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Minor Versionm

by Procheta Mallik


Fix a shirt button in the centre of a card sheet. Pass a cotton thread through two holes of the button and knot the two ends to make a loop. This simple toy can spin at very high speed and hence, can be used to separate liquid mixtures. Manu Prakash, a professor of bio-engineering at Stanford University, first demonstrated and perfected using this 'toy' for this purpose. It has opened a host of possibilities for the bio-medical field. Anybody with some paper and string can now centrifuge blood for blood tests without electricity and need for an expensive machine.

Video Overview

    • Take care while using scissors as well as the paper cutter, avoid making contact with their sharp edges and points.

  1. Start by inserting the pencil in the compass and measuring out 6 cm.
    • Start by inserting the pencil in the compass and measuring out 6 cm.

    • Cut a 14 cm x 14 cm piece of cardboard.

    • Now, being careful not to move the compass arms, use the pencil end to draw two diagonal lines on the cardboard square. The point where the diagonals meet is the centre of the square.

    • Place the compass point on the centre and draw a circle with diameter 12 cm.

    • Cut out the circle

    • Now place a button in the centre of the disc you've cut out and draw its outline with the pencil.

    • Push the scissor into the centre of the circle and rotate to gouge out a hole.

    • Be careful not to make the hole bigger than the button outline you made before. In fact, a hole slightly smaller than the outline is ideal. The card can deform slightly and allows you to squeeze the button in for a tight fit.

    • Apply a small amount of glue around the edge of the hole.

    • Push the button into the hole. Now let the glue dry for 5-10 mins before moving to the next step.

    • Make sure the holes on the button are not covered with glue. If they are, use a safety pin to clear the glue from the holes after the glue has dried.

    • Insert one end of the string into one of the button holes.

    • If the string doesn't fit easily, use a pin to push it into the hole.

    • Now take the thread on the other side of the button and insert it through the hole that's opposite to the first hole you inserted the thread through.

    • You should now have both ends of the string on the same side of the cardboard disc.

    • Tie the two ends together securely with a double knot.

    • Repeat the process to add one more thread through the other two holes in the button

    • Add some fine soil to a small cup of water (50 ml). Stir well till the mixture is a uniform brown colour.

    • A darker brown colour is better because it will show a more dramatic seperation, this is because of the higher ratio of mud suspended in the water. This is also why fine mud is preferred, the smaller the individual particles of mud the more likely they are to remain suspended in the water.

    • Dip the vial into the mud water mixture.

    • Close the lid of the vial. There should now be a small amount of the mixture retained in the the vial.

    • Place the sample as close to the edge of the cardboard disc as possible, with the tapered end pointing towards the outside, and perpendicular to the edge of the disc.

    • Now tape the sample in place thoroughly with a transparent tape.

    • Place a counter-weight vial, in exactly the same way, diametrically opposite and on the other face of the disc as the first vial

    • Make sure you have taped the vials tight enough, and completely along both axes, such that they don't get detached from the disc while spinning it.

    • Use two ice cream sticks, or your fingers, to hold the thread loops at ether end.

    • You are now ready to separate the sample.

    • Hold each thread loop in either hand, woven through your fingers such that you are able to wind up the threads as you give an initial few turns to the string.

    • Then onwards, pull the threads apart, and release the pressure alternatingly, as the disc builds up momentum and gathers speed by forming more thread turns in either direction. It's when you hear a distinct whirring, buzzing sound that you know you've reached very high and desired rotational speeds.

    • Run the centrifuge for about 2 mins at a time and check the results.

    • Sample after 2 minutes. Note a slight lightening of its brown tint.

    • Sample after 4 minutes. The colour lightens further.

    • The mud particles have clumped to the bottom.

    • If the fuge wobbles too much after the sample is attached, mount an empty refill or capillary piece on the opposite side of the disc that the sample is on. This will serve to balance it.

    • Make sure the threads are put properly, if you face issues playing with it.

    • Which end of the vial does the mud settle to?

      • A) Narrow end of the vial

      • B) Wider end of the vial

      • C) Right side of the vial

      • D) Left side of the vial

    • When you pull the disc, the speed of the second rotation of the disc is more than the first rotation of the disc. (True/False)

      • The speed of 10th rotation is more than speed of 9th rotation. (True/False)

    • The centrifuge will work even if we spin the disc horizontally instead of vertically. (True/False)

    • The disc changes the direction of rotation with each pull on the thread. (True /False)

    • Estimate the speed of rotation of the disc in terms of "number of rotations per second". Tic one of the following:

      • A) < 5

      • B) > 5 but, < 25

      • C) > 25

    • An interesting variation to try would be to mount the button off-centre. Try changing its distance from the centre as well.

    • Instead of a circle, make the centrifuge using a different shape like a square or an octagon.

    • A centrifuge where we are using more holes on the disc instead of a button.

    • To learn and make the variations for DIY Centrifuge, please go to DIY Centrifuge (Variations).

Finish Line

Kailash NR

Member since: 05/02/2017

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