 

The parameter tm is introduced as a measure of foam stability, i.e. the lifetime of foam layer at a distance about 2 cm from the a-source (this distance corresponds to one half of the free path of the a-particles). Table 7.2 summarised the results obtained for foams from various surfactants. 

Figure 10. Foam half-life measurements of foam stability and foam height measurements of foaminess versus dilatational elasticity for aqueous foams stabilized by a-olefin sulfonates. (Reproduced from reference 22 Copyright 1986 American Chemical Society.)...

As a measure of foam stability, the rate of foam formation Wp was proposed when a foam is produced [19]. This parameter can be determined experimentally on the basis of consideration of the balance of air volumes used to obtain the foam. 

FIG. 6 (a) Foam volume (or height, h) as a function of time initial volume is a measure of foamability and the change with time a measure of foam stability, (b) A foaming column for creation of foam and measurement of its volume (see text). 

Experimental correlations have been established in a given LDL formulation between foam stability and interfacial tension [33]. For example, Fig. 15 shows the effect of increasing water hardness on plate washing performance of an LAS/AES blend. A small amount of Ca2+ ion helps substantially to stabilize the foam. Under the same conditions interfacial tension is also lowered substantially. The two curves show an inverse relationship where the minimum interfacial tension value corresponds to the optimum level of foam stability as measured by plate washing [33]. 

An improvement in foam stability was observed as R was increased to >0.15 (Figure 17). This was accompanied by the onset of surface diffusion of a-la in the adsorbed protein layer. This is significantly different compared to our observations with /8-lg, where the onset and increase in surface diffusion was accompanied with a decrease in foam stability. Fluorescence and surface tension measurements confirmed that a-la was still present in the adsorbed layer of the film up to R = 2.5. Thus, the enhancement of foam stability to levels in excess of that observed with a-la alone supports the presence of a synergistic effect between the protein and surfactant in this mixed system (i.e., the combined effect of the two components exceeds the sum of their individual effects). It is important to note that Tween 20 alone does not form a stable foam at concentrations <40 jtM [22], It is possible that a-la, which is a small protein (Mr = 14,800), is capable of stabilizing thin films by a Marangoni type mechanism [2] once a-la/a-la interactions have been broken down by competitive adsorption of Tween 20. 

Food foams are usually made by bubbling, whipping, or shaking a protein-containing solution. Several measures of foaming capacity of proteins and other stabilizing agents exist, such as overrun, which quantifies the amount of foam produced, defined as ... 

The method for investigating foams at high pressure drop in Plateau borders permits the estimation of foam stability under strictly defined conditions (see Section 7.2). This method enables measurement of foam lifetime tp at a certain constant pressure as well as at reaching the critical state of the film. 

The concentration of black spot formation in microscopic films Cm characterises not only the threshold concentration of the surfactant at which stable foams and emulsions can be obtained but it can also be used as an indirect measure of film stability. The relations between film stability and Cm of the emulsifier depend on the polarity of the organic phase of the emulsion films (aqueous and hydrocarbon) [58], on the hydrophilic-lipophilic balance of the surfactant mixture [59] as well as on other properties. 

Another measure of thermal stability is the limiting oxygen index (LOI). Polyimide 2080 has an LOI of 44% and a syntactic foam using the resin as the matrix has an LOI greater than 55%. The flame penetration test of the US Bureau of Mines is a severe test of thermal stability. A syntactic foam having a density of 0.27 g/an exhibited a flame penetration of 1.1 hr/in. Thermal conductivity of the foam is in the range of 0.38 to 0.46 Btu/hr-ft - F/in over a density range of 0.22 to 0.30 g/cta . 

Figure 5. Measurements of foaming ability and foam stability for samples of raw foam, and humic substances from stream, foam, and foam extract samples from Como Creek and Suwannee River. Final foam heights were recorded after 5 minutes.

Some foams that have a drop-size distribution that is heavily weighted toward the smaller sizes will represent the most stable foam. In such cases, changes in the size distribution curve with time yield a measure of the stability of the foams. The bubble size distribution also has an important influence on the viscosity. For bubbles that interact electrostatically or sterically, foam viscosity will be higher when bubbles are smaller (for a given foam quality). This condition results because the increased interfacial area and thinner films increase the resistance to flow. The viscosity will also be higher when the bubble sizes are relatively homogeneous, that is, when the bubble size distribution is narrow rather than wide (also for given foam quality). 

Table I. Antifoaming Efficiency of Different Types of Antifoaming Agents Measured by Foam Stability Tests...

The static stability of a foam refers to the ability of the foam to resist bubble breakdown. The foam s static stability can be quantified by measuring its half-life. This is the time required at static conditions for the foam to drain half of its liquid volume. As parameters such as type of stabilizer, containment pressure, or foam generation process change, the stability will also change. Foam half-life is not a direct measurement of stability. Variations of foam stability will occur under different conditions. 

The experimental evidence of the role of liquid viscosity in determining nonaqueous foam stability has been provided by several researchers. Brady and Ross (2) found that the foam stability of refined mineral oils increased linearly with kinematic viscosity. McBain and Robinson (5) showed that a high bulk liquid viscosity was often associated with high foam stability. Callaghan and Neustadter (4) measured the foam stability of crude oil foams and reported that the average lifetimes of such foams are almost linearly dependent on bulk viscosity. 

Generally, foam height in a blender test deseribes the ease of foam formation, due to surfaee tension lowering and surfaee elastieity, the ability to ereate very thin lamellae and the speed with whieh foam ean be created or recreated. Half-life gives an indication of foam stability. Both measurements are important in drilling operations. Stability is required... 

A simple method for measuring the effectiveness of foam stabilizers in, for example, polymer foam systems... 

The whippability of egg white can be assayed by measurement of foam volume and foam stability (amount of liquid released from the foam in a given time). 

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