PRODUCING TINY DROPS USING THE TOOLS OF MICROFLUIDICS

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PRODUCING TINY DROPS USING THE TOOLS OF
MICROFLUIDICS

• Brina Crnko
• Advisor prof. dr. Slobodan Žumer

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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

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MICROFLUIDICS

• Studying flows in small channels (r50µm).
• Manipulation of small amounts of fluids
  (10-9-10-18 l).
• 1nl10-9l(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).

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MICROFLUIDICS

• Example
• Microfluidic (proof-of-principle demonstration)
  chip, that synthesizes FDG (2-deoxy-2-18Ffluoro
  -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

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FLOWS IN MICROCHANNELS

• Reynolds number ratio of inertial to viscous
  forces
• Water ?103kg/m3, ?10-3kg/ms
• v1µm/s-1cm/s
• L50µm
• Re10-6-10

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FLOWS IN MICROCHANNELS

• Remember for pipes with smooth walls, flow
  becomes turbulent for Regt2000
• For L50µm, flow is laminar for vlt10m/s
• Flow 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.)

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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)? d(area)
• Jet VjetpR2L, Sjet2pRL
• Drops Vdropsn4pr3/3, Sdropsn4pr2,
  VdropsVjet
• When rgt3R/2, surface energy is lower for drops.
• A cylinder of water in air is unstable.

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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).

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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
• ljettpinch off vinterface
• Capillary numberCaviscous drag/surface
  tension?v/?
• ?viscosity (outer fluid), ?interfacial
  tension, vvelocity (inner fluid)
• Transition between dripping and jetting Ca1

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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

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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.

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RADII OF JETS AND DROPS

• QOF...flow rate of the outer fluid
• Qsum...sum of flow rates
• of middle and inner fluids
• Dripping
• Solid circledrop
• diameter
• Open circleinner drop
• diameter
• Half-filled circlejet
• radius
• Jetting
• Solid triangledrop
• diameter
• Open triangleinner
• drop diameter
• Half-filled trianglejet
• radius

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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 profile
• Model
• Solid linepredicted drop size (dripping)
• Dashed linepredicted drop size (jetting)
• Dotted linepredicted jet radius (flat velocity
  profile)
• Dash-dotted linepredicted jet radius (parabolic
  velocity profile)

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TRIPLE EMULSIONS

• Cascaded microcapillary devices drops within
  drops within drops number and size of all steps
  can be controlled.

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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

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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

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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.