Periodic Reporting for period 4 - MUCUS (Modelling revolUtion for Complex flUid flow over Surfaces and walls)
Período documentado: 2024-09-01 hasta 2025-08-31
Everybody has experienced in daily life that complex fluids such as ketchup, mud or hair gel, behave differently and more erratically than simple fluids like water. The reason is in properties such as yield stress and elasticity, which makes the flow more complex to predict and understand. Yield-stress fluids are very common in nature (mud flows, volcanic events, biology) and industries (food industry, pharmaceutics, process industry, chemical industry, building industry). Processes designed for Newtonian fluids do not work for complex fluids, and there is an urgent need for improved theories and models.
The MUCUS project aimed to bring forward the state-of-the-art understanding of complex fluid flow over surfaces, by computer simulations which only became possible due to recent development of high-performance computing algorithms. Very little was known so far about time-varying yield-stress fluid flow over surface hills, grooves and how complex fluid droplets wet or are repelled by different surfaces.
The objectives of MUCUS project were to:
i) reveal new insight of the time-varying and unsteady (inertial) transport, mixing of different fluids and particles/droplets, spreading and impact of complex fluids on surfaces,
ii) create the first database of yield-stress fluid simulations, experiments and their cross-validation, and
iii) develop novel analysis tools, and couplings between micro- and macrostructure, to enable controlled design of complex fluids processes in the future
The conclusions of the project were:
i) Complex fluid droplets impact, and spread, or bounce from surfaces in a qualitatively different way from Newtonian droplets such as water. The type of behaviour is governed not only by surface wettability (hydrophobic/hydrophilic), but the microscale surface structure and complex fluids interact to create new phenomena. This has to be considered in design of surfaces in contact with complex fluids, such as, self-cleaning surfaces, or microfluidics.
ii) Particle and droplet suspensions in a yield-stress carrier fluid were studied for the first time, and their dynamics and distribution (near the channel centre or near the corners) were sensitive functions of the carrier fluid rheology.
Behaviours not present in Newtonian fluids were identified.
iii) Computational methods were developed that enabled us to simulate real three-dimensional flows of yield stress fluids and surface interactions of 3D viscoelastic droplets for the first time. We pioneered machine learning tools for analysis and prediction of yield-stress fluid flows.
The MUCUS project aimed to bring forward the state-of-the-art understanding of complex fluid flow over surfaces, by computer simulations which only became possible due to recent development of high-performance computing algorithms. Very little was known so far about time-varying yield-stress fluid flow over surface hills, grooves and how complex fluid droplets wet or are repelled by different surfaces.
The objectives of MUCUS project were to:
i) reveal new insight of the time-varying and unsteady (inertial) transport, mixing of different fluids and particles/droplets, spreading and impact of complex fluids on surfaces,
ii) create the first database of yield-stress fluid simulations, experiments and their cross-validation, and
iii) develop novel analysis tools, and couplings between micro- and macrostructure, to enable controlled design of complex fluids processes in the future
The conclusions of the project were:
i) Complex fluid droplets impact, and spread, or bounce from surfaces in a qualitatively different way from Newtonian droplets such as water. The type of behaviour is governed not only by surface wettability (hydrophobic/hydrophilic), but the microscale surface structure and complex fluids interact to create new phenomena. This has to be considered in design of surfaces in contact with complex fluids, such as, self-cleaning surfaces, or microfluidics.
ii) Particle and droplet suspensions in a yield-stress carrier fluid were studied for the first time, and their dynamics and distribution (near the channel centre or near the corners) were sensitive functions of the carrier fluid rheology.
Behaviours not present in Newtonian fluids were identified.
iii) Computational methods were developed that enabled us to simulate real three-dimensional flows of yield stress fluids and surface interactions of 3D viscoelastic droplets for the first time. We pioneered machine learning tools for analysis and prediction of yield-stress fluid flows.
We made significant progress in experimental and computational studies of how a complex fluid (viscoelastic) droplet wets different surfaces. Knowledge from this could be important for instance when designing surfaces that should retain or repel viscoelastic droplets, such as mucus.
Yield-stress fluids have stick-slip behaviour on smooth surfaces. We applied an experimental technique to directly measure the wall slip on different surfaces for different yield stress materials. Furthermore, a new computational algorithm enabled us to study this so-called slip yield stress, and how it influences the flow of yield-stress fluid through a rough or smooth porous medium. For example, facial masks, human body tissues, and gravel below Earth surface can be represented as porous media. Yield-stress fluids are known to experience channelization, where the flow happens along one path only, and no flow in rest of the porous medium. We found that on smooth surfaces with slip yield stress, more paths were open, if other parameters were constant. Furthermore, we analysed the pressure drop needed to push the flow of yield-stress fluids through and found some universal scalings for this.
Regarding time-varying, unsteady flows, we revealed in detail how elasticity and yield-stress together influence a turbulent channel flow of yield-stress fluids. One of the effects we observed was a significant reduction of drag, compared to fluids with only viscoelasticity or only yield stress. We showed how complex fluid nature leads to unstable and unsteady flows for a three-dimensional flow around a cylinder, for the first time replicating experimental findings from 1991.
Yield-stress fluids often have particles, droplets or bubbles in them that are retained because of the yield stress. Knowing how to process them in order to achieve desired bubble distribution is important for instance for properties of concrete, and taste of food products (whipped cream). Therefore, we study particle and bubble behaviour in yield stress fluids under different flow conditions. We pioneered studies of droplet and particle suspensions in yield stress fluids.
It is known that superhydrophobic surfaces can repel small droplets efficiently, since they can merge and jump away from the surface. We studied how superhydrophobic surfaces perform when the fluid is complex (non-Newtonian). One example is "self-propelled jumping" behaviour for viscoelastic droplets, where we found that they jump at different conditions compared to Newtonian droplets. We did experiments and simulations on rapid wetting and high-speed impact of polymeric and Newtonian droplets, finding surprising new impact regimes not found for Newtonian droplets.
Yield-stress fluids have stick-slip behaviour on smooth surfaces. We applied an experimental technique to directly measure the wall slip on different surfaces for different yield stress materials. Furthermore, a new computational algorithm enabled us to study this so-called slip yield stress, and how it influences the flow of yield-stress fluid through a rough or smooth porous medium. For example, facial masks, human body tissues, and gravel below Earth surface can be represented as porous media. Yield-stress fluids are known to experience channelization, where the flow happens along one path only, and no flow in rest of the porous medium. We found that on smooth surfaces with slip yield stress, more paths were open, if other parameters were constant. Furthermore, we analysed the pressure drop needed to push the flow of yield-stress fluids through and found some universal scalings for this.
Regarding time-varying, unsteady flows, we revealed in detail how elasticity and yield-stress together influence a turbulent channel flow of yield-stress fluids. One of the effects we observed was a significant reduction of drag, compared to fluids with only viscoelasticity or only yield stress. We showed how complex fluid nature leads to unstable and unsteady flows for a three-dimensional flow around a cylinder, for the first time replicating experimental findings from 1991.
Yield-stress fluids often have particles, droplets or bubbles in them that are retained because of the yield stress. Knowing how to process them in order to achieve desired bubble distribution is important for instance for properties of concrete, and taste of food products (whipped cream). Therefore, we study particle and bubble behaviour in yield stress fluids under different flow conditions. We pioneered studies of droplet and particle suspensions in yield stress fluids.
It is known that superhydrophobic surfaces can repel small droplets efficiently, since they can merge and jump away from the surface. We studied how superhydrophobic surfaces perform when the fluid is complex (non-Newtonian). One example is "self-propelled jumping" behaviour for viscoelastic droplets, where we found that they jump at different conditions compared to Newtonian droplets. We did experiments and simulations on rapid wetting and high-speed impact of polymeric and Newtonian droplets, finding surprising new impact regimes not found for Newtonian droplets.
We have developed novel computational algorithms that have enabled us to study time-varying, turbulent flows with yield-stress and elasticity (elastoviscoplastic), and new wetting problems for viscoelastic droplets such as rapid wetting. We have also for the first time studied elastoviscoplastic droplet behaviour in shear flows, and how droplets in yield-stress fluid affect rheological measurements. Furthermore, we have found a way to model the stick-slip behaviour for yield-stress fluids on smooth surfaces and studied its effects in channel flows, and on flow paths in porous media.
Our experiments of droplets of complex fluids impacting and sliding on surfaces have revealed distinct and surprising behaviours of complex fluid droplets, and new possibilities to control the behaviours. We found that at high impact speeds, viscoelastic droplets rebound fully and without satellite droplets, but that this behaviour critically depends on surface microstructure; an important lesson for design of superhydrophobic surfaces. Rapid wetting (without impact) on the other hand was not at all affected by viscoelasticity; the viscoelastic droplets spread at the same rate as Newtonian droplets, despite having higher viscosity.
Studies on particle suspensions and emulsions in inertial flows have shed light on pressure drop and flow behaviours. We also pioneered the application/development of analysis and prediction methods for turbulent and unsteady flows of complex fluids that have been lacking in the field, whereas have been available for many years for Newtonian fluids. The development will continue and expand to experimental applications after the project.
Our experiments of droplets of complex fluids impacting and sliding on surfaces have revealed distinct and surprising behaviours of complex fluid droplets, and new possibilities to control the behaviours. We found that at high impact speeds, viscoelastic droplets rebound fully and without satellite droplets, but that this behaviour critically depends on surface microstructure; an important lesson for design of superhydrophobic surfaces. Rapid wetting (without impact) on the other hand was not at all affected by viscoelasticity; the viscoelastic droplets spread at the same rate as Newtonian droplets, despite having higher viscosity.
Studies on particle suspensions and emulsions in inertial flows have shed light on pressure drop and flow behaviours. We also pioneered the application/development of analysis and prediction methods for turbulent and unsteady flows of complex fluids that have been lacking in the field, whereas have been available for many years for Newtonian fluids. The development will continue and expand to experimental applications after the project.