How to Make a Cheap, Portable Magnetic Stirrer





Introduction: How to Make a Cheap, Portable Magnetic Stirrer

About: My name is Alex Ngai. I am currently an electrical engineering student at Cornell University. I’m interested in bio-mimetic walking, running, swimming, and flying robots, automation, and 3D bioprinting. Cy...

I needed a magnetic stirrer to keep some cells agitated for cell culture, but didn't want to spend the thirty or so dollars needed to buy one. This magnetic stirrer uses an old computer fan and some cheap neodymium magnets with a moldable plastic stir bar.

The parts are:
- computer fan
- small magnets: (free shipping)
- moldable plastic (polymorph, instamorph, shapelock)
- lexan
-superglue (cyanoacrylate)

Step 1: Glue the Magnets to the Fan

Prepare and clean the fan for use. Your fan may have two wires (red for positive and black for negative). If it has three wires, the white wire is a tachometer and is not needed. Glue the magnets on opposite sides of the fan head. The magnets in the fan's motor may push the magnets around a bit, so arrange accordingly. I arranged the magnets to have opposite polarities facing up.

Step 2: Add Spacers

The magnets will increase the profile of the fan, so you will need spacers before putting a flat surface on top. The spacers can be anything, I used coins initially. Eventually I snapped off a few pieces of spare plexiglass I had lying around and glued them to the edges of the fan. Either way, make sure the spacers are higher than the profile of the fan with the magnets glued on.

Step 3: Add the Main Platform

I cut a small square of plexiglass over the fan as a platform for any containers I will need to stir. Glue or attach any thin, waterproof material to the spacers.

Step 4: Make the Stir Bar

The stir bar is made of two magnets embedded in the moldable plastic. Simply place the plastic in a container of hot water until it turns clear. Place two magnets (polarities opposite facing) onto the plastic and surround with more of the moldable plastic until the magnets are covered. Then round off the edges and mold until the bar is roughly cylindrical with the embedded disk magnets still inside.

Step 5: Powering Your Magnetic Stirrer

The magnetic stirrer can be powered by any voltage source the fan is rated to. However, make sure your magnets are strong enough to couple together from the fan to the stir bar with the rated speed. If the speed is too high, the stir bar will not spin correctly. I used a 9V battery at first, but used this set of 3 AAA batteries I had lying around. I didn't need torrential currents for mine, so I used a lower voltage. This is a magnetic stirrer, not a blender. The lower voltage will result in a slower fan speed, and the stir bar will better be able to better keep up with the fan speed.

Step 6: Use It

Place any container large enough to allow the magnetic stir bar to fit and start agitating/stirring! Since this unit is so small, you can place it on your desk and use it to stir your cold drinks. I wouldn't recommend hot drinks though, heat will cause the polymorph stir bar to soften and lose shape. If you want an improvement, you can try embedding magnets in ceramic and glazing the stir bar.



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    I guess old age has finally kicked into the grey matter.

    I kinda thought it was the magnets that were the primary factors,

    Not air movement!!!

    5 replies

    Did you even read it?

    Read it over several times. As I said, it may be my old brain.

    Still don't see what air movement has to do with a magnetic stirrer.

    Perhaps you might enlighten me?

    While the fan's air-moving function isn't inherent to the stirring, the air movement by the fan may affect the liquid if it is hot and you are trying to keep it that way. The air blowing on the base could potentially cool your liquid somewhat, so if stirring hot liquid is your intent, I would recommend planning your material's design around that potential issue.

    The fan is just being used as a conveniently shaped, and readily available, motor. The blades aren't important to this.

    "I guess old age has finally kicked into the grey matter. I kinda thought it was the magnets that were the primary factors, Not air movement!!!"

    you were right the first time. Its all about the magnets and nothing to do with air movement.

    For clarity, I'll cover the instructable. It is about re-purposing a throwaway old CPU fan into about the cheapest magnetic stirrer that can be found. There is an elongated piece of plastic covering small magnets inside the glass with the solution to be sired, and is spun by the rotating magnetic field from magnets on top of the fan. In the picture with the glass, it is hard to see as anything more than a white blur at the bottom because it is spinning so fast. But, you can easily see the vortex it generated. The actual fan blades serve no purpose in the stirring process except maybe to keep the rotor from spinning too fast.

    Side note: Like an unbalanced tire will cause increasing vibrations when spun faster, slightly unbalanced magnets glued to the rotor can cause excessive vibrations if spun too fast like say after cutting the fan blades off so that it is no longer loaded by air resistance, the magnets on top may cause excessive vibrations if it spins faster and there the magnets are not balanced.

    Normally, you don't want to use a stir bar with cells since the action of the stir bar and the bottom of the beaker will lyse them. Roller bottles are the common way this sort of thing is done at bench scale.

    8 replies

    If there are electrolytes in the water you could induce a flow using emf maybe? I don't know if that would kill them or not though. I've done it in a coffee cup. Really cool, but not terribly easy to apply.

    hrm. "If there are electrolytes in the water you could induce a flow using emf"

    I have always seen Magnetohydrodynamics use an emf to generate currents between plates conducted by ions through magnetic field causing a deflection in the ions that pushes the fluid. But, can an alternating magnetic fields induce currents in an electrolyte and those currents induce a reverse magnetic field that opposes the initial change in the magnetic field?

    I guess probably. Important factors would include but may not be limited to:
    1. The rate of change in the magnetic field so either you want really powerful magnets or your magnets have to be moving really fast or both.
    2. Resistance to current flow in the electrolyte.

    or maybe we wont get the the kind of movement we are hoping for. take sodium chloride for example. In water it becomes (Na+) sodium ions and (Cl-) chloride ions. A moving magnetic field would cause + ions to move in one direction while - ions move in the other, in a plane that is transverse to both the magnetic field and the magnetic field's direction of motion. I am moving beyond my area of expertise but overall wouldn't the two momentums cancel leaving little more than a heating effect?

    You are right about the motion of the ions. However, if we are not picky about the direction of stirring then a pulsed axial magnetic field would alternately push negative ions out and positive ions in, and then when the field collapses reverse directions. If the field also had a rotation whose direction depended on the phase of the pulse, then a toroidal current would be set up. Conservation of angular momentum would create the same vortex pattern of current that the magnetic stir bar creates.

    "However, if we are not picky about the direction of stirring then a pulsed axial magnetic field would alternately push negative ions out and positive ions in, and then when the field collapses reverse directions." This will happen regardless of direction of stirring. During the pass of one magnet with a magnetic field in the axial direction, ion of one type will first be pushed into the center and ions of the other type will be pushed to the outside and return after the magnet has passed.

    "If the field also had a rotation whose direction depended on the phase of the pulse, then a toroidal current would be set up" Some times it seems english is poorly equipped to discus changes in currents and magnetic fields in three dimension of space and one of time, not to mention adding in their interaction on + and - ions so i had a hard time visualizing what you concluded from your statements. But If I may step back for a second and ask "If the ion +- momentums balance, and individually both the + and the - ion to fluid displacement also balanced then aside from induced net alternating electric currents, are we not left with a net 0 fluid current".

    Maybe I should try this on a solution of sodium hydroxide and another solution of sodium chloride to resolve it.

    Also, one would definitely need to use electro magnets. Probably not possible using solid magnets.

    then that will be my next instructable xD

    a cheap multi tier cell roller would be awesome!!!

    If you raise the fain up off the surface it'll run more efficiently by allowing the air to flow through.

    4 replies

    It turns out this is NOT true. Fans run easiest in a vacuum. If that doesn't make sense, envision the opposite case whereby you try to push heavier air, which would cause the fan to work harder. Having the fan inlet sit flush on the table is perfectly acceptable.

    Not being too critical but try plugging in some reasonable values on air pressure. If the air pressure in ones lab is 1 bar, and given a single stage radial fan can at best only generate a 0.1 bar pressure differential (while radial fans are good for high flow rates, they are terrible creating high pressure differential), then all you will have is a partial vacuum of around 0.9 bar. Consequently, one ends up still having lots of air mas working on the blades as they spin generating lots of turbulence, and the drag approximating some function that is the cube of the rate of rotation.

    But even if a fan could achieve 0.9 bar pressure differential, rather than .1, then you have a reasonable vacuum on the underside, but you still have an average 1 bar pressure above of the fan blades with each passing of a fan blade pushing air up only to have it return after the fan blade has passed. All that movement of air is called turbulence, and is wasted energy that becomes nothing more than heat.

    Without speed controls, like a potentiometer or implementing PWM, the fan blades may still prove useful in preventing the rotor from spinning too fast where a slight imblance in the magnet's placement would introduce excessive vibration.

    I guess I was arguing on efficiency of no blades vs having blades when the initial argument was questioning the efficiency of placing the fan on table vs letting air flow through.

    While sitting on the table top would create lots of turbulence, maybe it is not using anymore energy than if air were allowed to flow through the fan. But there will be less vibration with air flow than if the fan were sitting on the table top; and one might find a way to put air flow can be put to good use.

    Air flow potential uses:
    1. Thermal regulating the contents of the container being stirred to room temperature. or
    2. One can have an inverted fume hood if air flow went down.

    I am not too sure of that.

    Putting the fan close to a surface will not make air less dense.
    On the contrary, putting a surface in close proximity to the fan will create a situation in which the fan will have to push the air against a surface, or suck it up from an enclosed space underneath. The latter might create a vacuum to an extent, but I am not convinced that would work the way you suggest, bit like sucking through a straw.
    If the fan was to operate in a sealed enclosure with most of the air removed, I agree it would spin faster.