Light Bulbs from Mixing

Assistant professor Ngai Yin Yip (EEE department) delivered an excellent talk for the Chemical engineering department on his PhD thesis and current research, which delves in the intersectionality between renewable energy and climate change. I was quite amazed because his research harnessed energy from the most unlikely source—mixing. Who would ever think that you can get energy from mixing salt in water? What I also found cool was that the energy from salt gradients does not come mostly from enthalpy of mixing, but the entropy of mixing. Salt prefers to dissolve because it can be more places when dissolved as opposed to in a crystal. I find that a really fundamental idea that makes so much sense. In a way, this research is the opposite of desalination—rather than putting in energy and separating salt from water, it is using the natural release of energy of mixing. I have since told multiple people (friends and family) about the idea that mixing is a renewable energy source and they were all immediately confused. It seems that it is a very non-intuitive idea for most of those who do not have a science background.

But what is the mechanism for gathering this energy? It’s none other than pressure driven flow! Basically, there is less salty water and very salty water separated by a semi-permeable membrane that only allows water to traverse. The water from the less salty water then crosses to the other side and the energy of this pressure driven flow is harnessed through a piston. The talk primarily focused on details about how to improve this process, and included some interesting techniques including running the process upside down in order to clean the membrane of gunk (called fouling). But what I found interesting, and a little suspicious about the whole thing, was that Professor Yip only spoke about the mechanical energy harnessed. Yet he compared the mechanical energy harnessed from this source to electrical energy from solar power and other renewable energy sources. Albeit, doing my own research, the efficiency in converting water turbine energy into electrical energy is as high as 90%, this still marks an important further loss that he didn’t take into account.

Lastly, how can this source of energy be used practically? Professor Yip gave one way, which is when fresh water and sea water meet, and there is a water potential between them. However, I found that scientists have proposed another use—which is how this research applies to climate change as well. Desalination results in salty water, and chemically treated wastewater has very little salt content. So scientists have proposed combining the two, and harnessing energy from that! Thus this process has real applications and can be a great energy source for the future. 

Bonus Blog: Surface Tension and Lotus Leaves

Do you ever wonder what happens when a Lotus leaf gets wet? What if it rains, and the leave gets soaked? Why doesn't the leaf get wet and, due to the added weight of the water, sink into the pond? The answer to these questions are much more complicated than one would think, and it has a lot to do with surface tension.

A Picture of Lotus Leaves

In class we learned that surface tension is the tendency of water to minimize its surface area. So if given a chance, water will bead up into as much of a sphere as it can. The lotus uses the surface tension of water to its advantage. Lotus leaves are ultrahydrophobic--they can't get wet. So when you put water on the top of a lotus leaf, it will bead up and roll out of the leaf! So rain water simply rolls away. How does this happen? Well first let's go through the most common guess people have when I have asked them this question--wax. One could think that maybe some sort of wax on the leaf is causing the water to bead up. Wax is non-polar, so water does not want to mix with wax and therefore rolls of the leaf. This only partially true, since Lotus leaves do have a coating of epicuticular waxes, however this does not explain the entire story. Waxes are temporary and keep regenerating, but they are not a full proof way to keep the leaves from getting wet. Again, Lotuses are ULTRA-hydrophobic.

The answer actually lies in the very geometry of Lotus leaves. They may look like plain old leaves from afar, but if you look a little closer (okay a lot closer), something really interesting is observed. Scanning electron microscope images of Lotus leaves show the following structure:

These structures are little bumps and hills, and they are only seen on the side of the lotus leaf facing the sky. The surface of a lotus leaf is in fact very rough; you just can't see or feel the roughness. These bumps reduce the contact area between water and the leaf. The less water is in contact with the leaf, the more it will want to turn into a sphere due to its high surface tension. The graphic below gives a good idea of what I mean:

This rough structure, along with the wax coating, makes it impossible for the lotus leaf to get wet. But there's more. The lotus leaf is always really clean--a reason why in Asia it is associated with cleanliness, beauty, and also sometimes holiness. This is because the water droplets that fall on the leaf are cleaners! The picture below shows how a water droplet can pick up dirt and clean the leaf as it rolls away.

So not only do Lotus leafs not let water touch them, they also make the water clean them as well, allowing for more exposure to the sun. Now, its obvious where this is going--do we have technology like this for humans? The answer is yes--you can actually get clothes and materials that mimic the lotus leaf. So you can actually buy clothing that is ultrahydrophobic, so no matter how awful you are at drinking, your clothes will be stain free. Look at this cool video as an illustration:

So if you have kids who always spill things on themselves, the answer lies in the lotus leaf!