LL and GLL flows in patterned capillaries

Two (LL) and Three-Phase (GLL) Flows and Extractions in Patterned Capillaries

The enhancement of mass transfer/extraction efficiency of liquid-liquid (LL) extraction systems using an inert gas phase (GLL) is an interesting phenomena with applicability to a wide number of systems. More recently, work has been done to measure the extraction performance and mechanism by studying the process at smaller length scales in capillaries (mm scale) by performing extractions and measuring the resulting flow patterns by visualization (high-speed camera). At these scales, the interfacial forces cannot be neglected and play an important role in phase stability and the nature of flow through the system; parallel-flows or two- and three-phase slug flows may arise, which have important implications for the mass transfer efficiency in these types of systems. The mechanism behind this enhancement is presently under investigation, requiring detailed measurements at varying flow conditions in order to determine the gas effect on various process parameters. These include interfacial area between liquid phases and the stability of flow regimes for example.


Figure 1:
 Two-Phase Slug Flow for Water-Octanol

The purpose of the proposed assignment is to investigate the influence of patterned wettability gradients in a capillary on the two- and three-phase flows (LL and GLL) and liquid-liquid extraction in these systems . Switching between hydrophilic and hydrophobic domains in a periodic fashion can lead to phase inversions/mixing patterns which may greatly enhance mass transfer. This will be accomplished through image/video capture using a high-speed camera, with subsequent flow patterns characterized using image analysis. Study of these flow patterns is then highly relevant for quantifying what possible enhancements can be realized through the use of such patterned-capillaries. Corresponding liquid-liquid extractions will be investigated using an established model system (acetic acid-water-octanol) initially, with an emphasis on determining the role of gas and wettability patterning on the resulting extraction efficiencies vs. a two-phase flow system.

If you are interested in this project or wish further details, please contact:

Jeff Wood Prof. Dr. Rob Lammertink

Email: j.a.wood(at)utwente.nl
Office: Meander 313
Phone Extension: 2961

Email: r.g.h.lammertink(at)utwente.nl
Office: Meander 314
Phone Extension: 2063

Electro-kinetic flow visualization by means of 3d μ-piv

Electro-kinetic flow visualization by means of 3D µ-PIV

Charge selective membranes or interfaces play an important role in membrane technology, such as desalination, separation, electrodialysis and blue energy. To fully understand and optimize these operations at macroscale, high-resolution measurements at small scale are crucial, which is the aim of this research. In systems containing a charge selective interface ion concentration polarization (ICP) is observed under the application of an external electric field. Fluid flow is influenced by this ICP and theoretical research predicts all kinds of turbulent effects and formation of vortices, see for instance figure 1 [1].

Figure 1 Time dependent instabilities in ICP near a charge-selective interface [1]

This turbulent behavior is predicted theoretically, but has not been observed in detail experimentally. In this research, we aim to visualize electro-kinetic flows in a 3D space by means of µ-PIV (Particle Image Velocimetry). Flow visualization in this system at micro-scale is challenging as a result of the small field of focus, the required particle density and size and the application of an electrical field. In this assignment, you will develop the technique for 3D electro-kinetic microflow visualization using the existing infrastructure in our labs (µ-PIV system).

Please do not hesitate to contact us for additional information!

Anne Benneker
Email: a.m.benneker@utwente.nl
Office: ME 318
Phone: 2962

  1. Druzgalski, C.L., M.B. Andersen, and A. Mani, Direct numerical simulation of electroconvective instability and hydrodynamic chaos near an ion-selective surface. Physics of Fluids, 2013. 25(11).
Charge selective membranes/interfaces in microfluidics devices

Charge selective membranes/interfaces in microfluidics devices

Charge selective membranes or interfaces play an important role in membrane technology, such as desalination, separation, electrodialysis and blue energy. To fully understand and optimize these operations at macroscale, high-resolution measurements at small scale are crucial, which is the aim of this research. In systems containing a charge selective interface ion concentration polarization (ICP) is observed under the application of an external electric field, see figure 1 [1]. As a result of this ICP a limiting current regime is observed upon increasing electrical field strength. An overlimiting current can be observed at even higher voltages, indicating an increased flux of ions towards the charge-selective material. Many mechanisms for this increased flux have been proposed (see for instance [2-4]), and thus systematical experimental investigations are important and needed to provide insights into the underlying mechanism. In this assignment we will study the charge flux and flow dynamics of a salt solution adjacent to a charge selective material (for instance a membrane or nanochannel) under an electric field using microfluidics. Different points of focus are possible, e.g.;

  • Influence of salt concentration (Debye length)
  • Confinement of microfluidic channels
  • Influence of inlet flow conditions (flow rate etc.)
  • Fabrication of an optimal charge-selective interface

Figure 1 - Schematical representation of ICP in systems containing a charge selective material [1]

Do not hesitate to contact us for additional information!

Anne Benneker
Email: a.m.benneker@utwente.nl
Office: ME 318
Phone: 2962

References

  1. Kim, S.J., et al., Concentration polarization and nonlinear electrokinetic flow near a nanofluidic channel. Physical Review Letters, 2007. 99(4).
  2. Nikonenko, V.V., et al., Intensive current transfer in membrane systems: Modelling, mechanisms and application in electrodialysis. Advances in Colloid and Interface Science, 2010. 160(1-2): p. 101-123.
  3. Zangle, T.A., A. Mani, and J.G. Santiago, Theory and experiments of concentration polarization and ion focusing at microchannel and nanochannel interfaces. Chemical Society Reviews, 2010. 39(3): p. 1014-1035.
  4. Kim, S.J., Y.-A. Song, and J. Han, Nanofluidic concentration devices for biomolecules utilizing ion concentration polarization: theory, fabrication, and applications. Chemical Society Reviews, 2010. 39(3): p. 912-922.
Diffusio phoresis of photocatalytic particles

Diffusio-phoresis of photocatalytic particles under self-generated concentration gradients

Aura Visan, Rob Lammertink

Soft matter, fluidics and interfaces group

a.visan@utwente.nl

The generation of flow within the interfacial structure due to concentration gradients was described quantitatively for the first time by Anderson [1] and extended by Ajdari and Bocquet [2] for solvophobic surfaces. The flow is driven by an osmotic pressure gradient which builds up inside the interfacial layer where the interaction potential between the chemical species and solid spans (Fig. 1). Moreover, if the surface is reactive, the diffusio-osmotic flow could be promoted without any external input, through self-generated concentration gradients. If the solid surface is not immobilized, e.g in a colloidal system, the surface flow will propel small particles.

In this project, the migration of photocatalytic particles (TiO2) under self-generated concentration gradients will be studied systematically in a microreactor where an aqueous solution of an organic contaminant is contacted under continuous flow with a particle suspension containing various concentrations of the same contaminant. When UV light is turned on, the photocatalytic particles decompose the contaminant lowering the concentration inside the colloidal stream. The difference in concentration that is generated via the photocatalytic reaction leads to the migration of particles toward the higher concentration site (Fig 2).

Figure 1: Concept illustration for diffusio-osmosis

Figure 2: A methylene blue (MB) solution is contacted with a TiO2 suspension stream containing the same MB concentration in a 200 µm width channel. The dotted line is used as a guideline to show the spreading of the colloidal stream upon illumination. a. UV off b. UV on

Please do not hesitate to contact Aura Visan for additional information! (a.visan@utwente.nl)

[1] J. L. Anderson, Annu. Rev. Fluid Mech., 1989, 21, 61–99.

[2] A. Ajdari and L. Bocquet, Phys. Rev. Lett., 2006, 96, 1–4.

Interfacial transport for hydrogen peroxide decomposition at pt/au surfaces

Interfacial transport for hydrogen peroxide decomposition at Pt/Au surfaces

Aura Visan1*, Rob Lammertink1, Pieter Bruijnincx2

1Soft matter, fluidics and interfaces group, University of Twente

2Inorganic chemistry and catalysis group, Utrecht University

*a.visan@utwente.nl

Mass transfer is always an issue in heterogeneous catalysis as reactions at liquid-solid interface involve the migration of species to the active catalytic sites. While macroscopic mixing can help in the bulk, the limiting factor lies in the boundary layer transport where concentration depletion or enrichment occurs due to the viscosity related velocity decrease towards the solid wall. The resolution could come from a surface driven flow which would impact globally the conversion capacity of the system.

Fig. 1 H2O2 decomposition on a Pt/Au Janus particle
Fig. 2 Concept illustration for catalytic heterogeneity

In this project a model reaction will be studied, namely the hydrogen peroxide oxidation and reduction on Pt and Au, respectively. The proton production and consumption leads to a charge distribution which translates into a self-generated electric field which in turn enhances electro-migration. The fast kinetics combined with the self-generated electro-osmosis leads to very intense flows with vortex structures. The focus in literature for this type of system is mostly on migration of engineered particles which are propelled by the flow at the surface (Fig 1). The interest here is to observe directly the fluid dynamics and mass transfer at immobilized surfaces. The interfacial transport is studied on length scales comparable with the boundary layer using a microfluidic platform (Fig. 2). Niche analytical techniques such as µPIV and FLIM will make possible the direct visualization of velocity and concentration profiles. Both µPIV and FLIM require complex data processing giving the student the opportunity to acquire valuable skills. The challenge is to develop a 3D particle tracking protocol.

Please do not hesitate to contact Aura Visan for additional information!(a.visan@utwente.nl)

Catalyst design for biomass conversion

Catalyst design for biomass conversion

Aura Visan1*, Rob Lammertink1, Pieter Bruijnincx2

1Soft matter, fluidics and interfaces group, University of Twente

2Inorganic chemistry and catalysis group, Utrecht University

*a.visan@utwente.nl

Catalysis is ever-present in industry, up to 90% of the processes use catalysts. However, this valuable experience covers mostly the oil based feedstock processing. For well-known reasons, there is a global interest for more sustainable resources. The catalytic conversion of biomass is a promising alternative for chemical and fuel production. Especially for chemical synthesis this route is highly promising due to the richness and complexity of the chemical composition of biomass waste. This is a clear distinction with respect to oil composition. While oil consists of hydrocarbons and, hence, is hydrophobic, biomass is high in oxygen content and consequently hydrophilic. Therefore, catalysts will have to fulfil completely different requirements in the case of biomass conversion.

Fig. 1 Microchannel 500 µm wide with catalyst patches
Fig. 2 High temperature, high pressure setup

The current project attempts to investigate key reactions in the valorification of biomass waste involving platform molecules such as levulinic acid to γ-valerolactone and glucose/fructose to hydroxyl-methylfurfural. The starting point is screening different materials, e.g. probe the potential of abundant metal oxides as catalysts instead of more expensive noble metals. The challenges that will be addressed are the stability of catalyst under aqueous conditions, the effect of hydration as well as other surface functionalities. Nevertheless, the main idea is to study the effect of surface heterogeneity on interfacial transport. It starts from the intuitive idea that consecutive reactions would benefit from alternating catalysts in order to consume intermediates that could potentially poison the catalyst and to generate local gradients that would shift the equilibrium of the reaction increasing overall conversion.

In order to design the optimum interface, the implications of solute – catalyst and solvent – catalyst interactions will also be investigated by changing the catalyst wettability (also alternating solvophobic and solvophilic surfaces), as well as probing the concentration profiles for various catalyst arrangements.

In this project, the effect of catalytic surface heterogeneity (Fig. 1) on interfacial transport is studied on length scales comparable with the boundary layer using a microfluidic platform (Fig. 2). The most challenging aspect for this system is the slow reaction rate. In order to provide a significant consumption flux for the reactant, a high catalytic surface area is necessary. Here the difficulty is to pattern porous materials inside microchannels.

Fig. 3: SEM of a sputtered ZrO2 layer
Fig. 4: Levulinic acid to GVL over alternating catalysts

The patterning of the catalytic material using deposition methods compatible with photolithography is straightforward. The important drawback for these deposition methods is the dense material that is obtained. The non-porous layer (Fig. 3) has a low surface area rendering a low catalytic activity to these films. Different approaches will be pursued in order to pattern porous layers inside the microchannels.

The next step after of the fabrication of porous discontinuous layers is alternating two porous materials for tandem reactions (Fig. 4). The effect of catalyst wettability will also be investigated. The catalyst will be hydrophobized by silane surface functionalization.

The oxide porous materials can act as a catalyst directly or serve as a support for metallic nanoparticles. In the later situation, the metal has to be wet impregnated in a second step. Obviously, every fabrication step has to be followed by a thorough characterization procedure. Here, the student will have the opportunity to get acquainted to different characterization techniques from XRD, SEM, EDX, XPS to ellipsometry and TEM.

Please do not hesitate to contact Aura Visan for additional information!

(a.visan@utwente.nl)

Overall vs. intrinsic reaction rates

Investigation of Titania Overall vs. Intrinsic Reaction Rates

Titanium dioxide (titania) is an extremely common photocatalytic material, used for degradation of pollutants in water. Literature on this material dates back for decades, with a wide range of values reported for the effective rate constant of different functionalities of titania (crystallinity, surface charge, etc.). These values typically range in the order of 10-7 to 10-4 s-1. Even after normalizing for the effective surface area, there is still a wide range of values (10-8 to 10-6 m s-1).

Recently, we have investigated the kinetics of degradation of methylene blue by titania in a custom fabricated microreactor. Using this microreactor, we can precisely decouple the effective diffusion at the interface from the intrinsic reaction rate. Doing this, we found reaction rates of the order of 10-1 s-1 (10-5 m s-1). This is orders of magnitude higher and opens up the question: why? There are many possible hypotheses: temperature/natural convection effects from local heating under UV illumination, buoyancy driven natural convection, oxygen limitations at the interface, substrate inhibition (p vs. n-type semiconductors), etc.

It is the purpose of the proposed research to investigate the kinetics of degradation of MB using prepared titania films in a variety of conditions. Specifically, under stagnant flow conditions and probing temperature/oxygen, what effective/intrinsic rate is observed? This will involve experimental work, as well as theoretical analysis of the relevant transport phenomena.

Temperature effects on ion transport through membranes in the limiting current regime

Motivations

Previously, we have investigated the influence of temperature gradients on ion transport and overall performance in electrodialysis (ED) and reverse electrodialysis (RED) membrane stack systems. Our studies found that the power requirements/generation (ED/RED) could be improved by heating either the concentrate or diluate stream, albeit at much larger cost of energy. These studies were conducted in the Ohmic region, where no significant diffusional limitations are encountered. However, operation in the limiting plateau where concentration polarization occurs is predicted to have much larger impacts.

Specifically, the selectivity of the process (desalination efficiency) can theoretically be tuned by imposing such a gradient. Additionally, multivalent ions show different temperature responses vs. monovalent and this may have a large impact on performance. The higher temperatures have lower electrical resistances, which is beneficial but also can increase water transport due to osmosis, which would have negative implications on performance. All of these effects can occur naturally in these types of systems, for example considering the case of river/sea water for RED. Finally, the effect of different membrane stack configurations (counter, co- and cross-flow) can be investigated as this affects both mass and heat transfer in the system.

Project Description/Aims

The proposed project will be focused on investigation of the effect of temperature gradients on ion transport in monovalent and mixed mono and multivalent ion solutions in ED and RED operation. This will involve experimental work with a membrane stack system, to characterize electrical, heat and mass transfer characteristics in various operation modes. Additionally, a strong numerical/theoretical component will be involved in order to predict temperature profiles in these systems, as well as overall system modeling via the Nernst-Planck-Poisson with Navier-Stokes equation.

Investigation Methodology

  • Electrical Characterization via Potentiostat/Frequency-Response Analyzer
  • Ion Concentration Profiles using ion chromatography
  • Numerical modeling of temperature, flow, electrical potential and ion profiles in the system

Contact Information

Anne Benneker a.m.benneker@utwente.nl

Dr. Jeff Wood j.a.wood@utwente.nl