 I explore in this video how engineers work their way around uncertainty because engineers solve practical problems before they have full scientific knowledge. I start with one of the most complex phenomena in nature, yet one of utmost importance to engineers, as seen when smoke rises from burning incense. The smoke exposes the movement of the air as heat from the incense drives the air to rise. The issuing smoke reveals two types of flow. Near the incense, the smoke wavers a bit, but the flow is smooth and even. This is called laminar flow. If we move up the smoke stream, we see that the smooth stream becomes chaotic. That churning flow is called turbulent. To this day, a fundamental understanding of when that transition from laminar to turbulent flow occurs puzzles scientists. Yet engineers must know when the transition occurs to control which type of flow occurs. Of prime importance is the smooth laminar flow of air over an aircraft wing. Yet without a fundamental scientific understanding of how to achieve that laminar flow, we have flown across the Atlantic Ocean routinely since the first commercial passenger flights in 1939. Although 21st century science cannot fully understand turbulence, a 19th century engineering professor, Osborn Reynolds, built an apparatus designed a formula used by engineers to predict the transition from laminar to turbulent flow. This modern version features a long tube with an inlet at the top where water enters. Water flows down the tube, then exits from an outlet at the bottom. At the top is the key piece. This round vessel is filled with green dye. This dye is released through this nozzle, which is very small compared to the diameter of the tube. This small section of the device in operation, a section just below the tip of the nozzle. What you see here is the dye falling because of gravity. The water is not flowing yet inside the tube. I let water into the tube at a low flow rate and increase it slowly. As the flow rate increases, the flow looks the same. While not very exciting, it is an important observation by Reynolds. As the flow rate increases, keep your eye on the smooth flow of the dye and then, boom, the dye traces out the chaotic flow of water, now turbulent flow. The dye reveals that now the water violently spirals around. With this experiment, Reynolds highlighted the key properties of the transition from laminar to turbulent flow. To see that, let's watch the flow change again as the flow rate increases. First, below a particular flow rate, no turbulence occurs. Second, that the transition occurs abruptly. And third, that there's an upper limit to the flow rate above which smooth flow cannot be sustained. To understand this behavior, Reynolds compared the flow of water to a military troop. If that troop stayed in formation, that was to his mind like laminar flow. If the troop became disordered, soldiers going in every direction, that mimic turbulent flow. He reasoned that the orderliness of marching troops depends on three characteristics. Speed, the number of soldiers in the troop, and discipline. A troop would struggle to stay in formation if it were fast moving, large or poorly disciplined. Although with discipline, a faster large troop could stay in order. The speed of the troop corresponds to the flow rate of the fluid and the size of the troop to the diameter of the pipe. And the discipline is something called viscosity. It's the resistance to flow. To see that, compare water and honey. The water flows readily, while higher viscosity honey flows slowly. Reynolds get these three characteristics of fluid flow. The diameter of the pipe, the speed or better the velocity of the fluid's flow, and its viscosity into a single relationship. The diameter times the velocity divided by the viscosity. By using different size pipes, changing the flow rate, using fluids with differing viscosities, he observed that when this combination of variables was less than about 2100, some textbooks today say 2300, the flow was laminar, and above that value, the flow could become turbulent. Although by 4000 the flow is turbulent. With this relationship, engineers could know what to change to achieve laminar or turbulent flow. For example, if the flow were just at that transition value of 2100, and you wanted to be sure you had turbulent flow, you could increase the pipe diameter so this ratio is above 4000. To ensure laminar flow, decrease the pipe diameter to lower the ratio below 2100. You could also, of course, adjust the flow rate of the liquid, or change to a liquid with a different viscosity, and adjust the flow so it was laminar or turbulent. This simple relationship among a few properties of a fluid informed the designs of engineers for over a century. I mentioned its application to flight, but it's used throughout our engineered world to fully mix pharmaceuticals, engineers design vessels that use turbulent flow to cool the strips of steel fashion into every type of household metal object, a laminar flow of water cascades over the hot metal. Turbulent flow cools unevenly and induces defects. And to conserve fuel, these vessels channel the air smoothly around a truck. Reynolds' approach, a scientist might well complain, doesn't describe turbulence at a molecular level. This underlines the striking difference between science and engineering. The scientific method strives to reveal truths about the universe while the engineering method seeks solutions to real-world problems. We might, though, think that today's science would subsume all of engineering. Yet scientific breakthroughs never remove the need for engineering. Humankind developed the engineering method to reach beyond codified scientific knowledge. Instead, the advance of science only pushes out the boundary between the certain and uncertain, and so resets the boundary where engineers work. This molecule illustrates that idea. It's an enzyme. In nature, it enables a chemical reaction to happen. Our body is filled with thousands of enzymes that help our cells feed, grow, reproduce, move, and communicate with other cells. This particular enzyme, called subtelicin, breaks down organic matter as shown here. At the center of each petri dish is a lump of gelatin. On the left, it's covered in only water. On the right, this enzyme subtelicin is dissolved in the water. In time-lapse, taken over nearly 30 hours, note that the gelatin in the water remains intact, but that in the enzyme it dissolves. The enzyme subtelicin is a protein-digesting enzyme. It breaks the chemical bonds holding the gelatin together. This ability to break bonds is of the utmost importance to engineers. Breaking and reforming bonds is what creates the engineered world around us in manufacturing chemicals, paints, pharmaceuticals, and plastics. And so it's no surprise that an engineer, Francis Arnold, decided to use these natural enzymes in industrial processes. But here's the problem. Enzymes like subtelicin work only in water and only under a narrow range of temperatures. In the industrial uses envisioned by Arnold, she needed it to work at all temperatures and in harsh solvents. So, of course, when Arnold put it in organic solvent called dimethylformamide, think of paint stripper, and you have a good idea of what this chemical is like, the enzyme no longer digested proteins. To engineer this enzyme, Arnold drew on a stunning scientific advance, a detailed molecular understanding of enzymes. To illustrate that, let's look at this section, this helix. It's formed from 14 amino acids linked in a chain. I'll go through them quickly here because the only things to notice are first that each of the amino acids is a simple molecule, three or four carbon atoms with a few oxygens, hydrogens, or nitrogens attached. Second, that we denote each type of amino acids with a single letter. And third, it's the order of these amino acids known as a sequence that determines the task performed by the enzyme. Seven different amino acids compose the sequence, but the whole enzyme contains only 14 more, 20 different types of amino acids in total. So we could think of this unique enzyme as just the sequence of the amino acids that compose it, some 275 amino acids. The enzyme has one long chain all curled up, and so this list, this sequence, shows the amino acids in order from the beginning of the molecule to the end. We can see here the task that faced engineer Arnold. This sequence of amino acids works in water, but which would she need to change to modify subdilicent so that it worked in paint thinner? Would it be a single amino acid? If so, which one? Or maybe two of them needed to be changed. She couldn't know because it's impossible to calculate from first principles the activity of a particular sequence of amino acids. The choices she faced were astronomical. 275 amino acids make up subdilicent, of which there are 20 different types. So the number of possibilities is 20 raised to the 275th power, a number far greater than the number of stars in the universe, something estimated as 200 trillion. To engineer subdilicent to work in harsh environments, she used nature's own mechanism, evolution. Here, in essence, is an illustration of what she did. She randomly mutated naturally occurring subdilicent. She used a method which replaces an amino acid or two or three here and there. I've illustrated this here with just 10 mutations. She set out 10 dishes filled with diluted paint thinner, as I've called the solvent, dissolved the mutated enzymes, added a block of gelatin, then washed the dishes for a day or two. Most of the mutated versions showed no ability to digest the gelatin, but at least one managed to partially dissolve it. She selected that enzyme, created from it 10 more mutated versions, filled the dishes with a stronger solution of paint thinner than tested again. This time, two showed activity, one of them more than the other. She mutated that version 10 times, filled the dishes with an even more concentrated solution of paint thinner than tested again until nature engineered for her an enzyme that worked in a harsh chemical environment almost as well as the original did in water. In the decade since her work, enzymes engineered by this directed evolution diagnose and treat disease, reduce farm waste, improve textiles, synthesize chemicals and pharmaceuticals, and remove stains. They're a key ingredient of laundry detergents. Arnold noted when accepting the Nobel Prize for Chemistry that even today, we struggle to explain how her evolved enzymes work. A clear reminder that scientific advances don't remove uncertainty, they simply move the borderline between certainty and uncertainty, the perfect space for an engineer to work. In the next video, I explore the relationship of engineering to science. I'm Bill Hammack, The Engineer Guy.