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How do you design, test, and build a small
and completely edible boat that propels itself

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through your cocktail? This is a complicated
problem, but it--and many other problems like

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it--can be solved systematically, by breaking
the problem down into several key phases.

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In this video, we'll explore how two MIT graduate
students created these Cocktail Cruisers.

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This video is part of the Problem Solving
video series.

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Problem--solving skills, in combination with
an understanding of the natural and human-made

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world, are critical to the design and optimization
of systems and processes.

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Hi, my name is Lisa Burton and I am Nadia
Cheng. We are graduate students in Peko Hosoi's

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lab in the Department of Mechanical Engineering
at MIT.

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Today, we're going to tell you about a class
project that we worked on that turned into

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a product called cocktail cruisers.

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After watching this video, you should be able
to identify the steps of the problem solving

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process and recognize that the problem solving
process is iterative.

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Take a moment to think about your approach
to problem solving. What steps do you generally

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go through?

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Please pause the video and take a moment to
think about it. Then continue playing the

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video to hear about one approach. [PAUSE]
While different people might approach a problem

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in different ways, using an explicit strategy
for problem solving can be very helpful. You

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can find different problem solving strategies
in the literature. The strategy that we will

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present in this video is adapted from Don
Woods and Philip Wankat.

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The first step is to define the problem. What
are the criteria? What are the constraints?

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After defining the problem, you need to gather
information. What are the knowns and unknowns?

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What content knowledge is needed to solve
the problem? Have others worked on a similar

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problem before?

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The next step is to explore. Look at the problem
in different ways. Brainstorm different approaches

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to meeting the criteria given the constraints.
After exploring, you need to pick one approach

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to solving your problem and plan your strategy.
Determine the resources needed to carryout

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your plan. A flowchart, outline, or schematic
might be useful.

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After the planning phase, it is time to act.
In this step, you follow your plan to create

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your solution.

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Once you have a solution, you need to check
it. Does it make sense? Does it meet the criteria?

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Where does it fall short? Is any troubleshooting
required? If the solution isn't satisfactory,

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it might be necessary to explore some more
and create a new plan.

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Once you have your final solution, what have
you learned that you can generalize to other

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contexts?

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Finally, disseminate your results so that
others can learn from your solution. This

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could be through a class discussion, a formal
presentation, or a scientific paper.

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Now that you are familiar with the problem
solving process, see if you can identify where

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I used these steps as I worked on a class
project.

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This project began in my Fluid Dynamics class,
which focused on interfacial phenomena and

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was taught by my other advisor, Professor
John Bush. The main criterion, or requirement,

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for the project was to conduct an experiment,
perform a simulation, or design a product

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fundamentally based on a topic covered in
class. There were also some constraints placed

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on the project. We only had three months to
work on it and a limited budget. Other than

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that, it was very open--ended.

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It was really up to me to define the problem
from there. Learning about the Marangoni effect

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inspired my design. The Marangoni effect arises
from a gradient, or difference, in surface

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tension.

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We can see the Marangoni effect by adding
a drop of soap solution to a container of

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water. The soap solution reduces the surface
tension of the water locally, causing flow.

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Using black pepper flakes as a tracer, we
can visualize the flow away from the area

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of low surface tension.

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Surface tension is simply a property of liquid
that results from intermolecular forces. In

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the bulk of a uniform liquid, each molecule
experiences a zero net force because molecules

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that exert equivalent force on each other
surround it. At the surface of a liquid, molecules

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from the liquid experience a non-zero net
force pulling them toward the bulk of the

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liquid because they are not completely surrounded
by like molecules. This causes the surface

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to minimize its area.

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The high surface tension of water allows objects
denser than water, such as a paper clip or

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a water strider, to float on the surface.

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If an object has a high surface tension liquid
on one side of it and a low surface tension

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liquid on the other side, the difference in
surface tension will cause the object to move

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toward the high surface tension liquid.

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We can see that happen in this demonstration
of a toothpick floating on water. If we add

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a few drops of soapy water to one side of
the toothpick, the toothpick moves away.

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I decided that I wanted to make a miniature
boat that would be propelled by a surface

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tension gradient. I wanted to propel the boat
in a water-based environment, with the ultimate

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goal of using it in a food setting. This required
that the second liquid be safe to eat since

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it would mix with the water. At this point,
I wasn't concerned with making the boat edible,

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because it could simply be removed by hand
from the food.

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Therefore, the problem we needed to solve
was how to design a boat that could be propelled

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by the Marangoni effect. This included the
identification of a liquid that was edible

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and had the appropriate surface tension for
use as the propellant.

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I started gathering information about different
fluids that had a lower surface tension than

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water. I also looked at the existing designs
of other boats, noting what propellants they

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used and their method for releasing the propellant
into the surrounding water.

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To identify important physical features of
the boat, I did some calculations, starting

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with a simple force balance diagram. We can
represent the boat as a rectangular prism

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for now. Viewing the boat from the side, we
see just a rectangle.

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In the vertical direction, the forces acting
on the boat are the force due to gravity,

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or the weight of the boat, and the buoyancy
force, which is equivalent to the weight of

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the water displaced by the boat and the meniscus.
You've probably noticed that water in your

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drinking glass slightly climbs up the wall
of the glass. This is the meniscus and it

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depends on the material of the object (for
instance, the boat or the glass), the liquid,

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and the gas (in our case air).
Therefore, to make the boat float, we must

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ensure that the weight of the water displaced
balances the weight of the boat. This is Archimedes'

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principle.

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In the horizontal direction, we have the force
of surface tension of the "fuel" on the left

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side and the force of surface tension of the
surrounding fluid, which we will assume is

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water. If these forces aren't equal, the boat
must accelerate, according to Newton's Laws.

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Therefore, to make the boat move faster, we
want the difference in surface tension to

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be as great as possible.

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Once the boat is moving, drag becomes important.
Drag opposes the direction of motion and increases

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with velocity and the cross sectional area
of the boat. A smaller cross-sectional area

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will allow the boat to move faster.

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To ensure that the boat remains stable, the
center of mass must be below the center of

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buoyancy. Otherwise, the boat will tip over
and sink. As I explored different ideas, it

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became clear that I needed to do some testing.

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To find an edible, low surface tension liquid
to act as the boat's fuel, I tested different

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liquids I found in my kitchen, from hot sauce
to sugar water to vinegar. From these tests,

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I decided that I needed a low surface tension
liquid that was also volatile, or evaporative.

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This is necessary for the boat to continue
to move; otherwise, the fuel surrounds the

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boat equally on all sides so that no surface
tension gradient exists and therefore the

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boat does not move.

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The best fuel I found was liquor. The alcohol
content of the liquor and the surface tension

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are inversely related. Therefore, using a
very high alcohol content liquor will result

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in a faster boat.

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I started sketching ideas for what the boats
might look like. Drawing on previous research,

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the initial design had an open fuel reservoir
in the middle of the boat with a small slit

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that allows the liquid to leave the reservoir.
The overall shape was a very simple boat shape.

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After testing materials for a while, it was
time to come up with a plan. I decided that

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I needed to create a prototype so I enlisted
the help of my lab mate Nadia. She suggested

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using a 3D printer to create the boat in the
Edgerton Center student shop.

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The only way we were really going to know
if the boat design was going to work was to

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actually try it. It was time to act, and having
rapid prototyping technologies—such as 3D

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printers—available significantly expedites
the testing and iteration process. The 3D

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printer requires that we develop 3D representations
of our boat design in CAD. There are various

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types of 3D printing technologies; the one
we used utilizes fused-deposition modeling,

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or FDM. The way it works is that there is
a spool of a thin plastic "thread" that is

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fed through a nozzle. The nozzle heats the
plastic to soften it so that the plastic can

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be laid out to build a 3D object. Basically,
it is like piling a piece of string on top

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of itself to make a 3D shape. Because the
boats needed to be less dense than the liquids

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they were to float on, we specified that the
boats were to be printed "sparsely", such

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that the plastic thread was to be laid out
loosely whenever possible in the interior

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of the boats.

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Once we had our boat, it was time to check
and see if it did want we wanted it to. Of

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course, our very first boat wasn't the ideal
solution. The reservoir was too small and

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the boat was too large; it didn't hold enough
fuel and moved slowly because of the mass.

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But it gave us insight into what we needed
to change. We modified our plan, created a

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new design, and tested it again. After some
troubleshooting, we went through several design

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iterations. We examined many different boat
designs and liquors to use as fuel until we

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had a design we were happy with.

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I presented my project to the class at the
end of course. I successfully stayed within

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the design and project constraints. I created
a boat that used edible materials for propulsion

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through the Marangoni effect.

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After completing the course project, we continued
to pursue this idea. We wanted to improve

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our design and also make the boat out of edible
material so that they could be used as a fun

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garnish for drinks. This required us to generalize
our solution to a new situation. We constructed

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soft, flexible molds out of silicone by 3D
printing rigid plastic molds that the silicone

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molds could be made from.

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After creating the molds, we tried several
edible materials for the boat including candy,

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chocolate and marshmallows. We quickly discovered
that the density of the material was a common

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limiting factor.

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After further testing, we found two materials
that worked: edible wax and gelatin, which

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we foam as it sets to reduce the density of
the boat. We've since disseminated our product.

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I won a design award at a competition at MIT
and we're currently working with Chef Jose

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Andres to incorporate our design into his
restaurant. Additionally, an article in the

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American Society for Engineering Education
PRISM magazine helped spread our ideas and

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product to a broader audience.

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To recap, in order to solve this problem,
we had to

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define the problem
gather information

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explore
plan

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act
and then check our solution

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We were able to generalize our solution to
a new context and then disseminated our results.

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The problem solving process really helped
us approach this problem and break it down.

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By consciously using a problem solving process,
you will find that you will become a more

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confident and better problem solver. Best
of luck!