A Study on the Coffee Spilling Phenomenon in the Low Impulse Regime
∗
Jiwon Han
Korean Minjok Leadership Academy, Gangwon-do, Rep. Korea
(Dated: November 2, 2015)
When a glass of wine is oscillated horizontally at 4Hz, the liquid surface oscillates calmly. But
when the same amount of liquid is contained in a cylindrical mug and oscillated under the same
conditions, the liquid starts to oscillate aggressively against the container walls and results in sig-
nificant spillage. This is a manifestation of the same principles that also cause coffee spillage when
we walk. In this study, we experimentally investigate the cup motion and liquid oscillation during
locomotion. The frequency spectrum of each motion reveals that the second harmonic mode of
the hand motion corresponds to the resonance frequency of the first antisymmetric mode of coffee
oscillation, resulting in maximum spillage. By applying these experimental findings, a number of
methods to suppress resonance are presented. Then, we construct two mechanical models to ratio-
nalize our experimental findings and gain further insight; both models successfully predict actual
hand behaviors.
I. INTRODUCTION
Rarely do we manage to carry coffee around without
spilling it once [Fig 1]. In fact, due to the very common-
ness of the phenomenon, we tend to dismiss questioning
it beyond simply exclaiming: “Jenkins! You have too
much coffee in your cup!”
Figure 1: Rarely do we walk without spilling coffee.
However, the coffee spilling phenomenon is deceivingly
simple [1]. As a counter-intuitive example, prepare two
liquid containers with distinct geometrical structures;
here, we consider a wine-glass and a normal sized cylin-
drical mug. Pour the same amount of coffee inside each
glass (this is to ensure that Jenkins does not have “too
much coffee” in his cup). Since the human walking mo-
tion consists of periodic movements on the plane parallel
to the walking direction, we will oscillate each cup at a
fixed frequency in order to simulate such oscillatory mo-
tion. Using the mechanical device shown in Fig 2a, we
impose a horizontal excitation X = X
0
cos(2π × 2t) to
∗
No institutions were involved in this study
each liquid container. According to common sense, since
the amount of coffee is the same inside each cup, the
amount of coffee that spills from the oscillation should
be fairly similar as well. However, this is not the case.
As it is clearly shown in Fig 3b, the coffee motion inside
the wine glass is aggressive while that of the cylindrical
cup is comparatively steady; consequently, the amount
of coffee spilt is significantly different. When the driving
frequency is changed to 4Hz, we are again surprised. Es-
sentially, the liquid behavior inside each container is com-
pletely reversed: while the coffee inside the wine glass re-
mains close to equilibrium, the coffee inside the cylindri-
cal cup oscillates violently [Fig 3c and Fig 3d]. Although
we yet do not have sufficient knowledge of the human
walking motion, such experiment results are enough to
show that the amount of liquid may not be the sole rea-
son behind spilling coffee.
(a) (b)
Figure 2: (a) A mechanical setup to maintain a fixed
freqeuncy during oscillation. (b) A diagram of the effective
cup height.
Indeed, the spilling of coffee is a manifestation of mul-
tiple interactions, ranging from the body-hand coordina-
tion to the resonance properties of the cup-coffee inter-
action. Thus, in order to gain clearer insight, the cof-
fee spilling phenomenon is divided into two regimes: the
low impulse regime and the high impulse regime. The
term “impulse” indicates the maximum magnitude of the
impulse that the cup experiences. Not surprisingly, the
physical properties of each regime are significantly dis-
2
(a) (b)
(c)
(d)
Figure 3: Oscillations at (a), (b) 2Hz and (c), (d) 4Hz.
tinct. In the low impulse regime, the interaction between
the cup and coffee is considered as a periodic function;
thus, the oscillation properties are researched extensively.
However, in the high impulse regime, the interaction be-
tween the cup and coffee is momentary and aggressive.
Oscillation properties carry less importance in such a
regime. Spilling from casual walking falls under the for-
mer regime; spilling after tripping on a stone falls under
the latter. In the present paper, the low impulse regime
is set to be the main focus of study.
Also, the effective cup height (which is defined to be
the height of the cup subtracted by the liquid equilib-
rium level [Fig 2b]) is not considered as a variable in
this study for two reasons. First, the role of the effective
height of the cup in spilling is rather straight forward. If
the effective height of the cup is large enough, the coffee
is unlikely to spill unless it is flipped over. On the other
hand, if the effective height of the cup is close to zero,
that is, if the cup is filled to its brim, the liquid is much
more likely to spill. Thus, the taller the cup and lesser
the coffee, the less you spill. Such a relationship is not
investigated to further extent in this study [2]. Second,
as much as it is simple, the role of effective cup height
is also absolute; thus, it should be considered more as a
classification than a variable. Again, such an extra classi-
fication is not included in this study, for it will complicate
the research more than deepen our understanding of the
phenomenon.
Thus, in this paper, we study the conditions that max-
imize the amplitude of coffee oscillation under the low
impulse regime. In the Experiment Studies section, the
liquid oscillation properties and the cup’s motion prop-
erties are investigated. Here, a surprising feature of the
cup (hand) movement during walking is observed from its
frequency spectrum. Then, combining the results from
each investigation, it will be revealed how the interplay
between the cup and coffee leads to spilling. By apply-
ing this knowledge, a number of methods to reduce coffee
spilling are presented as well. Next, in the Model Stud-
ies section, two mechanical models of the “normal hand”
posture and the “claw-hand” posture are proposed. They
are each the oscillating-pivot single pendulum and the
oscillating-pivot double pendulum; both models are con-
structed upon the bold assumption that coffee, at least in
this study, can be treated as a simple pendulum. Surpris-
ingly, simulation studies reveal that both models success-
fully predict the important physical properties discovered
through experiment. We then conclude the paper with a
summary of our discoveries.
II. EXPERIMENT STUDIES
From experience, we know that our carrying hand is
usually strong enough to be essentially unaffected by the
coffee’s impact on the cup. This subtle insight can im-
mensely simplify the situation: instead of analyzing both
directions of influence, we can limit ourselves to one.
Therefore, it is physically sound to interpret the coffee-
cup system as a forced oscillator. The driving force,
which is synchronized with the carrying hand’s motion,
is directly exerted on the liquid from the inner walls of
the cup. Since we are in the low impulse regime, this
driving force is considered periodic; if the driving fre-
quency corresponds to the resonance frequency of coffee,
the sloshing amplitude reaches its maximum and results
in spilling. Thus, the question that we must investigate
is clear: what are the resonance conditions of this forced
oscillator?
A. Liquid Oscillation Properties
In order to determine the resonance conditions, the
first and foremost information that must be acquired is
the resonance frequency of the oscillator. Here, the os-
cillator is coffee. From the assumption that our liquid in
consideration is incompressible, irrotational, and invis-
cid, the following equation predicts the natural frequen-
cies of the various modes of fluid oscillation in an upright
cylindrical container [[3], [4]].
ω
2
mn
=
g
mn
R
tanh(
mn
H
R
)[1 +
σ
ρg
(
mn
R
)
2
] (1)
Of the various modes of oscillation, our main interest is
the first antisymmetric mode. This is because of two rea-
sons: the first antisymmetric mode involves the largest
amount of liquid mass movement, and as we will see in
the following section, its frequency corresponds to the
3
driving frequency (at least partially). Thus, by substi-
tuting the parameters in equation 1 with the generic cup
dimensions [5] of 82mm diameter and 95mm height, the
generic σ and ρ values for coffee [6], and
11
= 1.841, we
calculate the first antisymmetric mode frequency to be
approximately 3.95Hz. Indeed, this value is dependent
on the specific dimensions of the cup, and it is helpful to
have a sense of how much the natural frequency would
change according to the radius of the cup. Such a rela-
tionship is illustrated in Fig 4. Interestingly, the equation
predicts a different amount of change in the natural fre-
quency when the radius is either increased or decreased:
an increase in the radius will not cause the natural fre-
quency to change as much as it would if it were decreased.
Figure 4: The natural frequencies as the radius changes.
The fist antisymmetric mode can also easily be ob-
served in the lab. Using the same mechanical device that
was utilized in the introduction, we give the cylindrical
cup a short “pump” and record its subsequent surface
waves. It is important to make sure that the given im-
pulse is at a reasonable magnitude; if the impulse is too
large, unnecessary effects such as the liquid surface break-
ing or other modes of oscillation being excited will be
observed as well. Here, the amplitude of the mechanical
vibrator was set to be 2cm and the frequency was fixed
at 2Hz, which are reasonable values that correspond to
actual dimensions of locomotion. By using the color dif-
ference between the coffee and the background, we track
one point on the liquid surface and plot its height rel-
ative to the equilibrium level. The final graph is pre-
sented in Fig 5a. Visually, the damping oscillation seems
to be monochromatic with an exponentially decreasing
envelope. The former observation can be easily verified
from the frequency spectrum [Fig 5b], which reveals that
the damping oscillation indeed has a single dominant fre-
quency of approximately 3.8Hz. This value is slightly be-
low the predicted frequency of 3.95Hz, most likely due to
the viscosity of coffee and other unconsidered frictional
forces that arise from the cup-coffee interactions [7]. The
second speculation is a bit trickier. The decreasing en-
velope is directly related to the damping coefficient γ;
however, without sufficient knowledge of the input en-
ergy and the rate of dissipation, the damping coefficient
defined as the following definition cannot be accurately
(a)
(b)
Figure 5: (a) The relative height of a point on the liquid
surface while it oscillates. (b) A FFT analysis of (a) reveals
that the oscillation is fairly close to 3.8Hz.
calculated [8].
γ =
˙
< E
l
>
2E
(2)
Instead, γ is determined by using an exponential curve-
fit of the enveloped curve of the damping oscillation. The
damping coefficient is revealed to be approximately 0.674
rad/s, with r-square value of 0.9774. A parameter that
can greatly increase this value is discussed in the Sup-
pressing Resonance section.
B. Cup Motion Properties
After investigating the oscillator properties, the next
step is to analyze the driving force: the cup. The cup is
synchronized with our hand’s motion, which is directly
influenced by our bodily movements. Such body-hand
coordination properties have been extensively researched
in biomechanical studies [[9], [10], [11], [12]], and it is
revealed that the hand’s swaying motion during loco-
motion is dictated by our lower body’s “up and down”
movements (H. Pontzer and Lieberman [9] coin the term
“passive mass damper” for our hand’s swaying motion).
However, we need to be cautious of the fact that the
specific mechanism of the hand’s control of the cup may
4
change according to how we hold the cup. While such
deviations will be investigated in the Suppressing Res-
onance section and the Model Studies section, for now,
we stick to the so-called “normal hand” posture, as illus-
trated in Fig 6a.
In this research, two methods were employed in or-
der to measure the acceleration of the cup during loco-
motion. The first method, which turned out to be un-
successful, was to utilize image processing tools. The
idea was to track the center of mass of the cup while
the cup holder casually walked. That way, it would be
possible to extract the time plot of the cup’s position,
and subsequently, the time plot of the acceleration of the
cup (by taking a second order derivative of the position
data). However, this method was unsuccessful due to
two main reasons. First, the image data was not sensi-
tive enough. If the data is obtained over a long distance,
one would inevitably have to zoom-out; this directly re-
duces the number of pixels by which the position data
is recorded, and results in an extremely “smoothed-out”
data plot. On the other hand, if we zoom in as much
as we want to, the data collection time span is greatly
limited. Unfortunately, we are stuck in a Heisenberg
uncertainty principle-like situation in which we cannot
achieve both measurements with desired quality at the
same time. Second, the visual data was limited to only
one plane of oscillation. Although the plane parallel to
the walking direction is indeed where most of the action
occurs, it would be better if data from all three planes
of oscillation could be acquired as well. Such issues were
solved by adopting the second method: utilizing an ac-
celerometer [13].
The second method proved to be quite successful. The
apparatus, as shown in Fig 6a, is straightforward. By
fixing an accelerometer (or, equivalently, a smartphone)
to the top of the mug, we record all three directions of
acceleration. Since the mug is a hard body, we expect the
acceleration measured on any part of the mug to be equal;
the accelerometer was also strapped to the bottom of the
mug in order to verify that the experiment results were
indeed independent of the position of the accelerometer
[14].
Representative acceleration plots in each orientation
and their respective FFT analysis results are presented
in Fig 6b and Fig 6c. Here, the y-axis is the walking
direction, the z-axis is the direction perpendicular to the
ground, and the x-axis is the remaining sideways direc-
tion. From the acceleration time plot, the difference in
the maximum magnitude of acceleration in each axis is
highlighted. The z-axis acceleration has the biggest mag-
nitude, and the x-axis acceleration is almost negligible in
magnitude compared to the other two. This matches our
expectations, since the up-and-down motion of walking
is visually much larger than that of sideways swaying.
According to the results of H. Pontzer and Lieberman
[9], the frequency of the z-axis oscillation should be syn-
chronized with our lower-body movements. Another in-
teresting observation can be made from the frequency
spectrum in each axis. In the acceleration time plot, the
z-axis oscillation seems to have a smaller frequency than
the y-axis oscillation; this is counter-intuitive, since we
expect the cup motion to be have the same frequency
as our body (up-and-down oscillation) itself. In order
to shine a light on such observation, a FFT analysis is
conducted on each acceleration plot.
Indeed, the FFT results are quite enlightening. Let
us first take note of the y-axis frequency spectrum [Fig
6c]. Evidently, the cup does not oscillate at the same fre-
quency of our body. In fact, the motion is not even close
to being purely sinusoidal: at least five or more distinct
harmonic frequencies are contained in the motion. This
directly goes against the daily assumption that our hand
simply goes up and down when we walk. Instead, the
cup-carrying hand undergoes a complex oscillation that
is less than perfectly synchronized with our bodily mo-
tions. We should note that such intricate oscillations do
not stem from the arm itself, but rather the extra degree
of freedom that the wrist allows in the cup motion. An-
other significant observation is made by examining the
specific values of the frequency components in the y-axis
oscillation. Among the distinct harmonic frequencies, the
second harmonic frequency coincides with 3.5 4Hz, which
is the resonance frequency of coffee in regular sized [5]
cylindrical cups. In other words, as we casually walk,
our hand oscillates in such a way that resonates with the
first antisymmetric mode of coffee oscillation; thus, the
likelihood of coffee spilling is maximized. It is impor-
tant to realize that resonance would not likely occur if
higher-frequency modes did not exist in our hand mo-
tion. For example, would one still spill coffee if the cup
was strapped around one’s waist? The answer is prob-
ably “no”, since, as we saw in the introduction, coffee
does not spill as much when it is simply driven at 2Hz.
Again, the particularity of the cup motion that allows
higher-frequency oscillation is highlighted.
Now we shift our focus to the other two results [Fig
6c]. First, the z-axis oscillation clearly exhibits a dom-
inant frequency close to 1.7Hz. There also exist higher
frequencies, but they are rather insignificant compared
to the dominant frequency. This is reflected in our ex-
perience that the walking motion is largely composed of
up-and-down motions, and that the frequency of such
up-and-down motion is what we normally perceive to
be the walking frequency. Although it cannot initiate
a significant level of coffee sloshing, the z-axis oscillation
at 1.7Hz can still amplify the first antisymmetric mode
in two ways. First, since 1.7Hz is close to half of the
resonance frequency, the z-axis oscillation can increase
the amplitude of the coffee once every two cycles after
the first antisymmetric mode is excited by y-axis oscilla-
tions. Second, there is the possibility of subharmonic res-
onance, as in the parametrically driven pendulum [[15],
[16]]. However, such behavior was neither experimen-
tally nor mathematically investigated thoroughly in this
research. Next, it is notable that the x-axis oscillation
has a dominant frequency of approximately 1Hz, which
5
(a)
(b)
(c)
Figure 6: (a) A simple apparatus to measure the
acceleration that the cup experiences during locomotion.
The acceleration data is recorded on the phone, which is
stably fixed on the cup. In order to ensure that the weight
of the cup did not change too much, the total weight of the
apparatus was set to be equal to that of a 2/3 full cup. (b)
The acceleration time plot in each orientation of
measurement. There is a clear periodic tendency. (c) The
FFT result in each orientation of measurement. The y-axis
oscillation clearly exhibits harmonic frequencies; the second
harmonic frequency coincides with the resonant frequency of
coffee. Due to our up-and-down motion during walking, the
z-axis oscillation exhibits strong periodicity of 2Hz; this is
our normal perceived frequency of walking.
is the half of the walking frequency itself. This reflects
the sideways swaying motion of our hands when we walk,
which, evidently, occurs once every two walking cycles.
The x-axis oscillation, combined with y-axis oscillation,
can cause the liquid to circulate around inside the cup.
C. Suppressing Resonance
So far, we have succeeded in uncovering the basic
mechanism behind coffee spilling: resonance. By inves-
tigating the frequency properties of the coffee and cup
motion, it is now evident that walking excites the first
antisymmetric mode of coffee oscillation. It was also re-
alized that such excitation is enabled by the biomechan-
ical particularity of the cup (hand) motion. Now we ask
the practical question. How do we stop spilling? The
suggested solution is rather straight forward. Since the
culprit behind spilling is resonance, preventing resonance
would be sufficient to significantly reduce the probability
of spilling. This can be achieved by altering either the
coffee’s resonance frequency or the cup motion itself. A
number of possible methods to implement such changes
are discussed here.
The first suggested method is to change the way we
walk. By walking backwards, we are able to significantly
change the frequency characteristics of our hand motion.
Using the same experiment setup shown in Fig 6a, we
conduct a FFT analysis of the cup’s acceleration when we
walk backwards. A representative result is shown in Fig
7. A notable change in the y-oscillation frequency spec-
Figure 7: Frequency spectrum of backwards-walking.
trum is highlighted. Compared to normal walking, the
frequency spectrum is more evenly distributed, and the
presence of higher frequency modes is greatly reduced;
in fact, there does not seem to be a dominant frequency
at all. Evidently, the resonance frequency of coffee is
no longer a significant component in the frequency spec-
trum of the cup. As a result, the first antisymmetric
mode now has a lesser chance of being excited, leading
to a subsequent decrease in the probability of spilling cof-
fee. Perhaps this is due to the fact that we are not used
to walking backwards: since we are not accustomed to
backwards walking, our motion in the walking direction
6
becomes irregular, and our body starts to heavily rely on
sideways swinging motion in order to keep balance. This
accounts for the subtle changes in the x-axis and z-axis
frequency spectra as well. Of course, walking backwards
may be less of a practical method to prevent spilling cof-
fee than a mere physical speculation. A few trials will
soon reveal that walking backwards, much more than
suppressing resonance, drastically increases the chances
of tripping on a stone or crashing into a passing by col-
league who may also be walking backwards (this would
most definitely lead to coffee spilling).
Figure 8: The “claw-hand” method of carrying coffee.
Figure 9: Frequency spectrum of the claw-hand posture.
Fortunately, the second suggested method is a bit more
realistic. By changing the way we hold the cup, it is also
possible to suppress resonance; the proposed method of
cup-holding is illustrated in Fig 13b, and it is named
as the “claw-hand” posture. As it will be explained fur-
ther in the Model Studies section, such a method of hold-
ing the cup is mechanically equivalent to adding another
oscillatory component to our system. Again, the same
mechanical device used in former experiments is used to
record the acceleration that the cup undergoes in the
claw-hand posture. Then, we investigate changes in the
frequency spectrum of the recorded data. A representa-
tive FFT analysis result is given in Fig 19. The change
in the y-oscillation frequency spectrum is similar to that
of walking backwards: the higher frequency harmonic
modes have been reduced greatly, although the domi-
nant frequency near 1.7Hz remains significant. Thus, we
expect the claw-hand posture to have similar effects on
the coffee oscillation as walking backwards.
We also propose the method of adding a foam layer
to the liquid surface. Such a method was extensively re-
searched by Sauret, Boulogne, Cappello, Dressaire, and
Stone [8]. Their study demonstrates that a relatively thin
layer of foam can be effective in damping liquid sloshing.
A similar but simplified experiment is conducted in this
study. The experiment apparatus illustrated in Fig 10a
is used to observe the surface oscillations when a layer
of foam was added—a Hele-Shaw cell is used due to the
technical difficulties of analyzing the surface oscillation in
a 3-dimensional container. Three samples are analyzed:
the no-foam sample, 1cm foam sample, and the 2cm foam
sample. The foaming solution is composed of 90% wa-
ter and 10% glycerol, and the experiments are performed
over a short timespan (about 1 second) so that the de-
cay of the foam layer would be negligible. Again, us-
ing the color difference between the coffee and the back-
ground, we track one point on the liquid surface. The
time plot of the relative height in each sample is pre-
sented in Fig 10b. From the frequency spectra in Fig
11a, we observe that the damping frequency decreases
from approximately 3.3Hz to 3Hz [17]. Then, from the
fitted curve [Fig 11b] of the no-foam sample 1cm foam
sample, we note that damping coefficient nearly triples
in its value (from 1.025rad/s to 2.928rad/s); according
to A. Sauret and Stone [8], this is a result of the energy
dissipation in the wall boundary layer.
There is also the method of changing the cup’s reso-
nance property itself. In Fig 4, it is evident that a de-
crease in the radius of the cup can significantly increase
the resonance frequency; by dividing the cup into smaller
cylindrical cells, as shown in Fig 12, the liquid oscilla-
tion is sufficiently displaced from the resonance domain.
However, such an effect is not quantified in this research
[18].
III. MODEL STUDIES
Although a full biomechanical description of the coffee
spilling phenomenon is beyond the scope of this study,
a simplified mechanical model is proposed and analyzed
in order to gain further insight into the dynamics of the
phenomenon. So far, we have examined two distinct ways
of carrying coffee: the “normal hand” posture and the
“claw-hand” posture. From the experiment findings in
the previous sections, we now know that the two postures
have distinct physical properties; thus, for each posture,
a mechanical model that encompasses such differences is
constructed and compared to experiment findings. The
proposed mechanical models are illustrated in Fig 13a
and Fig 13b. In both models, the coffee’s impact on the
cup motion is no longer negligible. Also, for simplicity’s
7
(a)
(b)
Figure 10: (a) A mechanical device to observe the effect of
adding foam. Due to technical difficulties, a Hele-Shaw cell
is used instead of a cylindrical container. (b) The time plot
of the relative height in each sample. A drastic decrease in
the amplitude as a foam layer added can be observed.
sake, the walking frequency is set to be 2Hz and the first
antisymmetric mode is set to be 4Hz (the actual values
are approximately 1.7Hz and 3.8Hz). For the normal
hand posture, the arm is depicted by a simple harmonic
oscillator of mass M. Since the arm itself is not flexible, it
is considered to oscillate at 2Hz, perfectly synchronized
with the bodily oscillations. Thus, M and the spring con-
stant k are decided so that the natural frequency of M
under a small displacement would equal 2Hz. Next, the
coffee is depicted by a simple pendulum; the simple pen-
dulum oscillating at f corresponds to coffee being driven
at an external driving force of frequency f. Here, l
1
is
decided so that the natural frequency of the simple pen-
dulum would equal that of the first antisymmetric mode,
which is 4Hz.
Indeed, at first, this does not seem to be a physically
sound analogy. However, as illustrated in Fig 14, the
first antisymmetric mode of liquid oscillation somewhat
resembles the simple pendulum in the sense that the cen-
ter of mass oscillates with respect to a fixed point above
the liquid surface. Also, as it will be revealed below, the
implications of this analogy is astonishingly consistent
with experiment results. The claw-hand model is simi-
larly constructed as well. The only modification for the
(a)
(b)
Figure 11: (a) The frequency spectra of the no-foam
sample (top) and the 1cm-foam sample (bottom). A shift in
the dominant frequency to the left can be observed. (b) The
fitted curves are plotted with the original data plots. When
1cm of foam layer was added, the damping coefficient γ
increased to 2.93rad/s from 2.23rad/s and the angular
frequency ω
d
decreased to 19.83rad/s from 20.16rad/s. The
R
2
value for each curve fit is 0.8877(no foam) and
0.7668(1cm foam).
claw-hand model is that the simple pendulum is now a
double-pendulum; the claw-hand posture essentially adds
one more degree of freedom to the generalized coffee-cup
coordinate system. Seemingly subtle, this extra degree of
freedom will later on prove itself to have a significant ef-
fect on the frequency spectrum of the cup motion. In the
following two subsections, the Euler-Lagrange equations
for each mechanical model is solved and investigated ex-
tensively.
A. Normal Hand Posture:
Oscillating-pivot Simple Pendulum
The generalized coordinate system for the oscillating-
pivot simple pendulum and their relevant parameters can
8
Figure 12: By dividing the cup into smaller cylindrical
cells, we can displace the oscillation from resonance.
(a) (b)
Figure 13: Two mechanical models are proposed. (a) is the
“normal hand” posture and (b) is the “claw-hand” posture.
be expressed as te following.
q
1
q
2
=
x
θ
1
M
m
1
l1
k
g
=
1kg
0.1kg
1.55cm
157.9137N/m
9.81m/s
2
Next, the system’s kinetic energy, potential energy, and
the Lagrangian are calculated to be the following equa-
tions.
L
normal
= T − V (3)
T =
M ˙x
2
2
+
m
1
2
˙x
2
+ (l
1
˙
θ
1
)
2
+ 2 ˙x
˙
θ
1
l
1
cos(θ
1
)
V = −m
1
gl
1
cos(θ
1
) +
kx
2
2
The corresponding Euler-Lagrange equations are inte-
grated by the Runge-Kutta method, with initial val-
ues set to be
x, θ
1
=
2cm, 0.1rad
and timespan
Figure 14: A depiction of the first antisymmetric mode
and a simple pendulum. The colored circles represent the
center of mass of liquid.
Figure 15: Phase diagrams for the normal hand model.
t = [0s, 60s]. The representative phase diagrams are il-
lustrated in Fig 15.
The arm, represented by x, clearly follows a monochro-
matic cycle. On the other hand, the coffee, represented
by θ
1
, follows a more complex trajectory. In order to
determine the frequency characteristics of each oscillator
(the arm and coffee), a FFT analysis is conducted again
on each data set. The result is illustrated in Fig 16. The
Figure 16: FFT analysis of the normal hand model. The
frequency spectrum of each generalized coordinate x, θ
1
is
shown from top to bottom. The timespan is 60s.
upper graph is the frequency spectrum of x and the lower
9
Figure 17: FFT analysis of the normal hand model. The
frequency spectrum of each generalized coordinate x, θ
1
is
shown from top to bottom. The timespan is 500s.
graph is the frequency spectrum of θ
1
. Indeed, the arm
oscillates monochromatically at 2Hz as our body would
during actual walking. What is surprising is the result
for θ
1
. Just as we have discovered in our former experi-
ments, the coffee is excited at 4Hz, even though the arm
itself oscillates at 2Hz. Although a definitive conclusion
cannot be made without further biomechanical insight,
our model suggests that such particularities of the cup
motion may indeed be a result of the “extra degree of
freedom” that the wrist provides. Also, our model shows
that the characteristics of walking can be simulated in
well-defined dynamical systems such as this one.
Another interesting observation can be made when the
timespan is greatly extended. For example, the frequency
spectra of x and m
1
are illustrated in Fig 17 when the
timespan is extended to 500s.
A conspicuous shift in the frequency spectrum has oc-
curred: the high frequency component (4Hz) of m
1
has
disappeared and a low frequency component (1 1.5Hz)
has newly appeared. In other words, as the time elapsed,
the system evolved into a stable state in which only low
frequency oscillations remained. This may be a potential
answer to the thought-provoking question: “if one were
to walk for an infinite amount of time, would coffee be
inevitably spilt at some point?” If we only considered
the fact that our hand motion resonates with coffee, the
answer appears to be “yes”. However, as our model sug-
gests, if our hand motion evolves into an oscillation state
that does not resonate with coffee, the answer may cer-
tainly be “no”. Of course, since we usually do not walk
for 500 seconds or more with a cup of coffee, such sta-
bilization hardly ever takes place; our mechanical model
merely suggests the possibility of it.
B. Claw-hand Posture:
Oscillating-pivot Double Pendulum
The generalized coordinate system for the oscillating-
pivot double pendulum and their relevant parameters can
be expressed as the following.
q
1
q
2
q
3
=
x
θ
1
θ
2
M
m
1
m
2
l1
l2
k
g
=
1kg
0.1kg
0.1kg
15cm
1.55cm
157.9137N/m
9.81m/s
2
Now, there is an extra m
2
and l
2
term involved. Con-
sequently, the system’s kinetic energy, potential energy,
and the Lagrangian take a more complex form than the
oscillating-pivot simple pendulum.
L
clawhand
= T − V (4)
T =
M
2
˙x
2
+
m
1
2
˙x
2
+ (l
1
˙
θ
1
)
2
+ 2 ˙x
˙
θ
1
l
1
cos(θ
1
)
+
m
2
2
˙x
2
+ (l
1
˙
θ
1
)
2
+ (l
2
˙
θ
2
)
2
+ 2l
1
l
2
˙
θ
1
˙
θ
2
cos(θ
1
+ θ
2
)
+ m
2
˙x
l
1
˙
θ
1
cos(θ
1
) + l
2
˙
θ
2
cos(θ
2
)
V =
kx
2
2
− m
1
g
x + l
1
cos(θ
1
)
− m
2
g
x + l
1
cos(θ
1
) + l
2
cos(θ
2
)
Again, the corresponding Euler-Lagrange equations are
integrated by the Runge-Kutta method. The initial val-
ues are set to be
x, θ
1
, θ
2
=
2cm, 0.1rad, 0.1rad
and
timespan t = [0s, 60s]. The representative phase dia-
grams are illustrated in Fig 18. Compared to the claw-
hand model, a visual difference in the phase diagram for
coffee (represented by θ
1
in Fig 15 and θ
2
in Fig18) is
evident. Such a difference can quantified through a FFT
analysis of the claw-hand model, as shown in Fig 19.
Figure 18: Phase diagrams for the claw-hand model.
10
Figure 19: FFT analysis of the claw-hand model. The
frequency spectrum of each generalized coordinate x, θ
1
, θ
2
is
shown from top to bottom. The timespan is 60s.
As expected, mass M oscillates at a frequency close to
2Hz. Although the motion is not as purely sinusoidal as
the normal-hand model, it is still fairly monochromatic.
But as we shift our focus to the next two frequency spec-
tra, a notable deviation from that of the normal hand
model is observed. While in the normal hand model,
the dominant frequency was increased as we shifted from
the main oscillator (M) to the subsequent oscillator (θ
1
),
the case for the claw-hand model is completely the op-
posite. Starting from approximately 2Hz, the dominant
frequency is halved for each subsequent oscillator (θ
1
,
θ
2
). Ultimately, the driving frequency of the cup (θ
2
)
is pronouncedly displaced from the resonance frequency.
Although our experiment results for the claw-hand pos-
ture suggest a less extreme effect, our model successfully
predicts that the claw-hand posture may indeed suppress
resonance. The slight deviation from actual experiment
results most likely arises from the fact that, in reality,
θ
1
and θ
2
cannot oscillate as independently as described
by the claw-hand model; the actual claw-hand posture
should be an in-between state of the normal hand model
and the claw-hand model.
Figure 20: A comparison of the angular acceleration that
θ
1
in the normal hand model(blue) and θ
2
in the claw-hand
model(red) undergoes.
Additionally, the difference in the magnitude of the
coffee’s acceleration in each mechanical model is com-
pared in Fig 20. Even though the amplitude of the M
oscillation is similar, as can be seen in Fig 21, the conse-
Figure 21: A comparison of the ampitude of oscillation
that M undergoes in each model. The blue line is the
normal hand model and the red line is the claw-hand model.
quent acceleration that the coffee oscillator undergoes has
a notable difference in magnitude. Since the magnitude
of acceleration is directly proportional to the magnitude
of impulse that an oscillator is given in one cycle, we ex-
pect such a difference in magnitude to have significant
effects on the consequent coffee oscillation. Although we
are currently under the low impulse regime, it is inter-
esting that such a difference in the magnitude of impulse
can be predicted by our mechanical model. Again, since
the magnitude of acceleration in the claw-hand model is
significantly smaller, the claw-hand posture is less likely
to spill coffee.
IV. DISCUSSION
In this paper, we have researched how the periodic
force imposed on the cup during locomotion excites the
first antisymmetric mode of coffee oscillation and results
in spilling. In order to do so, we approximate the coffee-
cup system to a forced harmonic oscillator, and deter-
mine the resonance frequency of coffee. The damping
coefficient is additionally measured in order to show that
the damping frequency ω
d
and the resonance frequency
ω
r
have almost the same value. Then, using a simple me-
chanical apparatus, we record the acceleration of the cup
during locomotion and analyze its frequency spectrum
to show that the walking direction acceleration contains
harmonic modes of higher frequency than the “up-and-
down” walking motion. Among these harmonic modes,
the second lowest frequency mode corresponds to the res-
onance frequency of coffee; thus, we verify that the first
antisymmetric mode of coffee is indeed stimulated during
walking motion. Moreover, we show that either walking
backwards or holding the cup with the “claw-hand” pos-
ture can lead to a significant change in the frequency
spectrum of the cup motion, suggesting that resonance
can be suppressed through such methods. The effect of
adding a foam layer in a Hele-Shaw cell is also examined;
we show that the damping frequency decreases and the
damping coefficient increases significantly when a foam
layer is added (extensive studies on the foam layer is done
11
by A. Sauret and Stone [8]).
Next, in order to rationalize the experiment results
and to gain further insight, we construct two mechanical
models of the normal hand posture and the claw-hand
posture. In both models, phase diagrams reveal a clear
periodicity in the base oscillator (M, which corresponds
to the arm) but a more complex cycle for the endmost
oscillators (θ
1
for the normal hand model and θ
2
for the
claw-hand model). From the frequency spectrum of each
endmost oscillator, we show that the model successfully
demonstrates the physical properties realized in previous
experiments: θ
1
in the normal hand model carries a 4Hz
component and θ
2
in the claw-hand model oscillates at
a frequency lower than 2Hz. Another theoretical finding
was that the endmost oscillator in the normal hand model
stabilizes to a lower frequency mode as time elapses—
this may perhaps account for the fact that spillage is less
likely to occur later on during walking motion. Also,
it was predicted that the former oscillator undergoes a
significantly larger magnitude of acceleration than the
latter.
[1] Note1. Coffee spilling has been extensively researched by
H. C. Mayer and R. Krechetnikov. However, the exper-
imental approach and the conclusion is quite different.
T. Kulczycki, M. Kwasnicki, and B. Siudeja have also
taken a different approach on the subject as well; based
on an appropriate Steklov eigenvalue problem, their re-
search puts a focus on the geometrical properties of the
fluid. Refer to [4] and [19].
[2] Note2. A statistical correlation between the probability
of spilling and the effective cup height may be an inter-
esting topic for future research.
[3] R. A. Ibrahim. Liquid sloshing dynamics: Theory and
applications. Cambridge University Press, 2005.
[4] H. C. Mayer and R. Krechetnikov. “Walking with coffee:
Why does it spill?” Physical Review E, 85(046117), 2012.
doi:10.1103/PhysRevE.85.046117.
[5] “Amplifer”. The standard coffee mug di-
mensions. URL http://blog.ampli.fi/
the-standard-coffee-mug-dimensions/.
[6] Note3. The surface tension of a generic cup of coffee has
been researched by V.Sobolik [20] to be approximately
0.037N/m at 40
◦
C.
[7] Note4. Indeed, the actual “natural frequency” should
be derived from the relation ω
r
=
ω
2
0
− 2γ
2
1/2
and
ω
d
=
ω
2
0
− γ
2
1/2
where ω
d
is 2π × 3.8Hz and ω
r
is the
resonance frequency. However, the damping coefficient is
determined to be approximately 0.674rad/s. Considering
that ω
2
d
is around 570rad
2
/s
2
, the difference between ω
d
and ω
0
is negligible.
[8] J. Cappello E. Dressaire A. Sauret, F. Boulogne
and H. A. Stone. “Damping of liquid sloshing by
foams”. Physics of Fluids, 27(022103), 2015. doi:
10.1063/1.4907048.
[9] D. A. Raichlen H. Pontzer, J. H. Holloway and D. E.
Lieberman. “Control and function of arm swing in hu-
man walking and running”. The Journal of Experimental
Biology, 212, 2009. doi:10.1098/rspb.2009.0664.
[10] P. G. Adamczyk S. H. Collins and A. D. Kuo. “Dy-
namic arm swinging in human walking”. Proceedings
of the Royal Society B: Biological Sciences, 2009. doi:
10.1098/rspb.2009.0664.
[11] Stella F. Donker. “Flexibility of Human Walking: A study
on interlimb coordination”. PhD thesis, 2002.
[12] M. G. Pandy F. C. Anderson. “Dynamic Optimization
of Human Walking”. Journal of Biomechanical Engineer-
ing, 5(123), 2001. doi:10.1115/1.1392310.
[13] Note5. Unfortunately, accelerometers are infamous for
their inaccuracy and high level of noise. However, since
we are mostly concerned with information related to
frequency, and noise signals are random by definition,
the most essential data extracted from the accelerometer
would be fairly reliable. A noise test was conducted in
order to confirm that no dominant frequency was shown
in a FFT analysis.
[14] Note6. There exists the issue that the cup also undergoes
a “nodding” motion as we walk, which would mean that
the “x, y, z” orientations recorded by the accelerometer
slightly change during motion. And, as we will mention
later, such an extra degree of freedom is what allows the
cup’s intricate oscillation. However, the magnitude of the
nodding motion itself is much smaller in scale compared
to the other orientations of oscillation. Thus, we ignore
the changes in the axis during locomotion.
[15] Eugene Butikov. “Subharmonic Resonances of the Para-
metrically Driven Pendulum”. Journal of Physics A:
Mathematical and General, 35, 2002.
[16] Joseph Rudnick. “Subharmonics and the Transition to
Chaos”, Lecture Notes in Physics. Springer, 1969.
[17] Note7. The deviation from the values in Fig 5a is due
to the distinct geometries of the Hele-Shaw cell and a
cylindrical cup.
[18] Note8. The cleaning of such a cup would indeed be quite
a tedious job.
[19] M. Kwasnicki T. Kulczycki and B. Siudeja. “Spilling
from a Cognac Glass”. 2013. URL http://arxiv.org/
abs/1311.7296.
[20] M.Delgado R.Zitny K.Allaf V.Sobolik, V.Tovcigrecko.
“Viscosity and electrical conductivity of concentrated so-
lutions of soluble coffee”.