PRODUCING TINY DROPS USING THE TOOLS OF

MICROFLUIDICS

Brina ČrnkoAdvisor: 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 L~50µm

Re~10-6-10vL

Re

FLOWS IN MICROCHANNELS• Remember: for pipes with smooth walls, flow

becomes turbulent for Re>2000For L~50µm, flow is laminar for v<10m/sFlow in microchannels is laminar.

• Navier-Stokes:

• Nonlinearity is absent (Stokes flow).• Laminar flow, no turbulence. • Fluids can flow parallely, no mixing, only diffusion.

(Mixing has to be achieved otherwise.)

fvpvvt

v

2).(

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 emulsione.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 DROPSQOF...flow rate of the outer fluid

Qsum...sum of flow rates of middle and inner fluids

Dripping:Solid circle…drop diameterOpen circle…inner drop diameterHalf-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

Rdrop from Navier-Stokes for a flat profile

– Jetting:Rdrop from

Rjet from Navier-Stokes for a parabolic profileModel: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)

22

2

jetorifice

jet

OF

sum

RR

R

Q

Q

offpinchsum t

RQ drop

_

3

3

4 /20_ OFjetoffpinch Rt

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.’