Iron core barely boosting field strength

[Vanadium 50]

A random piece of iron ot steel has μ=150 or so. So 3mm of steel has the same effect as less than half a millimeter of air. So the steel doesn't matter. Just the air. So you need to get as much of the air gaps out of your system as possible.

The alternative is to crank up the current. The problem is you are already near the 20 AWG limit. OK, so instead increase the number of turns - but that increases the resistance and now you need more voltage. So I think you are stuck with fixing the geometry.

 

[hutchphd]

I have mentioned this before, but I was tasked with a similar problem (also involving magnetic particles) and thought "no sweat" how hard can this be?. My best decision was to get a program (called Vizimag as I recall) and start playing with it. One should not skip this step...I learned so much about this stuff in a few days of just farting around. Unfortunately I think the software does not exist anymore, but surely equivalent ones do. I too tried for super large permeability (not useful) and confirmed the nasty idea that size (volume) and voltage matters because more turns always means less current otherwise. I ended up producing a good solution despite my initial ignorance.

 

[canuck123]

I have mentioned this before, but I was tasked with a similar problem (also involving magnetic particles) and thought "no sweat" how hard can this be?. My best decision was to get a program (called Vizimag as I recall) and start playing with it. One should not skip this step...I learned so much about this stuff in a few days of just farting around. Unfortunately I think the software does not exist anymore, but surely equivalent ones do. I too tried for super large permeability (not useful) and confirmed the nasty idea that size (volume) and voltage matters because more turns always means less current otherwise. I ended up producing a good solution despite my initial ignorance.

I am certainly feeling the stupidity now. I know programs like COMSOL would be good but those licenses are like 4000$ or something. I initially wanted to make the exact same setup as pictured in the research paper, but my microscope objective is so frickin wide that I can't get a coil close to the sample. What did you end up doing?

 

[Vanadium 50]

There exist free versions of software to calculate magnetic fields. OpenEMS is one.

Unfortunately, it doesn't make on an expert in designing magnetic systems, any more than Excel makes someone an accountant. It also sounds like it won't solve you problem - if you need a high field far from the coils, you are going to need ampere-turns. Plain and simple.

 

[berkeman]

Yes the dimensions are weird, I was trying to make it fit with my microscope objective and available fluid cells, since it is a pretty big objective and the fluid cell is machined precisely to fit some special components.

but my microscope objective is so frickin wide that I can't get a coil close to the sample.

Do you have any options to machine some different fixturing for your microscope? Or maybe use a different microscope? Maybe those would be better options comparing to going 10x on your coil drive power...

 

[hutchphd]

How much magnification do you need? Are the fluid cells complicated somehow?How much do you need to see? Does the objective need to be "in" the magnet (or vice-versa?)? There are many ways to get to were you need to be but you need to be facile with your tools.

 

[Baluncore]

Either you replicate the experiment by closely duplicating the magnetic circuit construction, or you optimise the magnetics and repeat the observations in a different magnetic fixture or jig.

Do you have any options to machine some different fixturing for your microscope? Or maybe use a different microscope?

The design will come down to identifying what area, under the microscope, is required to carry out the experiment. As reported, the size of the objective will also limit access. What range of magnification is needed? Can you use the objective from a stereo microscope that has a deeper field?

 

[canuck123]

I am using a zeiss 20x water immersion objective, I believe this is the one: https://www.micro-shop.zeiss.com/en...ective-W-Plan-Apochromat-20x-1.0-DIC-M27-75mm

They are quite expensive and have quite a small working distance from the sample so they have to be close. I may try making some 1/4" square mild steel cores with 38 degree downward angles to get in as close as possible, but I imagine they'll still be a cm or so away from the sample.

 

[canuck123]

How much magnification do you need? Are the fluid cells complicated somehow?How much do you need to see? Does the objective need to be "in" the magnet (or vice-versa?)? There are many ways to get to were you need to be but you need to be facile with your tools.

20x is the right size to be able to see my particles. Gives roughly a 500 micron square view which provides a good sample size to see how homogenous my particle movement is. Yes, the current fluid cells are complicated and somewhat large, but I am working on dumbing it down and making it smaller to allow for the coils to come closer from the side. The objectives for these microscopes are quite fat and have a short working distance with a 38 degree angle from the lens so there certainly isn't much space to come in from above either, but I'll see if maybe a skinny 1/4" square core could do it.

 

[berkeman]

Here is the link to the supplementary information from the paper, this has all the important stuff about the setup: https://www.rsc.org/suppdata/c5/lc/c5lc00294j/c5lc00294j1.pdf

Just for my curiosity, does the spinning 3-D magnetic field basically fling the magnetic particles apart in the suspension? Or is there some other magnetic interaction going on to make the magnetic particles repel each other? It does appear that the rotating component of "rotaphoresis" is important, otherwise the particles just clump up in a DC B-field as expected. Interesting stuff.

 

[Baluncore]

Just for my curiosity, does the spinning 3-D magnetic field basically fling the magnetic particles apart in the suspension?

A circular, or eddy current, is induced in the iron particles, by the changing magnetic field. The induced field is a counter field, that cancels the incident field. The incident field then changes orientation, so the particle reacts magnetically as the fields are misaligned.

Think of the model as an induction motor, where many small rotors, are turned in the same sense, by the external rotating quadrature field. The rotors also interact with each other as they turn.

Think of two close particles as being two small gear wheals, that are engaged magnetically. As they both turn in the same sense or direction, they are forced apart by the contrary direction of motion of their engaged surfaces. That is why the particles spread out evenly in the soup.

 

[berkeman]

Thanks @Baluncore that is very helpful.

 

[Tom.G]

Just a few comments:

Many Google hits for Visimag download (and tutorial):
https://www.google.com/search?hl=en&q=vizimag+download

If the 'scope objective is magnetic, that could mess up the magnetic field. Even if it is Brass, it will mess with the AC field shape due to induced currents (another case of "a shorted transformer secondary").

How about moving the X-Y coils below the sample and the Z coil around the objective. I don't know how practical, but maybe a larger diameter Z coil, either air core, or metal core with a hole in the center for the optical path.

Of course that may just transfer the magnetic problems to the Z coil but that's one coil instead of four.

Cheers,
Tom

p.s. Please keep us updated, even after success. We like to learn too!

 

[canuck123]

Update: I built a new microscope stage and new coils much like the ones in that paper (but a little improved). Everything works a treat. The cores are just mild steel and they have the same angle on the end as the objective so they can get in as close as possible to the sample. I put a mild steel yoke around the outside that connects the coils horizontal coils.

I'm able to get plenty enough field strength (up to about 25mT with the bottom coil and up to about 10mT with the x or y coils) on my sample center. Now I just need to figure out how to actually make my particles do what I want, but that's another matter altogether. Should be fun! Thank you everyone for helping a newbie.