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 ( l). 1nl=10 -9 l=(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-[ 18 F]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~10 3 kg/m 3, η~10 -3 kg/ms v~1µm/s-1cm/s L~50µm Re~

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: 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: V jet =πR 2 L, S jet =2πRL Drops: V drops =n4πr 3 /3, S drops =n4πr 2, V drops =V jet 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 l jet =t pinch off · v interface 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 Q OF...flow rate of the outer fluid Q sum...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: R jet from the mass flux at the orifice R drop from Navier-Stokes for a flat profile –Jetting: R drop from R jet 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.’