An investigation of the microscale geometry and liquid flow through an isolated foam channel network

Clarke, Christopher ORCID: 0000-0002-5752-6412 (2020). An investigation of the microscale geometry and liquid flow through an isolated foam channel network. University of Birmingham. Ph.D.

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Abstract

Liquid foams represent an extremely diverse and highly functional form of soft matter, whose application is widespread throughout industry. These can range from luxurious and low calorie applications in food and beverages, structural and insulating properties in building and manufacture, right through to dynamic and transport abilities in the petrochemical industry, among others. A common feature of all of these foams is they are required to exhibit longevity, however as foams are thermodynamically unstable systems, this is not always a trivial feat.

Foams are highly complex systems, with dynamic processes occurring on the molecular scale that influence properties at the scale of individual bubbles and subsequently at the macroscopic scale of bulk foams. A particular challenge of foam research is to unite these length scale processes, requiring robust theoretical and experimental studies to be made at all size regimes. This PhD thesis is concerned specifically with the microscale process of liquid flow between bubbles, as these liquid channels form the primary network through which liquid ‘drains’ through a foam under the force of gravity; one of the key mechanisms governing foam instability.

The initial focus of this PhD thesis was the design and implementation of an experimental technique to isolate and image liquid foam channels formed under controlled liquid flow rates. This was developed with a view to producing highly accurate and reproducible measurements of the channel geometries, which would enable the comparison to theory derived to describe such systems.

Measurements of low molecular weight surfactants and higher molecular weight emulsifiers clearly demonstrated three previously unseen geometries of foam channel that could not be described using existing theory. Instead, a new geometric model was developed which was
able to account for these differences, relating the bulk and surface properties of the foam channel to its length and the rate of liquid flow passing through it. When used as a fitting parameter, the new model was able to clearly demarcate between the characteristic low and high surface viscosities of the surfactant and emulsifier species respectively.

The surface viscosity of the surfactant foam channel interfaces was examined throughout this PhD study, as the values extracted from model fitting were consistently lower than the majority found in literature, but in line with predictions made from hyper-sensitive measurement techniques. Ultimately, it was proposed that these differences could be attributed to a combination of the limited measurement sensitivity of commercial systems,
combined with a liquid flow velocity dependence of surfactant concentration at the channel surface.

It was suggested that, in the case of low molecular weight surfactants, a surface tension gradient can exist along the length of a foam channel, that is dependent upon the rate of liquid flow, the concentration of surfactant and the rate of surfactant adsorption to the interface. In the case of high liquid flow velocities, it was shown that surface tensions in some channel regions could be almost as high as pure water, despite surfactant concentrations being above the CMC. As such, this could have significant consequences for stability in macroscopic foams where these conditions are present.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):