Formation and interaction of foams with porous materials

Foams are a common occurrence in industry where many applications require the foam to interact with porous materials. Interaction of foams with porous media has previously been investigated experimentally and theoretically where it was found that there are three different regimes of the drainage process for foams in contact with porous media: rapid, intermediate and slow imbibition. This investigation led to the interest into how foam was formed within porous media and how this interacts with the media; these topics are the subject of the current thesis. The wetting properties of aqueous solutions of a commercially available surfactant at various concentrations on porous media were investigated using the KRUSS DSA100 shape analyzer and the ADVANCED software to process the data. Time evolution of both the contact angle and drop base diameter at each surfactant concentration after deposition were monitored. Three different porous substrates (sponges) were examined. The sponges used were a car sponge, dishwasher sponge and audio sponge. The sponges were investigated both dry and at different degrees of saturation, that is, the amount of water absorbed into the sponge. It was found that pure distilled water droplets deposited on the dry porous media showed non-wetting behavior. However, if droplets of surfactant solutions were deposited then a change to complete wetting was found at all surfactant concentrations used. It has been observed that all the sponges, no matter the degree of saturation, displayed a minimum contact angle after which the droplet rapidly absorbed into the porous media.

A compression device to compress sponges for foam generation has been developed, which allows standardisation of the process of foam generation using sponges. The foam mass generated as a function of the concentration of surfactant solution was obtained for each of the three porous sponges investigated. For all the sponges 5 presses of the compression device produced all the available foam. The porosity and pore size of the dishwasher and car sponge were found to be very similar whilst the audio sponge was found to have the same porosity but a significantly smaller pore size. It was found that the maximum amount of foam is generated with a 70% w/w% concentration of commercial dishwashing solution in water solution for all the sponges. The commercial dishwashing solution is a mixture of surfactants, thickeners and dyes. The dishwasher and audio sponge foamability was found to be independent of the hardness of water i.e. concentration of salt in water, whereas the car sponge foamability increases as hardness increases. The average amount of foam produced for the audio sponge and dishwasher sponge is similar indicating that pore size has no significant effect on the amount of foam produced.

To provide a comparison with the commercial surfactant a pure surfactant, sodium dodecyl sulphate (SDS) was used, which has the same critical micelle concentration as the commercial dishwashing surfactant. During the investigation, it was found that the mass of foam increased up to 10 times the critical micelle concentration (CMC) and that any further increase in concentration after 10CMC resulted in no further increase in the mass of foam generated. The results for SDS and commercial dishwashing solutions have been used in the development of a model determining the amount of foam produced during the compression/decompression cycle of sponges. The investigations of both commercial product and SDS solutions went into deriving for the first time a model to predict the amount of foam produced by compression of a soft porous media. The model plots were compared to experimental results for both commercial dishwashing product and SDS solutions and was found to have agreement with the trend demonstrated by the experimental data.

Potential future work to build upon the investigations conducted to date include the creation of a model for the quality of foam produced using these systems. The foam quality is a combination of bubble diameter, foam stability and liquid volume fraction. In this project the quality of foam has been investigated for SDS using a foam column and conductivity column. The next step is to use these systems with commercial dishwashing solutions and then to develop models that accurately predict the different properties that determine the quality of foam. These future studies have been facilitated by the development in this project of the conductivity column that measures conductivity across the foam which can then be converted to liquid volume fraction.