Measurement techniques for velocity and liquid fraction in flowing foams

This review discusses and compares different measurement techniques for liquid foam flow experiments. Particular emphasis is put on measurements of foam velocity and liquid fraction. Because foam is opaque, complex and fragile, many of the established tools of fluid mechanics are not directly applicable. Consequently, these techniques had to be adapted or new approaches had to be developed. This review elucidates the most common techniques and approaches based on optical imaging, electrical conductivity, X-ray and neutron imaging, ultrasound, magnetic resonance imaging, positron emission tomography, and small-angle neutron scattering. Each technique has its specific advantages and limitations, and needs to be chosen wisely depending on the measurement requirements. To that end, this review provides some guidelines to choose the most appropriate technique for a specific measurement.