Moving Iron-Coated Polymer Particles Uphill Using External Magnetic Field

Microscopy of PMMA ferromagnetic Janus particle as used in the study (Credit: Wilson-Whitford et al., 2023)
Microscopy of PMMA ferromagnetic Janus particle as used in the study (Credit: Wilson-Whitford et al., 2023)

Granular media such as sand have a range of interesting properties that make it extremely useful, but they still will obey gravity and make their way downhill. That is, until you coat such particles with a ferromagnetic material like iron, make them spin using an external magnetic field and watch them make their way against gravity. This recent study by researchers has an accompanying video (also embedded below) that is probably best watched first before reading the study by Samuel R. Wilson-Whitford and colleagues in Nature Communications.

In the supplemental material the experimental setup is shown (see top image), which is designed to make the individual iron-coated polymer particles rotate. The particles are called Janus particles because only one hemisphere is coated using physical vapor deposition, leaving the other as uncovered PMMA (polymethyl methacrylate).

While one might expect that the rotating magnetic field would just make these particles spin in place, instead the researchers observed them forming temporary chains of particles, which were able to gradually churn their way upwards. Not only did this motion look like the inverse of granular media flowing downhill, the researchers also made a staircase obstacle that the Janus particles managed to traverse. Although no immediate practical application is apparent, these so-called ‘microrollers’ display an interesting method of locomotion in what’d otherwise be rather passive granular media.

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Life On Contract: Estimating Project Time

You sit there, irritation bubbling deep within as minute forty-five of the meeting ticks past on the clock in the corner of the office. Fight or flight is in a contest with your attention span as you struggle to keep an interested look on your face while they drone on. Real work could be done in this time. Maybe if you go to the bathroom you could sort of… fast forward the meeting. Panicked thinking continues for a bit until your awareness snaps back to the babble of words in the room.

“How long will it take you to do this?” the manager asks.

“A couple of days maybe?” You reply in turn. The manager nods and you take your escape. Little do you know that you have failed.

The project swerves out of control. Two days on the dot the manager is there expecting results. How? How did this happen again? It felt right! Two days is all you’d need to do such a simple project. It ended up taking a week.

The next meeting you say two weeks just to be sure. Everyone nods gravely, upset that something would take so long, but the work must be done. Two days later you sheepishly wander into the manager’s office with a completed project. He looks pleased but confused. The next meeting, he insists that you can do it in half the time. You and your fragile pride bowl ahead only to deliver late. The mystery!

This was my life until I started bugging the more experienced around me. I learned a lot from them and I ended up distilling it down into a few rules.

  1. There Is No Other Unit Than Hours
  2. Be honest.
  3. Get Granular.
  4. Promise a Range. Give a Deadline.

Why?

Why does someone want a time estimate? What are they going to do with this information? When working on a contract job it often feels like sticking a foot in a trap when a time estimate is given. Are they going to hold me to this? What if it goes wrong? After all, we are not fortune tellers. Unless the manager is extremely bad or you show yourself to be extremely lax in your duties, it is unlikely that a time estimate will be used against you.

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