Leidenfrost Effect on Oobleck
Adira Balzac1,2, Samira Shiri2, Dr. James C. Bird2
Nashoba Regional High School, Bolton, MA1; BU Interfacial Fluid Dynamics Lab2
Abstract
When water droplets hit a hot surface, the bottom of the droplet
vaporizes. This vapor creates a cushion and prevents further
contact with the surface. The vapor cushion slows heat transfer
between the droplet and the surface, which allows the droplet to
persist. This is called the Leidenfrost effect. The effect appears
when the surface temperature is sufficiently higher than the
vaporization temperature of the liquid. Simple fluids, like water
and liquid nitrogen, behave in predictable ways at their
Leidenfrost temperatures. However, the effect of adding particles
to a liquid on the behavior of the fluid at and above the
Leidenfrost temperature of the pure liquid is less understood.
Here we show that even low concentrations of cornstarch in
water, less than 10% by mass, can cause the complex fluid that is
formed to stick to a surface well above the Leidenfrost
temperature of water. As the fluid impacts the hot surface, the
water begins to vaporize, which greatly increases the local
concentration of cornstarch and forms a skin on the surface of
the droplet. We hypothesize that this skin prevents further
vaporization and causes the droplet to remain in contact with the
surface with no vapor cushion to slow heat transfer. Our results
demonstrate that complex fluids can behave very differently near
their interface than in the bulk of the fluid compared to a simple
fluid under the same conditions. We anticipate that these results
will extend to other colloidal suspensions including blood and
may impact blood pattern stain analysis.
Introduction
From previous experiments, water droplets have been shown to
contact a surface with two possible behaviors. They can either
stick to (Fig. 1) or bounce off the surface (Fig. 2 and 3). The
droplet will bounce if the surface is above the Leidenfrost
temperature of water (Fig. 2) or if the surface is
superhydrophobic (Fig. 3). In a Leidenfrost case, when the
droplet initially impacts the surface, the bottom of the droplet
vaporizes and forms a vapor cushion. This cushion prevents
contact between the droplet and the surface and allows the
droplet to bounce off the surface2. In the superhydrophobic case,
the droplet is kept from wetting the surface by the presence
of microscopic protrusions on the surface. These protrusions
create a cushion of air between the droplet and the surface that
prevents wetting3.
Methods
In order to investigate the behavior of waterbased complex fluids above the Leidenfrost
temperature of pure water, we first prepared
solutions with varying concentrations of
cornstarch in water. We then used a syringe to
drop droplets of these solutions from a fixed
height onto a silicon wafer on a hot plate at
varying temperatures. The hot plate was tilted
slightly to encourage sliding of the droplet,
should a vapor cushion form. We used a Photron
high-speed camera to record the behavior of the
droplets as they impacted the surface (Fig. 4a).
We also examined the behavior of higher
concentrations of cornstarch in water being
dropped onto a superhydrophobic surface.
We used a layer of soot on a glass slide for
the superhydrophobic surface, and used a
syringe to drop droplets from a fixed height,
as before. The surface was not tilted. We
used a Photron high-speed camera to record
the behavior of the droplets as they
impacted the surface (Fig. 4b).
[1]
Figure 4: Experimental setup.
Results
We varied the concentration of cornstarch in each droplet from 0 to 10 percent, and
for each concentration, we varied the temperature from 24.2 ˚C to 400 ˚C. At low
temperatures, all of the droplets stuck to the surface. At 100 ˚C and 250 ˚C, some of
the droplets boiled away, but only at particularly low concentrations. At 305 ˚C, all of
the droplets bounced. At 340 ˚C, the 10% and 9% droplets stuck, and at 400 ˚C, the
7% and 8% droplets stuck as well. All the other droplets bounced at those
temperatures (Fig. 5a).
We also varied the concentration of cornstarch from 0 to 80 percent in increments of
10 percent. The 0 to 70% droplets bounced off the superhydrophobic surface, and the
80% droplet stuck (Fig. 5b).
Discussion
When a droplet impacts a surface above its Leidenfrost
temperature, a vapor cushion forms. The vapor cushion is
constantly being drained out from under the droplet by the
droplet’s weight, and constantly being replenished by the droplet
as more of the liquid vaporizes. A water droplet will eventually
vaporize completely as the cushion is maintained through the
vaporization of the droplet, and the droplet will not maintain
contact with the surface. However, if the vapor cushion is not
being replenished as fast as it is drained, it shrinks until the
droplet is in contact with the surface.
A natural explanation for the behavior of the cornstarch and water
solutions is that the energy required for bouncing is dissipated in
the bulk of the material due its shear thickening behavior.
However, the results on the superhydrophobic surface indicate
that this bouncing will occur for concentrations equal to or less
than 70%, far higher than the concentrations used in the
Leidenfrost case (Fig. 5).
An alternative explanation is that, as the droplets with cornstarch
impact the surface, the water on the bottom of the droplet
vaporizes, which causes them to bounce. At low concentrations of
cornstarch, there is enough water on the surface that the vapor
cushion can be maintained (Fig. 6a). At higher concentrations, the
cornstarch prevents further vaporization of the water, which
causes the droplet to stick to the surface as the vapor cushion
drains (Fig. 6b).
Figure 6: Schematic of suspected underlying mechanism
of concentration-dependent transition behavior
Conclusions
The results in our study demonstrate that drops of complex fluid
that might be expected to support their weight due to the
Leidenfrost effect may instead stick to the surface. This transition
between sticking or not sticking to the surface is consistent with
dynamics that are dependent on both surface temperature and
concentration. We expect that our results are relevant to a variety
of situations where colloidal particles could cause other complex
fluids to behave in ways that cause them to stick to surfaces when
simple fluids would not. These behaviors may improve the
efficiency of certain spray coolants or change the forensic
outcome of a bloodstain.
References
Figure 1: Droplet spreading on surface, at T=24.2 ˚C
(1) Fall, A.; Bertrand, F.; Ovarlez, G.; Bonn, D. Shear Thickening
of Cornstarch Suspensions. J. Rheol. 2012, 56(3), 575-591.
(2) Quéré, D. Leidenfrost dynamics. Annual Review of Fluid
Mechanics. 2013, 45, 197-215.
(3) Richard, D.; Clanet, C; Quéré, D.; Surface phenomena:
Contact time of a bouncing drop. Nature, 2002, 417(6891), 811.
Figure 2: Droplet bouncing off surface at T=250 ˚C
Acknowledgements
Figure 3: Droplet bouncing off superhydrophobic surface.
Figure 5: The behavior of droplets as they impact a surface.
5a images (left to right): Water boiling at 100 ˚C, 2% cornstarch sliding (bouncing) at 400 ˚C,
9% cornstarch sticking at 400 ˚C.
We thank Dr. Anna Greenswag and the RISE program for making
this collaboration possible. We also thank the members of the
Interfacial Fluid Dynamics Lab for support in various aspects of
this project. A. B. acknowledges financial support from her
parents.
Leidenfrost Effect on Oobleck
Adira Balzac1,2, Samira Shiri2, James C. Bird2
1
Nashoba Regional High School, Bolton, MA; 2Interfacial Fluid Dynamics Lab Boston
University, Boston, MA 02215
When water droplets hit a hot surface, the bottom of the droplet vaporizes. This vapor creates a
cushion and prevents further contact with the surface. The vapor cushion slows heat transfer
between the droplet and the surface, which allows the droplet to persist. This is called the
Leidenfrost effect. The effect appears when the surface temperature is sufficiently higher than
the vaporization temperature of the liquid. Simple fluids, like water and liquid nitrogen, behave
in predictable ways at their Leidenfrost temperatures. However, the effect of adding particles to a
liquid on the behavior of the fluid at and above the Leidenfrost temperature of the pure liquid is
less understood. Here we show that even low concentrations of cornstarch in water, less than
10% by mass, can cause the complex fluid that is formed to stick to a surface well above the
Leidenfrost temperature of water. As the fluid impacts the hot surface, the water begins to
vaporize, which greatly increases the local concentration of cornstarch and forms a skin on the
surface of the droplet. We hypothesize that this skin prevents further vaporization and causes the
droplet to remain in contact with the surface with no vapor cushion to slow heat transfer. Our
results demonstrate that complex fluids can behave very differently near their interface than in
the bulk of the fluid compared to a simple fluid under the same conditions. We anticipate that
these results will extend to other colloidal suspensions including blood and may impact blood
pattern stain analysis.