Transcript crnko-predstavitev

PRODUCING TINY DROPS
USING THE TOOLS OF
MICROFLUIDICS
Brina Črnko
Advisor: prof. dr. Slobodan Žumer
CONTENTS
• Microfluidics
– Definition
– Promises and applications
– Properties of flows in small channels
• Jets and drops
– Mechanism of formation
– Creating drops
– Double emulsions
– Measuring and predicting radii
– New materials
• Conclusion
MICROFLUIDICS
• Studying flows in small channels (r~50µm).
• Manipulation of small amounts of fluids (10-9-10-18 l).
1nl=10-9l=(100 µm)3
• Main attraction:
‘lab on a chip’
for biochemical
applications.
IDEAL: Cheap, small,
easy-to-use,
disposable device for
synthesis and analysis
(e.g. for blood tests).
MICROFLUIDICS
• Example:
– Microfluidic (proof-of-principle demonstration) chip, that synthesizes FDG (2’deoxy-2-[18F]fluoro-D-glucose), a tracer compound used in positron emission
tomography, a medical imaging technique
– Introduce reagents through micropipettes into a network of channels and
‘plumbing’, imprinted on
a polymer (PMDS).
– Needed: valves, mixers,
pumps, detectors, filters…
All adapted to peculiar
properties of flows in small
channels
FLOWS IN MICROCHANNELS
• Reynolds number: ratio of inertial to viscous forces
Water: ρ~103kg/m3, η~10-3kg/ms
v~1µm/s-1cm/s
vL
Re 
L~50µm

Re~10-6-10
FLOWS IN MICROCHANNELS
• Remember: for pipes with smooth walls, flow becomes turbulent for
Re>2000
For L~50µm, flow is laminar for v<10m/s
Flow in microchannels is laminar.
• Navier-Stokes:


v  
2
 (  v .v )   p   v  f
t
• Nonlinearity is absent (Stokes flow).
• Laminar flow, no turbulence.
• Fluids can flow parallely, no mixing, only diffusion.
(Mixing has to be achieved otherwise.)
JETS AND DROP
FORMATION
• Rayleigh-Plateau instability
– A thin jet of water breaks into droplets,
as the surface energy is lower for drops.
– d(surface energy)=γ d(area)
• Jet: Vjet=πR2L, Sjet=2πRL
Drops: Vdrops=n4πr3/3, Sdrops=n4πr2, Vdrops=Vjet
• When r>3R/2, surface energy is lower for drops.
• A cylinder of water in air is unstable.
CREATION OF EMULSIONS
• Similar: a cylinder of fluid flowing inside a cylinder of outer fluid
(immiscible fluids).
• Formation of drops: balance between surface tension and the viscous
drag of the fluid pulling on the drop.
• Desired outcome: either drops (emulsions…) or jets (ink jet
printers…).
CREATION OF EMULSIONS
• Setup: Immiscible fluids (e.g. water and oil)
• Regimes:
Dripping: Drops form at the end of
inner capillary
Jetting: If the speed of one fluid is increased sufficiently, the result is
a jet, drops form further downstream
ljet=tpinch off· vinterface
• Capillary number=Ca=viscous drag/surface tension~ηv/γ
η…viscosity (outer fluid), γ…interfacial tension, v…velocity (inner
fluid)
• Transition between dripping and jetting: Ca~1
HYDRODYNAMIC FOCUSING
• Flow of the outer fluid focuses the inner fluid
• Creating double emulsion
e.g. oil-water-oil
• Outer fluid focuses a coaxial stream of middle and inner fluid.
• Drops: uniform droplets within larger uniform drops
CREATING DOUBLE EMULSIONS
• Adjust flows and dripping-jetting transitions of both fluids - create
different structures
• Control drop diameter, control shell thickness, control number of
inner drops.
RADII OF JETS AND DROPS
QOF...flow rate of the outer fluid
Qsum...sum of flow rates
of middle and inner fluids
Dripping:
Solid circle…drop
diameter
Open circle…inner drop
diameter
Half-filled circle…jet
radius
Jetting:
Solid triangle…drop
diameter
Open triangle…inner
drop diameter
Half-filled triangle…jet
radius
RADII OF JETS AND DROPS
• Model:
– Dripping:
Rjet from the mass flux at the orifice
R 2jet
Qsum
 2
QOF Rorifice  R 2jet
Rdrop from Navier-Stokes for a flat profile
– Jetting:
4R 3
t pinch_ off  20R jetOF / 
Rdrop from Qsum 
drop
3t pinch _ off
Rjet from Navier-Stokes for a parabolic profile
Model:
Solid line…predicted drop size (dripping)
Dashed line…predicted drop size (jetting)
Dotted line…predicted jet radius (flat velocity profile)
Dash-dotted line…predicted jet radius (parabolic velocity profile)
TRIPLE EMULSIONS
• Cascaded microcapillary devices: drops within drops within drops:
number and size of all steps can be controlled.
POSSIBLE NEW MATERIALS
• Double emulsion of water-volatile oil with surfactant-water
– Surfactant: diblock copolymer or phospholipid
– Surfactant goes to the interfaces, oil evaporates
– Possible encapsulants of drugs etc.
• Add resin (‘glue’) and
harden (e.g. by UV light) –
solid shells
SHELLS OF LIQUID CRYSTALS
• Middle fluid: liquid crystal mixed with chloroform (to ensure isotropy
and lower viscosity)
• Chloroform evaporates – shell of liquid crystal
• Defect structures can be studied
CONCLUSIONS
• Drops could be used as microreactors for chemical reactions.
• Once made, drops can be manipulated in channels, imprinted in
PDMS.
• Production of drops and jets with coaxial flows lead to highly
monodisperse emulsions for many possible applications.
• Despite great expectations, commercial microfluidic devices are very
few.
• Active field of research, full of imagination, innovation and promise,
but still in its infancy. ‘As a field, microfluidics is a combination of
unlimited promise, pimples and incomplete commitment.’