1. Continuum flow and particle transport processes in separation sciences
Jeffrey F. Morris
CUNY City College of New York
National Academy of Science
A Research Agenda for a New Era in Separations Science-
Statistical Methods and Fluid Dynamics
August 23, 2018

2. Fluid flow in separations: Categories touched
Through-flow separations
Membranes
Small molecules (membrane separation and ion exchange): flow issues simple until pores approach nanoscale: continuum breakdown, fluid layering
Larger-molecules (e.g. proteins), and particulate systems
ultra- and nano-filtration
flow influence on fouling, cake structures
Phase transfer separations
Distillation / Extraction / Flotation
Complex fluid mechanics: interfacial flows or non-Newtonian behavior
Microchannel separations: particle (or cell) migration
Inertial
Viscoelastic

3. Small-pore flows of solutions
Ion exchange, ion-selective nanofiltration, forced wetting of nanoporous solids… All of these processes involve flows of solutions or electrolytes in tight pores: major research challenges associated with dominant role of fluid-surface interaction
Ion exchange: classic method, but new materials challenge the limit of continuum theory — we lack models
Return at end. Jaehun Chun will have more to say.
Nanometer or sub-nm pores (in zeolites or metal organic frameworks): electrolyte solutions exhibit extremely large wetting and drying pressures vs pure water
osmotic effects enter strongly [microscale reverse osmosis (RO)]
energy storage opportunities arise
theoretical understanding of flow processes and when they transition from continuum to discrete is lacking
Impacts on energy:
Electrostatic double layer capacitance storage methods
Fuel cells
Michelin-Jamois, M. , et al. PRL 2015

4. Membranes: small molecules
Wide-ranging applications and methods —Solution-diffusion: adsorption from liquids —Reverse osmosis: Ion rejection leads to polarization. Clearance mechanism (e.g. cross-flow) needed.

5. Concentration polarization Major issue in RO, membrane bioreactive water purification, food and beverage purification
Dammak et al Food and Bioproducts Processing

6. Many forces and phenomena
Mohammad et al 2012 Food Process Biotech

7. Critical flux
Hydrodynamic and fluid-mediated forces determine “fouling” Like colloidal coagulation / gelation: Control by pH, ions, and flow
Gkotsis et al 2014 Processes
Wyss et al. Phys Rev E

8. Mechanism — critical flux
Separation (nm)
Flow drag (red) overcomes electrostatic or steric barrier (green), entering vdW capture zone (blue)
Aggregation: sub-continuum flows — frontier area in physics of fluids.
Complex geometry, fluid flow and potential fields — highly unpredictable outcomes. Need for coupled algorithms to probe microscale and develop physically based models. Osmotic phenomena play major role.

9. Sub-micron pore clogging/unclogging: fixed pressure drop
Liot, O. et al (in press) Scientific Reports
Opportunities: sub100-nm particles, direct imaging of
process, precise geometric and chemical control, …
Challenges: Attachment flows are sub-continuum.
Porous medium + polarization layer.

10. Flotation: interfacial force-controlled separations
Mineral ore and recycled paper processes
Foam flow: poorly understood particle attachment and aggregation processes Interfacial control: surfactants and impurities
Flotation: interfacial force-controlled separations

11. Microscale molecular dynamics; contact line dynamics
Colosqui et al. Phys Rev Lett. (2013)

12. Liquid-liquid systems
Lende et al RSC Advances.

13. Microchannel mixture flow Example of inertial migration, dilute particles
Bhagat et al., PoF 2008
Segregation and “filtration”
Bhagat, Kuntaegowdanahalli & Papautsky Phys. Fluids 2008

14. Microchannel separations: inertial and viscoelastic migration
Migration (here inertial) can be followed with encapsulation & drop-based analysis
Edd et al. Lab Chip 2008
S.Q. Gu, Y.X. Zhang, et al. Anal. Chem. 2011

15. Spiral channel: Dean flow Rapid and tunable separation
Martel & Toner Sci. Reports 2013
Challenges:
Parameter space large and empirically known—unsatisfying models. Influence of concentration is unknown. Are processes scalable or just analytic?

16. Viscoelastic: Particles in 8% PVP soln
Particles in non-Newtonian liquids Lu, Liu, Hu, Xuan J Coll. Interface Sci Seo, Byeon, Huh, Lee RSC Adv. 2014
Viscoelastic: Particles in 8% PVP soln
Shear thinning: Particles in 1% PEO soln
Challenges: complex fluid adds another component; competing effects of non-Newtonian additives

17. Concluding thoughts Navier-Stokes analysis is not in itself a problem
Multiphase mixtures (foams, emulsions, dispersions) are insufficiently developed
Necessary complexity of many filtration (membrane bioreactor; ultra- and nano-filtration) systems leads to numerous competing forces
Clogging, membrane fouling and clearance mechanisms: need guideposts/validated models
Impact on water purification and availability is a major driver
Microchannel (microfluidic) problems
Separation of particles (or cells or micro-motors) due to fluid stresses
Geometric complexity and additives, questions of scalability:
Fluid mechanics/fluid physics in confined — small-pore systems or nanoparticle attachment — systems is a frontier
solubility (molecular or ionic) and nucleation impacted by interface
flow boundary condition, couple to surface hydrophobicity, electrochemistry
osmotic phenomena enter strongly—often poorly understood in their relevance to flow