Periodic Reporting for period 2 - MUCUS (Modelling revolUtion for Complex flUid flow over Surfaces and walls)
Reporting period: 2021-09-01 to 2023-02-28
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 proposal aims 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 is 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 proposal are 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 MUCUS proposal aims 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 is 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 proposal are 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 recruitment has progressed despite the pandemic, and the MUCUS team now consists of three PhD students and one postdoc. Some of the highlights of the work so far are listed below.
We have made significant progress in developing a flow solver that can be used to study how a complex fluid (viscoelastic) droplet wets different surfaces. Knowledge from this flow solver could be important for instance when designing surfaces that should retain or repel viscoelastic droplets, such as mucus. This flow solver is very efficient when run on large high-performance computing clusters. We are working with complementary approaches that could use even less computational resources in high-speed impact cases where the droplet interface becomes complex. Furthermore, we aim to include yield stress along with viscoelasticity in this prediction tool.
Yield-stress fluids have stick-slip behaviour on smooth surfaces. We created a new algorithm to enable 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.
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. So far, we have finished studies of single droplets and bubble chains, also in comparison with experiments. Studies of droplet and particle suspensions are ongoing and expected to be finished in the second half of MUCUS project.
It is known that superhydrophobic surfaces can repel small droplets efficiently, since they can merge and jump away from the surface. We have studied this "self-propelled jumping" behaviour for viscoelastic droplets computationally and found that they jump at different conditions compared to Newtonian droplets. Furthermore, together with colleagues at KTH, we have performed experiments and simulations on rapid wetting of polymeric and Newtonian droplets. Surprisingly, we concluded that polymers did not affect the microscopic dissipation at the contact line, even if the polymers increased the overall viscosity several orders of magnitude.
We have made significant progress in developing a flow solver that can be used to study how a complex fluid (viscoelastic) droplet wets different surfaces. Knowledge from this flow solver could be important for instance when designing surfaces that should retain or repel viscoelastic droplets, such as mucus. This flow solver is very efficient when run on large high-performance computing clusters. We are working with complementary approaches that could use even less computational resources in high-speed impact cases where the droplet interface becomes complex. Furthermore, we aim to include yield stress along with viscoelasticity in this prediction tool.
Yield-stress fluids have stick-slip behaviour on smooth surfaces. We created a new algorithm to enable 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.
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. So far, we have finished studies of single droplets and bubble chains, also in comparison with experiments. Studies of droplet and particle suspensions are ongoing and expected to be finished in the second half of MUCUS project.
It is known that superhydrophobic surfaces can repel small droplets efficiently, since they can merge and jump away from the surface. We have studied this "self-propelled jumping" behaviour for viscoelastic droplets computationally and found that they jump at different conditions compared to Newtonian droplets. Furthermore, together with colleagues at KTH, we have performed experiments and simulations on rapid wetting of polymeric and Newtonian droplets. Surprisingly, we concluded that polymers did not affect the microscopic dissipation at the contact line, even if the polymers increased the overall viscosity several orders of magnitude.
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 on flow paths in porous media.
Regarding the expectations until end of the project, we foresee that the ongoing developments of numerical algorithms in MUCUS project will allow us to study how complex fluids droplets impact and wet surfaces at higher flow speeds, and the effect of surface geometry and roughness. Ongoing studies on particle suspensions and emulsions in inertial flows will shed light on pressure drop and flow behaviours. We are also currently making advances in new analysis and prediction methods for turbulent and unsteady flows of complex fluids that have been lacking in this field, whereas have been available for many years for Newtonian fluids.
Regarding the expectations until end of the project, we foresee that the ongoing developments of numerical algorithms in MUCUS project will allow us to study how complex fluids droplets impact and wet surfaces at higher flow speeds, and the effect of surface geometry and roughness. Ongoing studies on particle suspensions and emulsions in inertial flows will shed light on pressure drop and flow behaviours. We are also currently making advances in new analysis and prediction methods for turbulent and unsteady flows of complex fluids that have been lacking in this field, whereas have been available for many years for Newtonian fluids.