Integrative and Comparative Biology
Integrative and Comparative Biology, volume 54, number 6, pp. 969–973
doi:10.1093/icb/icu052
Society for Integrative and Comparative Biology
SYMPOSIUM
The Cocktail Boat
Lisa J. Burton,* Nadia Cheng* and John W. M. Bush1,†
*Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA;
†
Department of Mathematics, Massachusetts Institute of Technology, Cambridge, MA, USA
From the symposium ‘‘Shaking, Dripping, and Drinking: Surface Tension Phenomena in Organismal Biology’’ presented
at the annual meeting of the Society for Integrative and Comparative Biology, January 3–7, 2014 at Austin, Texas.
1
E-mail: [email protected]
Synopsis We describe the inspiration, development, and deployment of a novel cocktail device modeled after a class of
water-walking insects. Semi-aquatic insects like Microvelia and Velia evade predators by releasing a surfactant that quickly
propels them across the water. We exploit an analogous propulsion mechanism in the design of an edible cocktail boat.
We discuss how gradients in surface tension lead to motion across the water’s surface, and detail the design
considerations associated with the insect-inspired cocktail boat.
Introduction
Marangoni propulsion in nature
Nature is adept at solving design problems across all
scales. The capillary length Lc prescribes the scale at
which surface tension and gravity are comparable at
the surface
pffiffiffiffiffiffiffiffiffiffiffi of a fluid. At an air–water interface,
Lc ¼ =g 0.2 mm, where s is surface tension,
is the water density and g is the gravitational acceleration. For creatures with characteristic dimension L that is small relative to Lc (equivalently, for
Bond number Bo ¼ ðL=Lc Þ2 1), interfacial forces
dominate gravity. Surface tension, thus, plays a critical role in the lives of water-walking insects, both
for their floatation and for their propulsion.
Traditional boats float by virtue of the hydrostatic
pressure acting along their submerged surface. For
propulsion, motorboats rely on the transfer of momentum to the underlying fluid by way of propellers,
and sailboats on wind-induced thrust. Objects, small
relative to the capillary length may call upon interfacial forces for floatation; indeed, for water-walking
insects, the dominant weight-bearing mechanism is
surface tension (Keller 1998). Such creatures also exploit surface tension to generate propulsive forces
using a variety of mechanisms (Bush and Hu
2006), one of which served as the inspiration for
our cocktail boat (Burton et al. 2013).
Water-walking insects are known to be waterrepellent, an essential feature that enables them to
stay on the interface (Dufour 1833). They are
water-repellent by virtue of their integument, which
consists of a dense matt of waxy, hydrophobic hairs
(Bush et al. 2007) as pictured in Fig. 1. Waterwalking insects are fastidious about keeping their
hair clean; a contaminated layer of hair reduces the
hydrophobicity of the insect’s surface and may lead
to submergence. Velia and Microvelia are waterwalking insects that typically move across the water
by rowing and using an alternating tripod gait
(Andersen 1976; Bush and Hu 2006). Using the
tripod gait, Microvelia’s walking speed is 8–9 cm/s
(Andersen 1976). The deformation of the water’s
surface is responsible for supporting the weight of
the insect. With a mass of 103 g, Microvelia must
displace just 1 mm3 of water to stay afloat.
When a quick escape is needed, for instance in the
presence of a potential predator, Velia and Microvelia
employ a completely different mechanism. By releasing a surfactant through a tongue-like protrusion
from the rostrum, these insects create a gradient of
surface tension that propels them forward (Andersen
1976; Linsenmair and Jander 1963). This so-called
Advanced Access publication May 22, 2014
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L. J. Burton et al.
Fig. 1 (a) Microvelia deforms the water’s surface. The weight of the fluid displaced is equal to the weight of the insect (from Bush
et al. 2007). The scale bar is 1 mm. (b) The dense waxy hair on the insect’s legs ensures its water-repellency and so prevents it from
sinking through the interface.
Marangoni effect is also employed by certain terrestrial insects, not suited to walk on water, when they
find themselves unexpectedly on the water’s surface
(Bush and Hu 2006).
Marangoni flows are those driven by a difference
in surface tension, as may arise from gradients in
temperature or in chemistry at an interface
(Scriven and Sternling 1960). A well-known tabletop
example showcasing the Marangoni effect is the
‘‘soap boat’’ experiment. When one end of a small
floating toothpick of width w is coated with dish
soap, the surface tension at the clean end is greater
by s and the toothpick accelerates, owing to the
Marangoni force of FM ¼ w (Nakata et al. 2005),
until balanced by hydrodynamic drag. Surfactants
such as dish soap are molecules that find it energetically favorable to reside at the free surface, and
decrease the local surface tension. Most soaps
decrease the surface tension at an air–water surface,
s ¼ 72 dynes/cm, by a factor of two, resulting in the
toothpick achieving speeds of 10 cm/s.
As observed in the soap boat experiment, using
soap as fuel results in relatively short travel times
of 10 s. In a closed geometry, the interface becomes
saturated in surfactant, which suppresses the propulsive gradient in surface tension. Nakata et al. (2005)
demonstrated that this limitation may be avoided by
using camphor, a volatile surfactant that evaporates
rapidly from the surface, thus permitting continuous
motion.
Marangoni propulsion in insects is analogous to
that of the soap boat: the surfactant released by the
insect reduces the surface tension behind the insect,
and the resulting chemically induced gradient in the
surface tension generates a propulsive force
(Hu and Bush 2010). Schildknecht (1976) studied
the terrestrial rove beetle and found that its surfactant reduced the surface tension from 72 to 49
dynes/cm. Microvelia reaches a speed of 17 cm/s,
about twice its walking speed on water (Andersen
1982). In Fig. 2, typical trajectories resulting from
Marangoni propulsion are shown by the cleared
path behind Microvelia. Hu (2006) observed a variety
of trajectories after the insect released surfactant, although motion was predominantly translational rather
than rotational. Using Marangoni propulsion,
Microvelia can release surfactant about three times
before it must regenerate its supply (Hu 2006).
Andersen (1976) reported that while releasing the surfactant, the insect braces its legs along its body, presumably to reduce drag and so maximizes its speed.
Design and mechanics of the
cocktail boat
The cocktail boat likewise moves by virtue of
Marangoni propulsion. A successful cocktail boat
has three key characteristics: It floats, propels itself
forward, and is edible. We first developed a plastic
version of the boat, as pictured in Fig. 3a, made of
acrylonitrile butadiene styrene (ABS) created on a
Stratasys Dimension 3D printer. We then collaborated with chefs to create an edible vessel.
While the insects rely on complex set of hairs to
avoid wetting, mimicking this dense two-layer structure via manufacturing was unrealistic (features on
the order of microns would be needed). At 1.5 cm in
The cocktail boat
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Fig. 2 Microvelia is a water-walking insect that uses Marangoni propulsion to avoid predators. The clean, white areas behind the insects
indicate the surface cleared by their surfactant.
Fig. 3 (a) Prototypes of the plastic cocktail boat were 3D-printed from SolidWorks models. The boat’s cavity holds fuel, which leaks
into the bulk fluid via a slit on the boat’s stern. (b) Cocktail boats with varying shapes, slit angles, and sizes were tested. The fuel’s
alcohol content was the most important design factor for performance, with higher proof alcohol resulting in faster and more vigorous
motion of longer duration.
length, the boat is considerably larger than the semiaquatic insects and relies primarily on buoyancy for
floatation. The boat design (Fig. 3) includes an
empty cavity to hold ‘‘fuel,’’ that is, the material responsible for lowering the surface tension. Adding
fuel on board considerably increased the weight of
the boat. To guarantee floatation, we designed the
boat to be hollow to minimize the vessel’s weight.
A small slit (0.5–1.5 mm in width) on the trailing
side of the boat allows the fuel to leak into the
bulk fluid. Fine-tuning the cocktail boat’s design
was completed after the boat’s fuel was chosen.
After seeking an edible material that was both
surface-active and volatile, we found a fuel that
far outperformed any other—alcohol. Alcohol
reduces both the surface tension and the contact
angle on the aft side of the boat, thus decreasing
the horizontal force relative to that on the front
(as illustrated in Fig. 4). Pure ethyl alcohol reduces
the surface tension at an air–water interface from
sbulk ¼ 72 dynes/cm to sfuel ¼ 22 dynes/cm.
The contact angle between water and plastic is
bulk 808 and between an alcohol–water mixture
and plastic is fuel 308.
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L. J. Burton et al.
Fig. 4 (a) Top and (b) side view of the cocktail boat elucidates design considerations and the propulsion mechanism. A lighter boat is
desirable, so that the boat’s intrusion depth d and induced drag are minimized. Note that the propulsive force depends on the fore-aft
difference in both surface tension and contact angle.
Fig. 5 (a) Edible boats were cast from flexible silicone molds, which in turn were cast from 3D-printed negataives of plastic molds. (b)
Each combination of design and liquor were filmed and tracked via a MATLAB image-processing algorithm. Here, a cocktail boat
propels itself, fueled by Bacardi 151 (75% alcohol). The boat’s length is 1.5 cm.
Consequently, a cocktail boat of width w ¼ 1 cm is
subjected to a propulsive force
Fprop ¼ bulk sin bulk fuel sin fuel w 27 dynes;
owing to the fore-aft difference both in surface tension and in contact angle. A horizontal balance of
force yields the resulting steady speed U. The propulsive force Fprop is balanced by the appropriate high
Reynolds number drag force Fdrag ¼ U2 wd, where
wd is the relevant projection of the boat’s submerged
exposed area (Fig. 4). We thus obtain
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
bulk sin bulk fuel sin fuel
:
U¼
d
Speed increases with a greater fore-aft difference in
surface tension and contact angle. A lighter boat
leads to a smaller intrusion depth d and therefore
faster speeds. Observed peak speeds of the cocktail
boat are 10 cm/s, comparable to the theoretically
predicted speeds of 4–6 cm/s. Similar to the camphor
used by Nakata et al. (2005), alcohol is volatile and
evaporates from the interface rapidly relative to the
timescale of the boat’s motion, allowing for sustained
motion of the cocktail boat until its fuel is spent.
Commercial spirits typically range from 30% to
95% alcohol by volume (ABV), resulting in gradients
of surface tension ranging from ¼ 35 to 50 dynes/
cm2. Inclusion of sugar and other flavorings typically
increases the spirit’s surface tension, thus decreasing
the difference with water. We tested five different
liquors, Domaine de Canton (28% ABV), Absolut
Vodka (40% ABV), 99 Bananas (49.5% ABV),
Bacardi 151 (75.5% ABV), and ethyl alcohol (100%
ABV) with different boat designs, slit sizes, and sizes
of the cavity for fuel. Figure 3b shows a sample of
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The cocktail boat
different boat designs. Each combination of boat
design and liquor was tested and filmed in a square
(23 23 cm) glass container filled with clean water at
room temperature. A MATLAB-based tracking algorithm was applied to trace the speed and trajectory of
each boat (Fig. 5), and mean and maximum speeds
were recorded. As expected, speeds were higher for
liquors with higher alcohol content. The effect of the
design of the vessel was also considered. Different
angles of tilt were tried, and found to alter the typical
trajectories, with greater tilt giving rise to spiraling
rather than to rectilinear motion. Motion is inhibited
when the boat encounters a boundary, so rotation is
preferred for longer travel times (typically 1–2 min),
as it tends to prevent the boat colliding with the wall.
Finally,
we
teamed
with
chefs
from
ThinkFoodGroup to explore various materials for
an edible boat. After 3D printing mold negatives
out of ABS plastic, we created flexible food molds
using silicone gels (Zhermack Elite Double 8). Edible
materials were then cast into the flexible boat molds
(Fig. 5a–c) and tested. An edible (but not digestible)
wax performed the best, resulting in the highest
speeds (11 cm/s) and travel times (2 min) recorded.
Gelatin boats were capable of sustained motion, but
this performance was improved by using a cold fluid,
which prevented the gelatin from dissolving.
Moreover, they tended to adhere to the walls. In
their favor, gelatin lends itself to taking on a wider
range of flavors.
Discussion
On the basis of a natural example of Marangoni
propulsion, we created the cocktail boat. Although
the underlying propulsive mechanism for the insect
and boat are similar, there are several fundamental
differences in design between the two. The cocktail
boat is relatively large and need not be hydrophobic;
it is thus partially submerged in the bulk fluid rather
than suspended on the interface like the insect
(Fig. 1). Unlike the surfactant released by the
insect, the boat’s fuel (alcohol) is evaporative and
so enables sustained motion. The choice of fuel for
the boat was the dominant design consideration,
with higher speeds resulting from spirits with
higher alcohol content. Some degree of rotational
motion (spinning) was observed with every boat
design, whereas insect trajectories were primarily
translational. Through a collaboration with chefs,
we created an edible vessel so that the cocktail boat
and its fuel are both consumable.
Acknowledgments
We thank José Andrés, César Vega, and
ThinkFoodGroup for their contributions to the
edible cocktail boats and Pedro Reis, Sunny Jung,
David Hu, and Christophe Clanet for valuable
conversations.
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