A guide to droplet generation

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Contents
INTRODUCTION ........................................................................................................................................ 4
Droplet generators ............................................................................................................................... 4
A choice of designs ............................................................................................................................... 4
DROPLET GENERATION............................................................................................................................. 5
Droplet generator geometry ................................................................................................................ 5
Flow rate control .................................................................................................................................. 5
Droplet sizes and production frequencies............................................................................................ 6
Fluid selections ..................................................................................................................................... 9
Surface wetting properties ................................................................................................................. 10
Surfactant additions ........................................................................................................................... 10
Recommended cleaning technique .................................................................................................... 10
FLUIDIC CONNECT PRO........................................................................................................................... 11
CUSTOM DROPLET GENERATORS ........................................................................................................... 133Rev 1.0

Introduction
The use of droplet generators as research tools ensure you have both an easy to set-up system and aversatile platform for your investigations. Droplet generator chips offer a way to create highly
monodispersed bubbles and droplets at user defined production rates.
Key features of droplet generator chips:• Selection of device for a range of droplet sizes up to140µm in diameter• Chemically inert materials• Large viewing area of channels• A building block in a modular system
Droplet generators
The droplet generator chips that are compatible with theFluidic Connect 4515 and the Fluidic Connect Pro interface aresupplied in the standard polypropylene (PP) Fluidic Slide. The off-the-shelf chips are made of borosilicate
glass, other materials can be used on request. The glass chips are naturally hydrophilic and can besupplied with a hydrophobic coating on the channel surface, allowing either oil or water based droplets.
A choice of designs
The focused flow droplet generators come in four versions. All havethe same chip size but the channels and nozzles have different sizes.
Chips are offered with nozzle sizes of 5µm, 10µm, 50µm and 75µm.
The droplets that are generated with these chips are in the range ofthe size of the nozzle.
Focused Flow
Droplet Generator

Do you want different features for your droplet generators?
• More inlets or outlets• Different channel dimension or parameters• Integrated electrodes• Other material choices: Fused Silica / Quartz glass / Polymeric materials

Contact us to have your own custom-made droplet generators. See page 12 for more information.

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Droplet generation
The choice of the correct droplet generator for your application is sometimes not an easy one. There
are many variables which can affect the size, frequency and consistency of the droplets produced
including:• Channel dimensions and geometry• Flow actuation stability• Flow rates of each fluid, both relative and total• Channel wetting properties: hydrophobic (coated), hydrophilic (uncoated), etc.
• Miscibility of the continuous (outer) and dispersive (inner) fluids• Viscosity and surface tension of the continuous (outer) and dispersed (inner) fluids• Surfactant type and concentration
Droplet generator geometry
Focused flow geometry
The focused flow design consists of a cross junction where the inner
fluid or dispersed phase enters through a single channel and theouter fluid referred to as the continuous phase impinges on thedispersed phase from two channels diametrically opposite eachother. This combination of dispersed phase surrounded bycontinuous phase flows through the output channel, via the orifice.
The orifice is a constriction in the channel used to create a controlled
break-up of the dispersed phase into droplets.

Focused FlowDroplet Generator

Flow rate control
Another key factor to the production of uniform droplets is stability of the flow rate. In order to havea system running with constant throughput, a settling time is needed between changing flow rate
parameters and obtaining droplets with low polydispersity. This time varies dependent upon theactuation system used, with the worst case being 20 minutes wait time for a system using a coarse
controlled syringe pump and large plastic syringes, with long sections of Teflon tubing forinterconnection to the system. Once the system flow parameters have been set and the stabilisation
period is finished, the production mechanism of the droplets shows only small variations. The time toreach stable production can be reduced by using shorter interconnection tubing, as well as pressure
driven flows or syringes pumps with reduced pulsations to the flow rate pumping. If the flow rate is
not stable it can help to add some back pressure after your droplet generator. At elevated pressures,
small variations are of less influence on the droplet system.

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Droplet sizes and production frequencies
Effect of flow rate ratios
The use of flow rate ratio variation as a control parameter for selecting droplet size is a well-establishedtechnique due to both its simplicity and robust repeatability. What can be seen is that the increase in
the continuous phase flow rate compared to the dispersed phase flow rate enables the creation ofsmaller droplets and conversely a decrease will lead to an increase in size. The system will reach anatural limit in terms of the variations, as too slow a rate of flow for the inner fluid will cause droplet
production to stop and two fast a rate will mean the dispersive phase will run parallel to the continuous
phase with no droplets formed. A further effect of varying the flow rate ratio is to alter the production
frequency. This however is not purely dependent on the flow rate ratio, but is also affected by theindividual flow rates as well as the fluid parameters themselves. Depicted in figure 1 and 2 are guides
to the range of frequencies possible for our DGFF.50.2 and DGFF.75.2 droplet generator chips when theflow rates of the fluids are varied. For these results FC40 oil, di-ionized water and tween-20 was used.

Focused flow droplet generator 10.2

Droplet result focused flow droplet generator 10.2

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Independent control of droplet Size (µm) DGFF.50.2
Continuous phase flow rate (µL/min)2001805716057

140616412068100688070747672776083774082

droplet size (µm)

200051015202530354045

Dispersed phase flow rate (µL/min)Figure 1: Production frequency as a function of flow rates for the FFDG.50.2 chip for a specific set of circumstances.

Independent control of droplet Size (µm) DGFF.75.2
Continuous phase flow rate (µL/min)

300672507375
82

20087859193891509698

10010010150

109111111

droplet size (µm)

139005101520253035

Dispersed phase flow rate (µL/min)Figure 2: Production frequency as a function of flow rates for the FFDG.75.2 chip for a specific set of circumstances.

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Figures 3 and 4 show the droplet sizes as a function of flow rates. What can be seen is that the droplet
size depends a lot of flow rates. Depending on required production rates not all droplet sizes areachievable.
A general rule of thumb for a particular microfluidic device, the smaller the droplets created the fasterthe frequency at which this can be achieved. Naturally, there is a lower limit to the size a particulardevice can achieve and this is based upon the physical size and individual geometry of that particulardevice.

Independent control of droplet frequency (1/s) DGFF.50.2
Continuous phase flow rate (µL/min)

200
180

1735

160

3534

140

1400

3676

120

2007

100

1018

80

2744

3154

1467

867

2757

60

1677
1401

40571

Droplet frequency (1/s)

2000

51015202530354045

Dispersed phase flow rate (µL/min)Figure 3: Droplet size as a function of flow rate for the DGFF.50.2 chip with a specific set of circumstances.

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Independent control of droplet frequency (1/s) DGFF.75.2
Continuous phase flow rate (µL/min)

300
526

250

818
385
882

200
244

522

208

392

894

150
543

681

785

100
153

50

371

464

230

Droplet frequency (1/s)

60

005

10152025

3035

Dispersed phase flow rate (µL/min)Figure 4: Droplet size as a function of flow rate for the DGFF.75.2 chip with a specific set of circumstances.

Effect of total flow rates
As a general rule, increased total flow rates lead to increased production frequencies for the droplets.
However, there is a natural limit to this trend as at sufficiently high flow rates the fluids will flow parallel
to each other with no real interaction or droplet formation occurring.
Fluid selections
A key factor in droplet size and production is the interfacial surface tension between the continuousand dispersive phases. The interaction between the two fluids can influence the production rates andsizes of droplets produced, with rates increasing and sizes decreasing as the value for interfacial tension
decreases.
The choice of fluids used in droplet generation is generally based on what is under investigation.
However, in order to ensure good droplet formation two fluids that are immiscible are necessary. It isalso recommended that the use of highly viscous fluids is avoided for small channel geometries as thishas an adverse effect on flow rates and pressures, as high viscosity fluids tend to break-up into larger
droplets than fluids of lower viscosity.

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Surface wetting properties
The contact angle between the fluids and the droplet generator
surface are key for defining the stability of the wetting of thecontinuous phase, if the wetting is more preferential for thedispersive phase then pinning of the droplets to the channel canoccur. The standard uncoated droplet generators of Micronit areglass based, thus hydrophilic and are suitable for making organic
droplets in an aqueous phase (oil-in-water droplets). It is possible forMicronit to provide a coating which renders the surface of the droplet
generators hydrophobic, thus suitable for making aqueous droplets in
Hydrophobic surface coating
an organic phase (water-in-oil droplets). This coating is based on afluorinated polymer and ensures the coated surface has a contact angle of more than 90° with water.

Figure 5: Droplet can be oil-in-water or water-in-oil.

Surfactant additions
The use of surfactant fluids, in very low concentrations of around 1-5% v/v, aids the stability of thedroplets produced and reduces the instances of droplet coalescence. The surfactant population
concentrates at the interface between the two fluids as the molecules naturally orient to have thehydrophilic head and hydrophobic tail in contact with the appropriate fluid. Choices of surfactant are
dependent on the fluids used for the production if the droplets:• Addition to organic fluids: Span 80 and Triton X-100• Addition to aqueous solutions: Tween 20, Tween 80 and SDS (sodium dodecyl sulphate)
Recommended cleaning technique
In general, it is advisable to use pre-filtered fluids in the droplet generators, as channel sizes are small
and blockages can easily occur. This should help to reduce blocked channels and prolong the lifetime ofthe chip. However, it is still necessary to maintain your droplet production system, including occasionallycleaning the droplet generator chip. In case this is needed we recommend to flush with de-ionised
water, a solvent or another compatible fluid. To remove larger blockages from the channels werecommend placing the chip in an ultrasound bath for 10-15 minutes before flushing fluids through the10Rev 1.0

system at high pressures (7bar) or flow rates (100-200µl/min). The use of chemicals for more abrasive
cleaning is possible if necessary, such as the use of sequential flushing with acetone, distilled water andfinally isopropanol or another alcohol solution.
Finally, for uncoated pure glass devices it is possible to use a solution of sodium hydroxide (NaOH). A
solution of 1M sodium hydroxide in water is effective, however lower concentrations might also be
sufficient. If traces of the cleaning solution remain inside the chip after cleaning and rinsing with water
is not possible then ammonia can be used instead. Please note this should not be attempted withcoated devices, as coatings will be removed when using for instance NaOH.

Fluidic Connect Pro
The Fluidic Connect Pro offers a user-friendly way ofcreating your own lab-on-a-chip setup within minutes.
The durable and robust platform design is compatible
with a large variety of microfluidic chips meaning droplet
generation, micro-reactions, cell analysis, and many more
applications can be carried out on one single system. The
chip-holders can be used in conjunction with standard
laboratory equipment such as syringe pumps and uprightand inverted microscopes. The load and seal design
assures tight connections without the possibility ofbreaking precious microfluidic chips. By cleverly making
use of inserts the holder can easily be adapted to chips ofdifferent sizes and thicknesses. It is even possible toconnect multiple chips at the same time.

Use the Fluidic Connect Pro for your lab-on-a-chip experiments and reduce your time to obtain results!
Key features of Fluidic Connect Pro:
• Fast, easy, and leak-free fluidic connections• Chemically inert materials• Large chip viewing area• Compatible with upright and inverse microscopes• A building block in a modular system• Standard and custom chips available• Holder defined sealing to prevent chip cracking• Simultaneous connection of multiple chips• Able to connect chips from 15 x 15 mm up to 30 x 90 mm

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Fluidic Connect PRO
Sealing Mechanism
Chipholder Material
Dimensions (L, W, H)
Max. Operating
Temperature
Max. Operating Pressure
Sealing Material
Compatible Chip Thickness

Compatible Chip Sizes

Compatible Tubing

Load ‘n' Seal
Aluminium
128 x 85,4 x 20 mm80ºC
10 barPerlast (FFKM)
1.1 – 2.0 mm
15 x 15 mm
30 x 30 mm
15 x 30 mm
30 x 45 mm
15 x 45 mm
30 x 60 mm
15 x 60 mm
30 x 90 mm
15 x 90 mm
25 x 75 mm*
1/16 inch OD Teflon, Stainless Steel, PEEK

* Microscope Slide Format

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Custom droplet generators
Are you looking for a variation on the available droplet generators, for example if you require a different
size nozzle, then take a look at our online prototyping service miproto. This prototyping service means
purpose-designed droplet generators can be created to match your product needs.
Miproto’s benefits:
A quick detailed quote• Ideal tool for budget management
High quality microfluidic solutions• Choose from various polymer or glass types• Free expert advice available
Save time• Doing the right things right, the first time
• Delivery within 3 weeks• Focus on your research, while we create your chip
Micronit has more than 15 years of experience in microfluidic chip manufacturing for science andindustry, making us your perfect partner to outsource your microfluidic chip needs.

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Micronit Microfluidics 2017
Address

Micronit (main office)Colosseum 157521 PV Enschede
The Netherlands

Phone+31 53 850 6 850

Fax+31 53 850 6 851

E-mailinfo@micronit.com

Websitewww.micronit.com

Webstore

store.micronit.com

Address

Micronit GmbH Dortmund
Konrad-Adenauer-Allee 11D-44263 Dortmund
Germany

E-mailinfo@micronit.de

Websitewww.micronit.de

Webstore

store.micronit.com

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