User:Brian P. Josey/Notebook/2010/05/18

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Variable Current Density

I am playing with the idea of mounting a yoke onto a microscope stage so that I can perform experiments in a flow cell and see the result as it happens. Friday, after calculating greater terminal velocities in the flow cell and tube for the iron electromagnet than I was for the neodymium yoke, I was hopeful that I could capitalize on the variability of the force generated by the iron. The one disadvantage that I can foresee with the neodymium yoke is that I cannot control the force because it is a permanent magnet. With an electromagnet, I can change the current flowing through it, and in turn the force. To see if this was viable, I ran a couple of simulations with differing current densities passing through the copper wires encircling the iron core in the electromagnet. Here are my raw data results for the force, and the terminal velocity of ferritin:

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Here, the current density, J, is measured in MA/m2 and is defined as the current per unit area perpendicular to the flow of charge. For my data point for a current density of -10 MA/m2, I instead used -9.69 MA/m2, which is the number that I have been using as my control in all of my calculations.

Here is a representative graph of the data. This graph shows the force near the tip of the cone as a function of the current density. All of the other graphs follow this general trend and were omitted to save space.

5-18-10 Force Near Tip.png

As you can tell from a quick glance of the data, both the force and the velocity, which is a scalar multiple of the force, begin by rising quickly until around a current density of 10 MA/m2. At this point, any increase in the current density is met with an insignificant increase in either the force or the speed. Going into the simulations, I knew that at some point I would hit the maximum saturation of iron, which occurs where the graph turns. From the data, it is clear that this occurs around 10 A.

From Friday, I have a calculated force of 0.975 fN, 0.186 fN and 0.041 fN for near the tip, flow cell and tube respectively for the neodymium yoke. In contrast, today, by increasing the current density, the force tends towards 0.54 fN, 0.3 fN and 0.06 fN for these points. As before, the neodymium yoke wins hands down near the tip, but surprisingly the iron experiences greater forces in the flow cell and the tube. For inside of the tube, both forces are so slight, that there are negligible, and as I said before, it is clear that the magnetic yokes have no real effect on this area. For the flow cell, it might be more advantageous to use the iron core electromagnet in place of the neodymium, something that I would like to try soon. However, when placing a cell near a magnet to perform an experiment, using the neodymium yoke is obviously the best possibility.

New Designs

The force acting on ferritin is more of a function of the gradient of the magnetic field than the strength of the field. Because of this, I am interested in creating a couple of new models that will explore different geometries. I want to know what would happen if I wrap the neodymium in copper wire and ran a current through it, what the effect very thin magnets would have on the force, and if I can bend the magnets and maintain the same forces. The addition of current to the neodymium magnets is to just see if I can magnetize it any more, which I doubt but it's worth checking, and is the least important idea I want to pursue. For the bending of the material, I am a little concerned with how the shape of the yoke could get in the way of the microscope, and if there is a way to redirect the field in a way that doesn't get in the way. And I want to use very thin magnets to check my understanding of the physics, and to see if that has anything to do with the gradient of the magnetic field.

Salad Dressing

I have also been thinking about Larry's idea about how to visualize the movement of individual ferritin molecules, the "salad dressing" idea. He suggested putting the ferritin into water with something that will label it, such as quantum dots or fluorescent tags, and then suspending droplets of the water in oil and flowing it into a slide. If after we apply a magnetic field to the ferritin and they move, then we should see all of the fluorescent labels, or quantum dots concentrated in a specific region. I don't believe that quantum dots are magnetic, and as long as I use an organic label, then there is no reason to believe anything other than the ferritin is attracted to the magnet. I figure with surface tension, it would be very difficult to suck the ferritin out of the water droplets, plus they are soluble in water and wouldn't go into a non-polar oil. The only issue that I am still thinking through is the creating droplets out of the ferritin-water-label solution. Could I place a very small amount of the solution into the oil and then sonicate it to break it up, or would the attraction between different water molecules prevent the separation of droplets? Could sonication break up a protein's shell?