Density and Centripet Demonstration
Date: Spring 2012
I am trying to create a teaching tool that shows how blood separates in a centrifuge. I want to be able to shake up three liquids in a sealed jar and then have them separate into three component levels after a minute or so--like blood does in a centrifuge. I can do this with water and red food coloring (representing red blood cells) and vegetable oil (representing blood plasma), but I cannot figure out what a third white liquid (representing white blood cells) would be and that third liquid needs to float above the water. Any ideas?
Use a solid material to represent blood cells and dye your water
white. Maybe a fine red sand for blood cells?
We prepare a tool that demonstrates the same, but not in the same way you describe. There is a little prep that we do not find in the science catalogs. We find it in the fish store.
1 bag of red fish gravel(small sand, but not silt) to mimic the red cells
Very fine white polystyrene (we grind up a dry polystyrene coffee cup in a blender. Be careful! The polystyrene goes everywhere!)
In terms of percent, hand load 51 mL of the red sand, add water up to the 60 mL mark, then fill it up to 100 mL with oil and last add just a pinch (< 1ml) of the polystyrene.
For demo, we do this in a large 1000 mL cylinder in front of the class. We give a vigorous shake and place the cylinder on the bench for all to watch while we are taking about the centrifugation step in blood fractioning. In about 10 minutes, we wind up with the red cell fraction (hematocrit) of about 50%, the buffy coat (very fine oil-water mix) of about 10%, the yellowish oil that looks just like plasma and the white polystyrene on the top if the lipids.
These ratios, the hematocrit, is normal for males. The buffy coat white cells is borderline abnormal, but it makes the point. Plasma is normal for males. The lipids are normal if you used very small amounts of polystyrene, hyperlipidemic if too thick.
To make this normal for females, cut red sand to 41 mL.
For a sealed jar, simply measure where you want the total volume to be, measure that with a rule, label total volume. Calculate 0.51 x total volume = length of Red Cell, measure that length on the side of the jar, mark and fill up to that line with red sand. Add water so all the sand is covered with water and then a little extra, so you can see the water clearly. Fill to top with oil. Add smidge of polystyrene and replace lid. Done!
We generally can use this for 2 years(8 quarter terms) stored at cool room temperature if we cover it tightly with Parafilm. After that the sand begins to pick up the oil and it gets very messy. It still works, but not as clearly as we need for a large class, so we remake it when the separation looks messy.
Peter E. Hughes, Ph. D.
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Update: June 2012