Comprehensive Review: Micromodels for Fluid Flow in Porous Media

This report provides a comprehensive review of micromodels, focusing on their fabrication and applications in studying fluid flow within porous media. The introduction highlights the importance of micromodels as tools for visualizing complex fluid dynamics at the microscale, particularly in systems where direct observation is challenging due to the non-transparency of real porous materials. The report discusses various fabrication methods, including photolithography and the use of materials like elastomers and thermoplastics. It also covers the application of micromodels in diverse areas, such as flow visualization, two-phase flow analysis, and the study of phenomena like capillary fingering and fractal dimensions. The paper examines how micromodels enable direct visualization of flow, velocity field retrieval using techniques like micro-PIV and micro-PTV, and the study of particle packing and two-phase flow dynamics. The paper also addresses the significance of the wettability of walls in two-phase flow, the influence of network geometry on porosity and permeability, and the use of advanced imaging techniques. The conclusion underscores the utility of micromodels in scientific research and their contribution to fields like environmental engineering and petroleum engineering.

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Introduction
Micromodels are devices with a connected
transparent porous network which allows the
easy vision of complex fluid flow dynamics
inside(Balbino et al.2013). In engineering
systems and natural processes, the complex
flow in porous media is ubiquitous. Some
Figure 1: Porous media of dolomite(Demirel
& Babur 2014).
examples include; solute transport and
subsurface flow of water, multiphase flow in
the recovery of oil, species of aqueous
vanadium redox or gas flow through porous
electrodes in fuel cells and interstitial flow
in biological tissues. Interfacial forces help
in governing the fluid flow physics because
of sudden reduction change in characteristic
system length to micrometer scale(Demirel
& Babur 2014).
Figure 2: Fliuid flow in porous material(Yue
et al 2015).
In such, there is a deviation in the behavior
of flow from the expectation in the scale of
the length larger than that of the micrometer.
Direct visualization of flow dynamics and
structure of the fluid are the key factors
considered the ineffective study of the flow
mechanisms(Yue et al 2015). This
visualization is somehow challenging as real
porous media are not transparent to visible
light.

Research activities that are conducted
with micromodels
Micromodels are used commonly in the
processes of investigation as well as in
visualization of small-scale chemical,
physical and biological
processes(Guckenberger et al.2015). One of
such models was developed and used in the
year 1952 in the investigation of microscale
fluid behavior mechanism in a porous
media(Kotnik 2015).
Figure 3: Porous material via templating
material (Kotnik 2015)
From that date, micromodels have been used
in the study of very many applications as
well as processes that involve two-phase
flow(Xu et al.2017). The practical examples
of such activities include:
 Capillary fingering effect
 The percolation dimensions
 Fractal dimensions
 Study of the fluid flow through a
nanometer scale channel
 Labyrinth patterns that are found in
confined granular-fluid systems.
There is an easy observation of the behavior
of complex flow using an optical
microscope in 2D micromodel because it
consists of single-layer microfluidic
channels of the arbitrary porous structure.
2D micromodel, however, may not capture a
critical feature for the porous media flow
which is physics associated with the porous
connection in 3D. E.g. an enhance tortuosity
and connectivity of the pore space led to a
reduction in the breakup of immiscible
fluids in 3D(Koo et al 2013). Features of
realistic porous media of 3D geometry are
best imitated by the 3D but a close match of

refractive index for fluids and the solid
matrix is required(Khandan, Stark, Chang &
Rao 2014). There are various approaches
available for any type of micro model for
tailoring the surface properties such as the
ability of a certain fluid to wet the
surface(Santhiago, Nery, Santos & Kubota
2014).
Materials and fabrication methods
(Photolithography)
Materials: Some elastomers, as well as
thermoplastic materials with the properties
like optical transparency, low-cost
fabrication as well as moderate chemical
resistance, have been utilized in the
fabrication of the micro models(Song &
Kovscek 2015)..
Fabrication: The 2D as well as3D micro
model fabrication can be effectively
implemented either additive or nonadditive
manufacturing apart from the direct packing
of the microparticles so as to give a 3D
porous media model(Yu et al.2017)
Photolithography can be described as in the
process below(Wang et al.2017).
Figure 4 : Graphical illustration of fluid flow
in porous material(Wang et al.2017).
Initially, a laser printer based on the design
of a computer is used to print the
photomask. Substrate spin-coated either
silicon wafer or micromodel materials to be
processed are used in transferring the mask
pattern to a photoresist layer whose
thickness depends on the spinning speed and
the photoresist viscosity(Tomazelli et
al.2014). The substrate is then placed on an
oven or a hot plate to separate it from
photoresist solvents which are volatile, a

step known as prebaking or soft
baking(Gauteplass, Chaudhary, Kovscek &
Fernø 2015). The properties of the materials
are then changed by exposing the photoresist
layer to ultra-violet light through the
mask(Ebrahimi, Withayachumnankul, Al-
Sarawi & Abbott 2014). A developer
solution is used to clear the area exposed or
the area not exposed to the UV light
depending on the photoresist type whether
positive or negative. Developing a negative
type of photoresist appears to be slow
therefore before its development,
postexposure baking is required to
accelerate photoresist polymerization. The
whole procedure of photolithography is
illustrated in the diagram below(Cao, Dai &
Jung 2016).
Figure5: Simplified diagram to illustrate
the process of photolithography using
positive or negative photoresists(Geistlinger,
Ataei , Mohammadian & Vogel 2015).
Photoresist master mold in most of the
occasions used as a patterned template for
replica molding (PDMS) and at times in
medium model porous construction it is
directly bonded with flat glass plate. Though
it is expensive and waste time, it cannot be
easily deformed as it is much stiff compared
PDMS(Daniele, Boyd, Adams & Ligler
2015).
Microfluidic Applications in Porous
media and two-phase flow.
Flow Visualization
A micromodel enables direct visualization
of flow inside a porous medium. Micro-PIV
and micro-PTV are the common methods
used in the retrieval of the velocity
field(Schlüter et al.2016). Tiny particles of
considerable density are added into the

flowing fluid and assumed to flow without
interference with the flow of the fluid. The
velocity of the particles can be regarded to
be the velocity of the fluid flow. The motion
of the particles is then visualized as the
images at short intervals of time are
obtained(Zhang et al.2016). Since the
development of first micromodels came in
place, there is an establishment of various
2D and 3D micromodels to study the
phenomena of transport and flow in porous
media based on the direct observation of
pore-scale and or structures of the
macroscale, matrix-fluid characterization
interactions and visualization of the flow
field(Rogers, Qaderi, Woolley & Nordin
2015).
The studies have contributed a lot to the
fields such as environmental engineering,
geologic and petroleum(Armstrong, Evseev,
Koroteev & Berg 2015). In this section,
there is a discussion on applications and
fabrication of micromodels 2D and 3D. The
existing methods of fabrication, properties
of materials and the materials used are also
introduced. There is also discussion on the
application of micro models on the study of
their phenomena of transport and flow
emphasizing on scientific research(den &
Onck 2013).
Packing particles
The construction of the 2D micro models
can be achieved through random packing of
the particles. Extraction of 2D or 3D of
high-resolution static images for the inner
porous structure has been done by the
advanced imaging techniques like X-ray
microcomputed tomography (micro-CT) and
focused ion beam scanning electron
microscopy (FIB/SEM)( Hauge, et al.2016).
The researchers have been discouraged from
the study of pore-scale flow dynamics by the
prolonged time of scanning, however, there
is current development of faster
versions(Waheed et al.2016). Magnetic
resonance imaging can also be used as an

alternative as it is seen to provide faster
dynamics' measurements but it also has
limitations in spatial resolution.
Figure:Oil/water displacement(Miralles,
Huerre, Malloggi & Jullien 2013) polymers
Higher connectivity, low porosity networks
in fluidic models usually represents a very
crucial subclass of the porous media that is a
representation of the biological as well as
geological structures(Xia, Si, & Li 2016).
Several studies have been conducted by the
use of a novel geometry generation
algorithm which is working on the principle
of two-dimensional Voronoi tessellation in
the definition of the networks(Ren, Zhou &
Wu 2013).
. Various techniques of manufacturing
additive and nonadditive or packing
materials such as glasses can be used in their
fabrication(Almajid & Kovscek 2016). A
micro model has a porous structure of either
2D or 3D depending on the method of
fabrication used(Miralles, Huerre, Malloggi
& Jullien 2013).
Two-phase flow
One of the typical examples of two-phase
flow in porous media is the oil displacement
by aqueous solution in tertiary as well as
secondary processes of oil recovery. This
kind of process can possibly be simulated
with the corner-to-corner flow in both
heterogeneous (E6–E10) as well as
homogeneous (O6–O10) networks.In the
case of the hydrophilic walls, the fluid
which is responsible for the displacement
will practically facilitate a wetting process
called imbibition. However, when the walls
are hydrophobic, there is active influence by
the non-wetting fluid which favors a process

called drainage(Mohajeri, Hemmati &
Shekarabi 2015).
This particular result is very crucial since it
is a clear indication of the relationship
between porosity and permeability in the
real porous media to account for the
geometry known to be responsible for the
porosity prediction(Alim, Parsa, Weitz &
Brenner 2017). Two-phase flow has
therefore been introduced in most of the
studies as a way of accounting for the
network geometry(Volpatti & Yetisen
2014). In the implementation of the two-
phase flow, the wettability of the wall has
been a predominant factor which affects the
displacement of one layer of the fluid by
another layer of different fluid(Krause et al
2016). The research paper has evaluated the
methodology which can possibly be used in
the creation as well as control of the realistic
and complex porous media on the
microfluidic substances(Jahn et al 2017).
Conclusion
Micromodels enables observation of flow at
pore-level and the structure of the fluid
under transient conditions by dividing them
using a camera and microscope(Zhang et
al.2016). They are made of materials
optically transparent like glasses and
transparent The permeability of the single-
phase flows as well as two-phase flow
dynamics has been discussed previously in
two types of networks that are
heterogeneous and homogeneous
characteristics (Warkiani, Wu, Tay & Han
2015). In the case of the single-phase flow it
has been discovered that the inclusion of the
heterogeneities whose form is usually vugs
increases permeability even beyond the
equations of material flow in porous
components.
References
Alim, K., Parsa, S., Weitz, D. A., &
Brenner, M. P. (2017). Local pore
size correlations determine flow
distributions in porous

media. Physical review
letters, 119(14), 144501.
Almajid, M. M., & Kovscek, A. R. (2016).
Pore-level mechanics of foam
generation and coalescence in the
presence of oil. Advances in colloid
and interface science, 233, 65-82.
Armstrong, R. T., Evseev, N., Koroteev, D.,
& Berg, S. (2015). Modeling the
velocity field during Haines jumps in
porous media. Advances in Water
Resources, 77, 57-68.
Balbino, T. A., Aoki, N. T., Gasperini, A.
A., Oliveira, C. L., Azzoni, A. R.,
Cavalcanti, L. P., & Lucimara, G.
(2013). Continuous flow production
of cationic liposomes at high lipid
concentration in microfluidic devices
for gene delivery
applications. Chemical engineering
journal, 226, 423-433.
Begolo, S., Zhukov, D. V., Selck, D. A., Li,
L., & Ismagilov, R. F. (2014). The
pumping lid: investigating multi-
material 3D printing for equipment-
free, programmable generation of
positive and negative pressures for
microfluidic applications. Lab on a
Chip, 14(24), 4616-4628.
Cao, Q., Han, X., & Li, L. (2014).
Configurations and control of
magnetic fields for manipulating
magnetic particles in microfluidic
applications: magnet systems and
manipulation mechanisms. Lab on a
Chip, 14(15), 2762-2777.
Cao, S. C., Dai, S., & Jung, J. (2016).
Supercritical CO2 and brine
displacement in geological carbon
sequestration: Micromodel and pore
network simulation
studies. International Journal of
Greenhouse Gas Control, 44, 104-
114.

Cheng, J., Jun, Y., Qin, J., & Lee, S. H.
(2017). Electrospinning versus
microfluidic spinning of functional
fibers for biomedical
applications. Biomaterials, 114, 121-
143.
Daniele, M. A., Boyd, D. A., Adams, A. A.,
& Ligler, F. S. (2015). Microfluidic
strategies for design and assembly of
microfibers and nanofibers with
tissue engineering and regenerative
medicine applications. Advanced
healthcare materials, 4(1), 11-28.
Demirel, G., & Babur, E. (2014). Vapor-
phase deposition of polymers as a
simple and versatile technique to
generate paper-based microfluidic
platforms for bioassay
applications. Analyst, 139(10), 2326-
2331.
den Toonder, J. M., & Onck, P. R. (2013).
Microfluidic manipulation with
artificial/bioinspired cilia. Trends in
biotechnology, 31(2), 85-91.
Ebrahimi, A., Withayachumnankul, W., Al-
Sarawi, S., & Abbott, D. (2014).
High-sensitivity metamaterial-
inspired sensor for microfluidic
dielectric characterization. IEEE
Sensors Journal, 14(5), 1345-1351.
Gauteplass, J., Chaudhary, K., Kovscek, A.
R., & Fernø, M. A. (2015). Pore-
level foam generation and flow for
mobility control in fractured
systems. Colloids and Surfaces A:
Physicochemical and Engineering
Aspects, 468, 184-192.
Geistlinger, H., Ataei‐Dadavi, I.,
Mohammadian, S., & Vogel, H. J.
(2015). The impact of pore structure
and surface roughness on capillary
trapping for 2‐D and 3‐D porous
media: Comparison with percolation

theory. Water Resources
Research, 51(11), 9094-9111.
Gruner, P., Riechers, B., Orellana, L. A. C.,
Brosseau, Q., Maes, F., Beneyton,
T., ... & Baret, J. C. (2015).
Stabilisers for water-in-fluorinated-
oil dispersions: Key properties for
microfluidic applications. Current
opinion in colloid & interface
science, 20(3), 183-191.
Guckenberger, D. J., de Groot, T. E., Wan,
A. M., Beebe, D. J., & Young, E. W.
(2015). Micromilling: a method for
ultra-rapid prototyping of plastic
microfluidic devices. Lab on a
Chip, 15(11), 2364-2378.
Hauge, L. P., Gauteplass, J., Høyland, M.
D., Ersland, G., Kovscek, A., &
Fernø, M. A. (2016). Pore-level
hydrate formation mechanisms using
realistic rock structures in high-
pressure silicon
micromodels. International Journal
of Greenhouse Gas Control, 53, 178-
186.
Jahn, I. J., Žukovskaja, O., Zheng, X. S.,
Weber, K., Bocklitz, T. W., Cialla-
May, D., & Popp, J. (2017). Surface-
enhanced Raman spectroscopy and
microfluidic platforms: challenges,
solutions and potential
applications. Analyst, 142(7), 1022-
1047.
Khandan, O., Stark, D., Chang, A., & Rao,
M. P. (2014). Wafer-scale titanium
anodic bonding for microfluidic
applications. Sensors and Actuators
B: Chemical, 205, 244-248.
Koo, C., LeBlanc, B. E., Kelley, M.,
Fitzgerald, H. E., Huff, G. H., &
Han, A. (2015). Manipulating liquid
metal droplets in microfluidic
channels with minimized skin
residues toward tunable RF

applications. Journal of
Microelectromechanical
Systems, 24(4), 1069-1076.
Kotnik, T., Frey, W., Sack, M., Meglič, S.
H., Peterka, M., & Miklavčič, D.
(2015). Electroporation-based
applications in
biotechnology. Trends in
biotechnology, 33(8), 480-488.
Krause, A. T., Zschoche, S., Rohn, M.,
Hempel, C., Richter, A., Appelhans,
D., & Voit, B. (2016). Swelling
behavior of bisensitive
interpenetrating polymer networks
for microfluidic applications. Soft
matter, 12(25), 5529-5536.
Lamberti, A., Marasso, S. L., & Cocuzza,
M. (2014). PDMS membranes with
tunable gas permeability for
microfluidic applications. Rsc
Advances, 4(106), 61415-61419.
Li, X. J., & Zhou, Y. (Eds.).
(2013). Microfluidic devices for
biomedical applications. Elsevier.
Mehling, M., & Tay, S. (2014). Microfluidic
cell culture. Current opinion in
Biotechnology, 25, 95-102.
Miralles, V., Huerre, A., Malloggi, F., &
Jullien, M. C. (2013). A review of
heating and temperature control in
microfluidic systems: techniques and
applications. Diagnostics, 3(1), 33-
67.
Miralles, V., Huerre, A., Malloggi, F., &
Jullien, M. C. (2013). A review of
heating and temperature control in
microfluidic systems: techniques and
applications. Diagnostics, 3(1), 33-
67.
Mohajeri, M., Hemmati, M., & Shekarabi,
A. S. (2015). An experimental study
on using a nano surfactant in an EOR
process of heavy oil in a fractured