Manu Prakash, inventor

[Pishwish]

Want to feel inadequate? a thread, interview, and article about a guy who invented a $1 microscope, a 20 cent centrifuge, and (really letting the spending go out of control here) a $250 microscope for detecting diseases 120 times faster than ordinary microscopes.

interview...we stumbled upon this toy which is called a button-on-a-string. Anybody can do this: You take a button, you put two strings in, you pull them back and forth, the disk spins.
When we started doing this, we realized that nobody actually understood how this simple toy works, and it happens to be the oldest recorded toy in the history of mankind, which I find remarkable. When we looked at the archeological data, 5000 years ago people had relics of almost the same object. Many cultures have discovered it again and again but have never understood it. That number of revolutions you quoted, 125,000, is powerful because we first arrived at that mathematically.

If I noticed that no-one had solved an engineering problem (I assume they solved it) that had been lying around for 5 thousand years, I think I would be pretty smug.

I'm not sure how well the automatic disease detection device is doing since the above links are from 2019, I couldn't find anything more recent. I imagine people might be concerned at AI and how accurate the diagnoses are.

[Boustrophedon]

Perfectly obvious how that toy works. I'm not knocking his inventions but that statement is just stupidly wrong.

[dyqik]

Yeah, I can draw the force vectors on it trivially. Calculating the torque and thus the speed is a little harder, and would probably have to be done numerically, but that's mainly because string is a pretty variable thing.

Frankly that statement calls into doubt all of his "inventions".

[Pishwish]

The article is behind a paywall, but the press release says:

Saad Bhamla, a postdoctoral research fellow in [Prakash's] lab...recruited three undergraduate engineering students from MIT and Stanford to build a mathematical model of how the devices work. The team created a computer simulation to capture design variables like disc size, string elasticity and pulling force. They also borrowed equations from the physics of supercoiling DNA strands to understand how hand-forces move from the coiling strings to power the spinning disc.

“There are some beautiful mathematics hidden inside this object,” Prakash said.

Once the engineers validated their models against real-world prototype performance, they were able to create a prototype with rotational speeds of up to 125,000 rpm, a magnitude significantly higher than their first prototypes.

“From a technical spec point of view, we can match centrifuges that cost from $1,000 to $5,000,” said Prakash.

[Boustrophedon]

Yeah, I can draw the force vectors on it trivially. Calculating the torque and thus the speed is a little harder, and would probably have to be done numerically, but that's mainly because string is a pretty variable thing.

Frankly that statement calls into doubt all of his "inventions".

Is the torque calculation difficult? You have a diameter string centre to string centre, a tension and an angle of twist?

[dyqik]

Yeah, I can draw the force vectors on it trivially. Calculating the torque and thus the speed is a little harder, and would probably have to be done numerically, but that's mainly because string is a pretty variable thing.

Frankly that statement calls into doubt all of his "inventions".

Is the torque calculation difficult? You have a diameter string centre to string centre, a tension and an angle of twist?

It's the unwrapping over time that's a little more difficult to deal with. Not terribly hard, but not trivial either.

[dyqik]

The article is behind a paywall, but the press release says:

Saad Bhamla, a postdoctoral research fellow in [Prakash's] lab...recruited three undergraduate engineering students from MIT and Stanford to build a mathematical model of how the devices work. The team created a computer simulation to capture design variables like disc size, string elasticity and pulling force. They also borrowed equations from the physics of supercoiling DNA strands to understand how hand-forces move from the coiling strings to power the spinning disc.

“There are some beautiful mathematics hidden inside this object,” Prakash said.

Once the engineers validated their models against real-world prototype performance, they were able to create a prototype with rotational speeds of up to 125,000 rpm, a magnitude significantly higher than their first prototypes.

“From a technical spec point of view, we can match centrifuges that cost from $1,000 to $5,000,” said Prakash.

That's my point. It's not hard to model, and we knew how it works. It doesn't need anything special, just application of known stuff.

[KAJ]

The article is behind a paywall, but the press release says:

Saad Bhamla, a postdoctoral research fellow in [Prakash's] lab...recruited three undergraduate engineering students from MIT and Stanford to build a mathematical model of how the devices work. The team created a computer simulation to capture design variables like disc size, string elasticity and pulling force. They also borrowed equations from the physics of supercoiling DNA strands to understand how hand-forces move from the coiling strings to power the spinning disc.

“There are some beautiful mathematics hidden inside this object,” Prakash said.

Once the engineers validated their models against real-world prototype performance, they were able to create a prototype with rotational speeds of up to 125,000 rpm, a magnitude significantly higher than their first prototypes.

“From a technical spec point of view, we can match centrifuges that cost from $1,000 to $5,000,” said Prakash.

That's my point. It's not hard to model, and we knew how it works. It doesn't need anything special, just application of known stuff.

I doubt that “From a technical spec point of view, [they] can match centrifuges that cost from $1,000 to $5,000,” Such centrifuges don't only have to reach a high rpm. Inter-alia they have to maintain that rpm for some time, and minimise vibration.

[shpalman]

I seem to remember seeing a "toy" centrifuge aimed at teaching (I don't think it was that one), which was basically fine for a lot of applications.

But instead I found one based on a salad spinner.

(Note though that a smaller one would need to go at a higher rpm to match the same centrifugal acceleration.)

[Boustrophedon]

I think I may have to back pedal on this little and cut Mr Prakash some (small and very twisted) slack. The neat twisted pair of strings model that I and Dyqik were assuming, is just too neat. In practice the string super coils into a bigger helix and even knots itself up with little twisted loops coming off the side. I understand now the reference to super coiled DNA. Modelling the physics of that state and how the pull on the ends of the string translates into torque is messy, difficult and pointless.

Pointless because there is a simple energy equation (assuming little friction, but the string does not slide against itself much so the process may be very efficient.) You pull outwards with a given force over a given distance inputting a given energy and the rotating kinetic energy of the disc is going to be nearly equal to but less than that.

The neat modelling of twisted super-coiled strings is at best an academic exercise.