Sciencemadness Discussion Board

Magnettic stirring is surprising

Chemgineer - 13-4-2025 at 04:27

My heating mantle has a magnetic stirrer which I always thought was a little weak and when I started it I would just turn it on slowly and the stirbar would often decouple and cascade around the flask.

Well I've just realised that magnetic coupling increases with velocity/acceleration so if I just whack it on faster it actually couples more strongly!

jackchem2001 - 14-4-2025 at 00:29

Interesting. In my experience using round bottom flasks higher speeds may decouple overtime, especially if solids are present. I generally turn up stiring to a moderate speed (maybe this is the effect you describe?), then turn down to find the minimum that consistently stirs and go slightly above that.

I am not sure how much mixing benefit there is to faster stiring speeds at lab scales anyway. I would speculate that (especially for viscous liquids) a bigger stir bar would do you better than faster speeds. Or for jointed glassware, an empty thermometer well could be used as a baffle.

MrDoctor - 14-4-2025 at 03:19

is it possible that instead of a high temp resistant magnet that instead it has been constructed with an electromagnet that is in series or parallel with the motor, receiving the same (albeit rectified) regulated power?
i have never observed what you describe, any kind of change in the coupling strength with speed. At most there is, where a particularly thick/heavy solution once spinning produces far less resistance compared to when stationary, but im sure you dont mean that

Chemgineer - 14-4-2025 at 11:48

Quote: Originally posted by MrDoctor  
is it possible that instead of a high temp resistant magnet that instead it has been constructed with an electromagnet that is in series or parallel with the motor, receiving the same (albeit rectified) regulated power?
i have never observed what you describe, any kind of change in the coupling strength with speed. At most there is, where a particularly thick/heavy solution once spinning produces far less resistance compared to when stationary, but im sure you dont mean that


I suspect you could be right! Just tested on 500ml nearly full flask of ethanol and I can get it stirring right up to top speed but then when I slow it down at some point it decouples!

Twospoons - 14-4-2025 at 17:54

You are probably seeing two effects:
1] at faster speeds there will be a thicker film of liquid between the stirbar and the flask, reducing friction between the stirbar and flask
2] at faster speeds the viscous drag on the stirbar will result in an increased angular lag behind driving magnet in the hotplate, which in turn shifts the magnetic force vector on the stirbar to be more sideways and less down, so once again less friction against the flask.