Micro Droplet Injector

Research into the possibility of injecting a low volume stream of liquid into a fast stream to create droplets 100 um across.

Similar projects worth following
This project is still very theoretical. So far I have only done the most basic proof of concept work.

One method of creating micro sized nano robots uses tiny droplets of ferrofluid. To make them, the NanoRobotics Club at the University of Alberta had previously used a microinjector, something that is not built to handle these volumes of fluid well. This new approach seeks to improve the diameter of the droplets we can create.

The picture to the right shows a droplet made with the microinjector, submerged in silicone oil. It is about 400 um in diameter.

The device I hope to build uses two linearly actuated stepper motors, which push the plungers of small volume syringes. One syringe is filled with oil and the other with a water based ferrofluid.

When something drips from the tip of a spout, its size is determined by the balance of forces between gravity and the inclination of fluids to cling to surfaces. The oil rushing past the ferrofluid should decrease the clinging action and induce the droplet to leave the needle faster, resulting in a smaller droplet.

After this project is completed, I hope to expand research into the effectiveness of different ferrofluids to act as micro object manipulators.

  • Notes

    Adam Gulyas08/20/2014 at 14:43 0 comments

    Calculations and general musings about droplet diameter control.

    Some sphere calculations:

    Diameter Volume
    1 mm 523.6 nL
    500 um 65 nL
    400 um 34 nL
    300 um 14 nL
    200 um 4.2 nL
    100 um 520 pL

    So if the goal is 100 um, the syringe should be able to dispense 520 pL of fluid with a certain plunder displacement. The minimum usable displacement will be decided by the minimum step size of the stepper motor.

    Needle point types:

    Some potential syringes:

    (Sigh, these links are now dead...)

    The lowest volume one can dispense 5 uL, which means I would want to dispense about 1/10000 of the volume at one time. If the plunger travel distance is 10 cm, I would need to be able to push it at 10 um increments. Can stepper motors do that?

    Another option would be to push the ferrofluid out slowly, relying on the speed of the oil to tear drops away from the needle quickly. If I can get 1 mm/second (not sure about this) I would need to produce 100 drops/second.

    Smaller gauge needles should let go of drops of lower volume, everything else being equal. My hypothesis is that a droplet will release when it is of larger diameter than the outside of the needle, given a flow of fluid past it. If that's true, I can control droplet size through needle diameter. Gauge 34 needles have an outer diameter of 184.2 um. (But would they be strong enough to pierce a plastic tube?)

    Apparently you can model pipe diameter and fluid pressure like resistance and voltage. If I get a small gauge needle, I might not have to worry about moving the plunger slowly. The goal might instead be to create a constant pressure in the syringe, which would create a steady fluid flow through the needle.

    It becomes apparent that a mechanical engineer would be useful here.

View project log

Enjoy this project?



PointyOintment wrote 07/27/2020 at 08:38 point

Are you sure you need linear steppers or servos, and that they would work? I usually see syringe pumps (even on here) using regular rotary motors and leadscrews. Leadscrews can be very precise—they're what's usually used in micropositioners. If you need super-fast actuation, you could instead use hard drive head-positioning voice coil motors, connected via a connecting rod or similar.

Or you could use a leadscrew for the slow motion (i.e. moving the plunger to continue to apply pressure as the liquid is depleted) and do the fast droplet-ejection pulses using a second actuator, such as a piezo stack between the leadscrew and the plunger, or a hammer that hits a membrane on a second liquid-filled chamber that communicates with the syringe and the nozzle. I think the latter is probably better, because it circumvents syringe plunger stiction, at least for the droplet ejection. You might be able to take up the pressure pulsation caused by syringe stiction using a second more flexible membrane, and somehow (acoustically) route the pulse from the hammer membrane toward the nozzle. Or just get rid of the syringe altogether and use one soft membrane with a leadscrew for slow pushing and a stiffer one with a hammer for ejection.

Also, maybe look at the mechanisms that inkjet printer heads use. Might be hard to replicate without industrial technology, but might inspire something easier to make.

  Are you sure? yes | no

Tim wrote 01/02/2020 at 17:42 point

If you want to generate droplets, there are systems available on the market. For example, the pressure driven system, it is able to generate droplets with very good CV.

  Are you sure? yes | no

Brian wrote 12/23/2015 at 20:25 point

I'm not quite clear on your application, but if you need a large number of a specific size droplet you could try ultrasonic ejection. An ultrasonic transducer agitates the ferrofluid causing microdroplet ejection into a stream of silicone oil. In may be possible to sort the microdroplets by size using an appropriate magnetic field strength in a manner similar to a mass spectrometer. 

If a magnetic field were used to hold the ferrofluid onto the surface of the ultrasonic transducer, then perhaps the ejected microdroplets could be ejected into free space.

  Are you sure? yes | no

Scott Powers wrote 12/20/2015 at 20:04 point

If you have the ability to cast parts out of PDMS I think this becomes a fairly easy project.  What you are doing is actually analogous to droplet formation in a T junction fluidic device.  There are a lot of papers on this sort of work, especially out of the Weitz lab.  This paper might help a bit: 

Alternatively if you are just collecting drops and using elsewhere you do not have to do this in fluidics.  You can do a mini emulsion chemistry and create droplets due to shear forces on immiscible fluidics.  The paper you want to start with there is titled Solvent Free Synthesis of Janus Colloidal Particles, Granick is the PI on the paper.  

  Are you sure? yes | no

benoit.callebaut wrote 07/29/2015 at 19:53 point

I would use a peristaltic pump. 

It has the following advantages:

  * No piston and no valve -> less leak.

  * Mechanically easier to to using a stepper motor

  * Ability to control very small volumes

  Are you sure? yes | no

Similar Projects

Does this project spark your interest?

Become a member to follow this project and never miss any updates