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I previously worked on Virtual Reality and other hardware at Valve.  I currently work at Google[x].

Prior to starting at Valve, I built computer peripherals such as keyboards, mice, and joysticks that were designed to be used inside MRI machines.  My company, Mag Design and Engineering, sold these devices directly to researchers at academic institutions who used them to publish scientific papers in peer-reviewed journals.

After work, I spend time on many different types of projects that usually involve circuit design, machining, material selection, and general fabrication/hacking.  My favorite place to be is my home workshop.

ben dot krasnow at gmail




Monday, January 21, 2013

Rheoscopic coffee table

I've received a few questions about my coffee table, which I built in 2006. I used fairly standard woodworking techniques (floating tenons, breadboard ends) with maple, walnut, and bloodwood for contrasting colors. I didn't use any stains or colored finishes -- all of the color is from the wood's natural appearance.

The table has a 30" diameter disc that contains about 2 gallons of water mixed with Pearl Swirl fluid. This fluid is essentially very fine glitter that becomes suspended in the water, and will show flow currents as the water moves within the disk. I positioned the disc on a 24" diameter lazy susan bearing for low-friction rotation. When the disc suddenly changes rotational speed, there will be turbulence in the water, which causes the rheoscopic fluid to show fluid vortices and eddies that are part of the turbulence.

The outer metal ring was custom-built by a shop that specializes in metal rings (see sources below). All of the other parts were bought from McMaster, eBay, and Steve Spangler Science for the Pearl Swirl fluid.

I got the idea for this table after see a tornado visualization exhibit at the Carnegie Science Center in Pittsburgh. The Exloratorium has an orb filled with a similar fluid to show flow patterns similar to those on a rotating planet with an atmosphere.

The main pieces are: 3/8" thick glass top, aluminum ring with L cross-section, a 3/4" thick O-ring with square cross-section and a 1/2" thick PVC plastic bottom.  There are a bunch of screws that pull the aluminum ring down onto the glass, which pushes onto the O-ring, which pushes on the PVC bottom.  There is no glue used anywhere in the disk.  The overall diameter is about 32".

The disk holds about 2 gallons of liquid, and weighs a total of 80 lbs (just a good estimate -- I never measured it).  There are two threaded fill-holes with plugs on the bottom, and the whole thing sits on a 24" lazy susan bearing.

These are my sources:
Glass top
3/8" thick tempered   $85 shipped

Outer metal ring
Custom 2" x 2" x 3/16" thick, angle-in, "L" cross section, 32" outer dia
about $150 shipped

3/4", square cross-section, O-ring cord stock -- I made a scarf joint
to make this into a ring, about $40

Bottom plate
1/2" thick gray PVC plastic  $100

Get some food coloring too

Screws, cushioning rubber, stuff to wire-brush the aluminum ring:

lazy susan bearing
eBay -- search for "aluminum lazy susan"
12" will not work, you need to find a 24" dia bearing on eBay


  1. Thanks, I'll try to make mine now :)

  2. it is a knock of of a kalliroscope created and btw patented by Paul Matisse ( he is a genius artist also a grandson of Henry Matisse)

  3. Thank you for making this and for sharing.

  4. ¨very nice product and thank you for beaing so kind to share the details,.. I'll maybe try to make sth similar :) .. could you please post a picture of a scarf joint you used for the o-ring please?

  5. Anonymous, I don't have photos of the scarf joint in the table, but it is very similar to this: http://benkrasnow.blogspot.com/2009/11/stainless-steel-conical-beer-fermenter.html

  6. Hi,

    Huge fan of this - I am considering making something similar (minus the wood bit). Silly question, but does the whole assembly rotate (ie glass top too), or just the outer ring. It has been too long since I've played with one, and videos of it in motion are inconclusive. Thanks!

  7. Adam, the whole assembly rotates. It's only the inertia of the fluid that causes the flow patterns. This is neat because the table can be spun at constant speed, and the fluid will become stable; then when the disc is suddenly stopped, interesting turbulence patterns are generated.