Hydrophobic barriers and casting crayons

There’s a neat, short paper called ‘Rapid Prototyping of Paper-Based Microfluidics’ that says if you want to create hydrophobic barriers in porous paper cheaply and easily, you can do it with crayons (which they tactfully call ‘wax pens’). Porous paper, like filter paper, if often used in microfluidics to wick fluid from one place to another; it can also be used to filter out particulates in the fluid. To create channels in the paper for the fluids to follow, you often use a complicated manufacturing technique, like photolithography, or unpleasant chemicals, like soaking your paper in a polystyrene and toluene solution and then printing the barriers in toluene.

Or you can draw with crayons.

The trick is to bake the paper, after you’ve drawn on it, for about 2 minutes at 135 degrees Celsius (the paper gives more details on the numbers). This is easily achievable in something as simple as a microwave oven as it doesn’t need to be precise. The only problem you might encounter with baking it too long is that the paper might burn – but that’s an order of magnitude longer than it takes.

Why do you need to bake it? When you draw on the paper with the wax, the fluid can still be wicked beneath the surface. By baking it you slightly melt the wax into the paper, creating a complete hydrophobic barrier.

So crayons do this perfectly. China markers and another commercial ‘wax pens’ less so. But the problem with crayons is that they’re so big and imprecise and keep changing diameter as you use them. When you bake the wax it also is wicked through the paper slightly, so if you draw a 1mm line in crayon you’ll probably end up with a 1.5mm hydrophobic region. I wanted a thinner crayon.

Crayons melt at a pretty low temperature, so I started trying to mold the crayon into approx. the size of the lead that goes in mechanical pencils. I used the ends of needles for syringes, because I had them on hand, they were about the right dimension, they were metal, and very smooth.

Macro shot. Diameter of crayon cylinders is ~.75mm.

Zoomed out shot.

This actually worked okay, in that I managed to produce some crayon lead, but it had some serious problems:

The needles were too small to pour the melted wax into, (surface tension stopped it from entering the needle,) so I had to put small shavings on solid wax into a funnel above the needle and put the entire apparatus in the oven. This allowed the wax to melt into the needle slowly, overcoming the surface tension problems. However the melting temp. of the funnel (plastic) is not that far from the wax, so I accidentally melted the funnel a little a couple times. It also seemed like I wasn’t getting the most solid cylinder: they weren’t very strong and would break under the pressure necessary to use them as a drawing device. Finally, I didn’t have a great tool for ejecting the cylinders, which was a serious problem.

I decided to increase the size of the cylinder to get a hold on some of the structural problems and stop melting my funnels. After about .9mm, the next commercially available lead holder is 2mm, so I drilled as smooth of a 2mm hole as a I could in a piece of stock aluminum. This allowed me to make the “funnel” in the aluminum as well.

It worked okay, but the structural issues remained, plus now my hole wasn’t nearly as smooth (compared to the professionally manufactured medical needles) so despite a better ejection device (back side of the drill bit  — perfect size) I had the same problems with ejecting.

If I wanted to take this further, there are a number of things to try:

Make the mold out of something smoother, like teflon, which would also held the ejection process as it would release better.

Build an extruder. This has the benefit of being able to pressurize the wax, which I think would fix some of the structural problems.

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