12. Microfluidic systems Pressure Driven Flow
There are two common methods by which fluid actuation through
microchannels can be achieved. In pressure driven flow, in which the
fluid is pumped through the device via positive displacement pumps,
such as syringe pumps. One of the basic laws of fluid mechanics for
pressure driven laminar flow, the so-called no-slip boundary condition,
states that the fluid velocity at the walls must be zero. This produces a
parabolic velocity profile within the channel.
13. Microfluidic systems Pressure Driven Flow
Influence of Channel Aspect Ratio
Three-dimensional velocity
profile of fluid in a
microchannel with an
aspect ratio of 2:5
Three-dimensional velocity
profile of fluid in a
microchannel with an aspect
ratio of 50:1.
24. Diffusion
2D solution for the diffusion of biotin (D = 340 µm2/s, left image) and
albumin (D = 65µm2/s, right image) through a T-sensor. A normalized
concentration of 1.0 enters at the left inlet where the velocity flow rate
is 125 µm/s. Buffer enters at the right inlet at the same velocity.
26. The Hfilter
The H-filter is an inherently microfluidic device. Developed at UW in the
mid-90's, the H-filter allows continuous extraction of molecular analytes
from fluids containing interfering particles (e.g., blood cells, bacteria,
microorganisms, dust, and viruses) without the need for a membrane
filter or similar component that requires cleaning or replacement.
Principle: differences in the diffusion coefficient of particles.
27. The Hfilter
The principle of the H-filter is shown at upper left. In the center
of the device, streams move in parallel, and diffusion causes
equilibration of small molecules across the channel, whereas
larger particles do not equilibrate during the transit time of the
device. Modified streams separate at the right edge..
28. Rhodamine solution were
introduced into the vertical
channel
Scanning fluorescence image : note that the Rhodamine
solution doesn’t enter into the horizontal channel
(only a small diffusion is visible)
30. Example : Design a long mixing channel
Requirements : Mix ethanol completely with water in a parallel micromixer
with two inlets (Y-mixer) at room temperature. Flow rate for both liquids are
10 μl/min. D of ethanol in water (25 °C)= 0.84×10-5 cm2/s.
Find : The required length of the mixing channel with channel diameter of
100 μm×100 μm.
Too much for
microsystems !!!
31. Mixing
Mixing is a basic process required for many biological
applications. At the microscale, laminar flow conditions
prevent mixing except by diffusion.
In a microfluidic device, there are two ways of mixing fluid
streams : Passive mixers and active mixers.
Passive mixers: use channel geometry to fold fluid streams to
increase the area over which diffusion occurs, such as:
distributive mixer, static mixer, T-type mixer, and a 3D mixer.
Active mixers: Active mixers use external sources to increase
the interfacial area between fluid streams. Examples of active
mixing include a PZT-based mixer, electrokinetic mixers, a
chaotic advection mixer, and magnetically driven mixers.
33. Mixing Passive - examples
Splits the fluid
streams and then
recombines them.
3-D
micromixer.
34. Example of an active chaotic advection mixer
Principle
Simulation examples for 2 different temps
Active chaotic advection mixer developed
at ECL (France) for hybridization of DNA
BioChips [F. Raynal et al., Phys. Fluids,
16, 9, 2004]
35. Surface Tension
The cohesive forces among the liquid molecules are responsible for the
phenomenon known as surface tension. In the bulk of the liquid, each
molecule is pulled equally in every direction by neighboring liquid
molecules, resulting in a net force of zero. The molecules at the surface do
not have other like molecules on all sides of them and consequently they
cohere more strongly to those directly associated with them on the
surface. This forms a surface "film" which makes it more difficult to move
an object through the surface than to move it when it is completely
submerged.
Surface tension has the dimension of force per unit length, or of energy
per unit area. The two are equivalent.
42. Hydrogen Bond Formation is Essential
for Interactions with Water
Molecules that dissolve or interact with water, such as
carbohydrates, are said to be hydrophilic, water loving, but
these molecules just dissolve in water, because they form Hbonds with water molecules. Each hydroxyl (-OH) group on a
carbohydrate can make hydrogen bonds to three different water
molecules. The hydrogen can bond to a pair of valence electron
on the oxygen of water and each of the two pairs of valence
electrons of the hydroxyl can bond to a hydrogen of water. Most
of the molecules within cells can form H-bonds and are
hydrophilic.
Typical hydrophilic molecules include :
·
proteins,
·
carbohydrates,
·
nucleic acids (DNA and RNA),
·
salts (form ionic bonds),
·
small molecules of metabolism (e.g. glucose, amino
acids, ATP)
43. Hydrophobic Molecules Don’t Make
Hydrogen Bonds with Water
In contrast, fats that float and don’t dissolve in water are called
hydrophobic, but that doesn’t mean that fats hate water or are
pushed away from water, it simply means that fats can’t form
hydrogen bonds with water. Since bonding means that energy is
released as molecules come into contact, then water molecules
forming hydrogen bonds are in a lower energy state than water
molecules in contact with a hydrophobic molecule, such as a fat.
Random movement of a mixture of fat and water will eventually
result in the water molecules sharing the minimal possible
surface with the fat, because that is the lowest energy
configuration.
Typical hydrophobic molecules include:
· fats,
· steroids,
· lipids,
· aromatic compounds (such as some drugs).
47. Transport Processes
Type of Transport :
Pressure driving flow (often generated mechanically by a
pump).
Entropy-driven transport (occurs only if a fluid is more
disordered after transport than before), e. g. diffusion.
Gradient induced flow (temperature gradient or
concentration gradient).
Electrophoresis flow.
Electroosmosis flow
Dielectrophoretic flow
48. Electroosmotic and Electrophoretic
Flow
Electroosmotic flow : Mobile ions present close to the walls drag the entire
liquid column towards one of the electrode.
Electrophoretic Flow : Electrostatic forces moves individually each ion in the
direction depending on the charge.
Total Electrokinetic Mobility of each molecule = Electroosmotic Mobility +
Electrophoretic Mobility
54. Pressure driven flow and Electroosmotic Flow
Charges very close to walls move with the field and drive the
entire fluid through the channel.
Can obtain uniform velocity profile.
57. Glass/Liquid interface and Electroosmotic Flow
1)A glass capillary or channel
2) At the surface of the glass are silanol groups (-Si-OH),
which, depending on the pH of the buffer solution, are
deprotonated to a greater or lesser extent (pK = 6).
3) Deprotonation results in charge separation (pK = 6).
- Negative charges (-Si-O-) immobilized on the wall.
- Protons immediately adjacent to the wall.
4) Diffuser layer: negative and positive charge carriers
form further into the bulk of the solution.
5) Fixed-layer and diffuse layers: electrical double layer
61. Glass/Liquid interface
and
Electroosmotic Flow
Zeta potential : the potential at
the shear plane between the
fixed stern layer and the
diffuse Gouy-Chapman layer.
Dependencies of the Zeta potential :
- The chemical composition of the wall (material, dynamic or
permanent coating, etc.)
- The chemical composition of the solution (pH,ionic
strength, additives etc.)
- Temperature
62. Glass/Liquid interface and Electroosmotic Flow
:1) When an electric field is applied the mobile positive charges drag the entire
liquid column towards the cathode-electroosmotic flow.
2) Electroosmotic mobility :
3) Electroosmotic velocity :
4) Advantages : simple, uniform velocity distribution across the entire
cross section of the channel.
5) Debye length (the thickness of the double layer)
73. Valves and Pumps
Pumps : Devices to set fluids into motion.
Valves : Devices to control this motion.
Type of valves :
1) Passive valves: utilizing energy from the flow
2) Active valve: needing external energy to function
Passive valve :
Passive valve devices normally consist of either cantilever or
membrane on the silicon surface, which open and close to
enable and disable fluid flow during forward and reverse
pressures.
Active valves :
The actuation principles : pneumatic, thermopneumatic,
piezoelectric, electrostatic, shape memory alloy,
electromagnetic...
84. Microneedles fabricated by fs laser
Stratum corneum (SC): a dead tissue
Viable epidermis (VE): It consists of living
cells which have blood vessels capable of
transporting drugs, but contains very few
nerves (painless).
The microneedle should penetrate into
the skin about 100 μm.
86. DIGITAL MICROFLUIDICS
(a) Bifurcating channel geometry used to halve droplets at each
junction
(b) Pillar in channels demonstrates asymmetric fission of waterin-oil droplets
(c–e) Active fission of droplets using DEP through surface
electrodes in EWOD system
87. Nanofluidics
Nanofluidics is the study of the behavior, manipulation, and control of
fluids that are confined to structures of nanometer (typically 1-100 nm)
characteristic dimensions. Fluids confined in these structures exhibit
physical behaviors not observed in larger structures, such as those of
micrometer dimensions and above, because the characteristic physical
scaling lengths of the fluid, (e.g. Debye length, hydrodynamic radius)
very closely coincide with the dimensions of the nanostructure itself.
When structures approach the size regime corresponding to molecular
scaling lengths, new physical constraints are placed on the behavior of
the fluid. For example, these physical constraints induce regions of the
fluid to exhibit new properties not observed in bulk, e.g. increased
viscosity near the pore wall; they may effect changes in thermodynamic
properties and may also alter the chemical reactivity of species at the
fluid-solid interface.
All electrified interfaces induce an organized charge distribution near the
surface known as the electrical double layer. In nanometer dimensions
the electrical double layer may completely span the width of the
nanopore, resulting in dramatic changes in the composition of the fluid
and the related properties of fluid motion in the structure.
91. Fluorescent images showing the stretching of 103 kb DNA
in the nanofluidic channels. DNA stretching reaches about
95%, and in some cases 100%. The scale bar is 20 µm.
94. Knudsen Number
The Knudsen number is useful for determining whether statistical
mechanics or the continuum mechanics formulation of fluid dynamics
should be used: If the Knudsen number is near or greater than one, the
mean free path of a molecule is comparable to a length scale of the
problem, and the continuum assumption of fluid mechanics is no longer
a good approximation. In this case statistical methods must be used.
