Dr Paul Grassia


Chemical and Process Engineering

Personal statement

Dr Paul Grassia joined Strathclyde in 2015 following an academic post at University of Manchester (1998--2015), postdoctoral research at Stanford University (1996--1998) and University of Chile (1994--1996), a PhD at University of Cambridge (1990--1994), and BSc (Hons) at University of Western Australia (1985--1989).

Working at the interface between chemical engineering, applied mathematics, physics and physical chemistry, Dr Grassia's research group studies foams, and does so, for two main reasons: first and foremost, because foams are interesting scientifically in their own right(!), and secondly because they are important in a multitude of engineering operations (e.g. froth flotation in minerals processing, foam fractionation in protein purification, polymer foam manufacture, foam improved oil recovery).

Just stop and think what truly amazing materials foams really are: if anyone told you they could make a substance that was 99% by volume air, and 0.999% by volume water, and yet that had flow properties total different from either air or water, would you believe them?

Dr Grassia's research on foams has focussed on two main areas treated at very different length scales: foam drainage and foam rheology. Work on drainage has primarily been at the continuum scale, whereas that on rheology has been at the bubble scale. One major research success in drainage has been modelling froth flotation systems, and demonstrating the role that thin capillary boundary layers play in flotation tank operation. Meanwhile a major success in rheology has been modelling bubble flows in confined systems (e.g. a microfluidic channel and/or a pore in an oil reservoir during foam improved oil recovery), demonstrating how viscous drag effects can lead foam structures to break up.

Dr Grassia's group also studies solid-liquid suspensions (in addition to gas-liquid foams). To a certain extent however these solid-liquid suspensions are merely `upside-down foams': solids in suspensions tend to fall, whereas bubbles in foam rise.

Dewatering of suspensions or sludges is in fact a particularly important engineering operation, that reduces the volume (and thereby facilitates the disposal of) solid-fluid wastes, whilst simultaneously providing a source of clean water. Robust design of equipment to dewater a given sludge relies on knowing the sludge's rheological properties, which in turn are sensitive to the local solid fraction and local microstructure. According to existing theories for a sludge under compression, two (phenomenological) material properties are sufficient to describe the dewatering behaviour: a compressive yield stress (describing the weight-bearing strength of the sludge) and a hindered settling function (describing its frictional resistance). The results of small scale laboratory tests (e.g. batch settling, centrifugation, pressure filtration) depend on the values of the phenomenological material properties.  Research in the group has focussed on solving so called inverse problems to extract sludge material properties from laboratory test data, as well as using the material properties thus obtained to improve designs of sludge dewatering equipment.


Effect of surfactant redistribution on the flow and stability of foam films
Vitasari D, Cox S, Grassia P, Rosario R
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences Vol 476 (2020)
The influence of different solid-liquid ratios on the thermophilic anaerobic digestion performance of palm oil mill effluent (POME)
Khadaroo Sabeeha NBA, Grassia Paul, Gouwanda Darwin, Poh Phaik Eong
Journal of Environmental Management Vol 257 (2019)
Applicability of various pretreatment techniques to enhance the anaerobic digestion of Palm Oil Mill Effluent (POME) : a review
Khadaroo Sabeeha NBA, Poh Phaik Eong, Gouwanda Darwin, Grassia Paul
Journal of Environmental Chemical Engineering Vol 7 (2019)
Is the dewatering of Palm Oil Mill Effluent (POME) feasible? Effect of temperature on POME's rheological properties and compressive behavior
Khadaroo Sabeeha NBA, Grassia Paul, Gouwanda Darwin, Poh Phaik Eong
Chemical Engineering Science Vol 202, pp. 519-528 (2019)
Motion of an oil droplet through a capillary with charged surfaces
Grassia P
Journal of Fluid Mechanics Vol 866, pp. 721-758 (2019)
Foam stability influenced by displaced fluids and by pore size of porous media
Shojaei Mohammad Javad, Osei-Bonsu Kofi, Richman Simon, Grassia Paul, Shokri Nima
Industrial and Engineering Chemistry Research Vol 58, pp. 1068–1074 (2019)

more publications

Research interests

Dr Grassia's research focuses on foams and allied multiphase systems including solid-liquid suspensions and liquid-liquid emulsions.

Chemical engineering applications abound. Foams are used in froth flotation for extracting valuable metals, in foam fractionation for purifying proteins, and in manufacturing polymeric materials to name a few.  Foams can also be a major nuisance in bioreactors, whilst even laundry wash detergents tend to contain additives to reduce the volume of foam.  Solid-liquid suspensions meanwhile are processed in minerals tailings operations, and also when treating sewage sludges so as to extract clean water from waste and reduce waste volumes. Liquid-liquid emulsions meanwhile form the basis of many foodstuffs, and household and personal care products.

What all of the above systems have in common is that they consist of a discrete phase (whether gas bubbles, solid aggregates or liquid droplets) suspended in a continuous liquid phase.  Moreover the energetics of the interface between the discrete and continuous phase dominate the system behaviour, whilst the individual discrete phase elements interact with their neighbouring discrete phases. These interfacial properties and interactions, make foam (second perhaps only to graphene!) simultaneously both very light but very tough.

To understand these systems one needs to understand both their physics and their physical chemistry.  Moreover one needs to understand the physics and chemistry at a multitude of scales ranging from the nanometre scale of individual surfactant molecules within a foam film to the kilometre scale over which foam might propagate in improved oil recovery operations. Multiple time scales also determine the behaviour: individual surfactant molecules can enter and leave molecular aggregates called micelles on the order of microseconds, whereas the supermarket shelf life of a chocolate mousse can be up to some weeks.

In summary, foams have attracted the attention of humanity ever since the goddess Aphrodite (literally `risen from foam') was said to have been born from the sea foam on the shore of the Mediterranean. In the 21st century, foams continue to exert both their scientific fascination and their engineering utility.

Professional activities

Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences (Journal)
Editorial board member

more professional activities


Really Small Science in Colombia
Grassia, Paul (Principal Investigator) Haw, Mark (Co-investigator) Leckie, Joy Susan (Co-investigator)
01-Jan-2018 - 31-Jan-2019
Exploiting microbiology & rheology to improve treatment of Palm Oil Mill Effluent - (Newton Fund)Poh-Phaik-Eong-Newton-Collaboration-Programme
Grassia, Paul (Principal Investigator) Knapp, Charles (Co-investigator)
01-Jan-2016 - 30-Jan-2017

more projects


Chemical and Process Engineering
James Weir Building

Location Map

View University of Strathclyde in a larger map