﻿ 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 that 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 twenty-first century science cannot fully understand
turbulence, a nineteenth-century engineering professor, Osborne Reynolds, built an apparatus
to find 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,
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. Let’s look at 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 you 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 is 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 mimicked 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 fast or 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 gathered these three characteristic 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 sized
pipes, changing the flow rate, and using fluids with differing viscosities, he observed that when
this combination of variables was less than about 2,100 — some textbooks today say 2,300 — the flow
was laminar and above that value the flow could became turbulent, although by 4,000 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 twenty-one hundred and
you wanted to be sure you had turbulent flow you could increase the pipe diameter so this ratio
is above four thousand, to ensure laminar flow decrease the pipe diameter to lower the ratio
below twenty one hundred. 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 fashioned 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, baffles channel the air smoothly around a truck. Reynolds’s 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 subsumes 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 subtilisin, 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 subtilisin is dissolved in the water. In time lapse, taken over nearly thirty hours,
note that the gelatin in the water remains intact, but that in the enzyme it dissolves.
The enzyme subtilisin 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 — Frances
Arnold — decided to use these natural enzymes in industrial processes. But here’s the problem:
enzymes like subtilisin 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 an 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 fourteen 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 acid with a single letter. And third, it is the
order of these amino acids — known as a sequence — that determines the task performed by the enzyme.
Seven different amino acids compose this sequence, but the whole enzyme contains
only fourteen more — twenty 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 is 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
subtilisin 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 subtilisin, of which there
are twenty different types. So the number of possibilities is twenty raised to the 275th power:
A number far greater than the number of stars in the universe! Something estimated as 200 trillion.
To engineer subtilisin 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 natural occurring subtilisin — she used a method, which replaces an amino
acid or two or three here and there. I’ve illustrate that here with just ten mutations.
She set out ten dishes, filled with diluted paint thinner, as I’ve called the solvent.
Dissolved the mutated enzymes, added a block of gelatin, then watched 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 ten more mutated versions, filled
the dishes with a stronger solution of paint thinner, then tested again. This time two showed
activity, one of them more than the other. She mutated that version ten times, filled the dishes
with an even more concentrated solution of paint thinner, then 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 decades 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 are 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 engineerguy.