January 11, 2016 at 02:50AM

Today I Learned: 1) Ants lay pheromone trails to guide them from nest to food and back. How do they tell which direction is which? Today I learned a pretty comprehensive list of orientation mechanisms. The big three are light from the sun, polarization of light from the sun, and landmarks. Also, more rarely, species use: * Chemical cues from the nest: specifically, CO2 gradients, as the nests are pretty CO2-rich * Watching other ants: ants carrying food are more likely to be moving towards the nest * Forks in the path: if an ant comes across a fork with two paths at acute angles to each other, it can pretty safely assume the other path leads home. 2) Bone is piezoelectric! There's some debate on how much bone is actually piezoelectric and how much the observed piezoelectricity is actually due to streaming current (more on that below), but there's definitely some piezoelectricness to bone. There's some thought that this contributes to the feedback mechanism that causes strained bones to become thicker -- strain causes an electric current around/in the bone, which signals nearby cells to deposit more bone, strengthening the bone. Another thing to note -- apparently helical molecules tend to be piezoelectric, especially if they contain regularly-spaced charged groups. Collagen, for instance, is a helical protein that was one of the first-known biological piezoelectric materials, and is what is thought to give bone its piezoelectricness. DNA is also piezoelectric, though it's not known whether cells use this property. 3) Streaming current is a weird kind of electric current caused by physically forcing ions through liquid. It happens when you force an electrolyte (which is just a liquid with charged ions, i.e. a salt) past a rigid, charged surface. For instance, imagine you put some water with sodium chloride (table salt) into a tube with a positively-charged inner surface. The tube will attract some negatively-charged chloride ions, which will more or less stick to the outside of the tube. This makes the rest of the liquid net *positively* charged. If you then force the liquid to move, say by flowing more salt water into one end, the chloride ions will stay stuck to the tube surface but the positively-charged sodium ions in solution will move down the tube, so you have a net movement of charge, so you've generated positive current (remember that positive current is defined as the relative movement of positive charge). Why would you care about streaming current? Well, if you're a material scientist studying bone piezoelectricity, you have to be aware that liquids streaming through bone could cause streaming current if the bone is charged (and collagen has a lot of positive charges), so some of the "piezoelectricity" you measure could just be current streaming thorugh all the pores of the bone. You can also measure streaming current over a surface to measure how charged it is -- the stronger the charge, the more streaming current. 4) Bonus: SpaceX is hiring. A lot. I didn't count all the job openings, but I estimate about 350 (based on counting out 50 jobs here and estimating how many of those would fill the scroll bar: http://ift.tt/1cZ6GHD). Among other things, they're looking for IT guys, programmers, managers, business developers, and a whole bunch of engineers of all kinds.