Aider, Jean-Luc

Benoit-Gonin, X

Dumy, G

Hoyos, M

English

Archive Ouverte en Sciences de l'Information et de la Communication

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Submitted on 22 Nov 2019
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Acoustic propulsion of metallic nano-cylinders:
contribution of the local vertical acceleration from micro
to hyper gravity
G Dumy, X Benoit-Gonin, M Hoyos, Jean-Luc Aider
To cite this version:
G Dumy, X Benoit-Gonin, M Hoyos, Jean-Luc Aider. Acoustic propulsion of metallic nano-cylinders:
contribution of the local vertical acceleration from micro to hyper gravity. 26th European Low Gravity
Research Association Biennial Symposium and General Assembly, Sep 2019, Granada, Spain. ￿hal-
02376725￿
HAL Id: hal-02376725
https://hal.archives-ouvertes.fr/hal-02376725
Submitted on 22 Nov 2019
HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d’enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.
Acoustic propulsion of metallic nano-cylinders:
contribution of the local vertical acceleration from micro
to hyper gravity
Jean-Luc Aider, G Dumy, X Benoit-Gonin, M Hoyos
To cite this version:
Jean-Luc Aider, G Dumy, X Benoit-Gonin, M Hoyos. Acoustic propulsion of metallic nano-cylinders:
contribution of the local vertical acceleration from micro to hyper gravity. 26th European Low Gravity
Research Association Biennial Symposium and General Assembly, Sep 2019, Granada, Spain. ￿hal-
02376725￿
26th European Low Gravity Research Association Biennial Symposium and General Assembly
14th International Conference on "Two-Phase Systems for Space and Ground Applications”
European Space Agency Topical Teams meetings
Granada (Spain), September 24-27, 2019
133. Acoustic propulsion of metallic nano-cylinders: contribution of the local vertical acceleration from 
micro to hyper gravity
26th European Low Gravity Research Association Biennial Symposium and General Assembly
14th International Conference on "Two-Phase Systems for Space and Ground Applications”
European Space Agency Topical Teams meetings
Granada (Spain), September 24-27, 2019
 
Acoustic propulsion of metallic nano-cylinders: contribution of the local 
vertical acceleration from micro to hyper gravity
G. Dumy1,2, X. Benoit-Gonin, M. Hoyos1, J-L. Aider2
1 PMMH Lab, ESPCI Paris, Paris Sciences et Lettres, Sorbonne Universités, Paris, France 
 2 Université Paris Descartes, Paris, France 
gabriel.dumy@espci.fr, mauricio.hoyos@espci.fr, jean-luc.aider@espci.fr 
Introduction 
Acoustic manipulation of microscopic objects can be 
performed using ultrasonic standing wave resonators and is 
called acoustophoresis. Provided that the wavelength Ȝ of the 
acoustic field (generated in this case by a piezoelectric 
transducer) and that one of the cavity dimension h verify Ȝ = 
2h, an ultrasonic stationary wave can be generated between 
two opposite walls of said cavity. This field can have two 
effects on the cavity components: on scattering particles it 
will exert a net body force on scattering particles, and it will 
transfer momentum to the fluid filling the cavity resulting in 
a streaming flow.  Thus, a suspension of microscopic objects 
submitted to such a field will see its constituents driven 
towards the pressure nodes (or antinodes depending on 
particles properties), where they can be trapped. The typical 
size of the objects that can be moved by acoustophoresis 
ranges usually from 500 nm to several tens of microns. 
A surprising finding in ultrasonic manipulation of 
micron-sized objects is the auto-propulsion of metallic nano-
cylinders when trapped in such a field (Wang et al. 2012): 
instead of aggregating like spherical particles, they start to 
swim while being confined in the pressure node of the 
acoustic field. The phenomenon has first been described with 
gold nano-cylinders (1µm long and 200nm in diameter). 
Trajectories ranging from straight lines to circular trajectories 
is observed. It is an intriguing phenomenon, since these quite 
small particles can propel up to hundredth of sizes per seconds 
(Ahmed et al 2016), while while moving into a Re ~ 10-4 flow 
(see Figure 2. d) ). In addition, these particles can organize 
themselves in chains hundredth of microns long because of 
their rotation, showing interesting self-assembling properties. 
Moreover, the power levels used here are on par with medical 
echography devices, and these nanorods have proven to be 
active even in cells, showing interesting potential for bio-
oriented applications. In the present work we investigated the 
contribution of the local acceleration of the gravity field on 
the orientation of the nano-cylinders, and its potential role in 
the propulsion of these objects. 
Reorientation of metallic nano-cylinders under different 
vertical accelerations 
Indeed, in standard gravity conditions, the aggregating 
position differs from the geometrical pressure node because 
of the equilibrium between the acoustic force and buoyant 
forces, which are non-negligible for metallic particles. As 
depicted on Figure 1, for metallic particles the aggregating 
plane is lower than the geometric center of the acoustic field. 
Calculations show that for gold microparticles with a typical 
acoustic energy of 10 to 100 J.m-3 under regular gravity, the 
levitation plane is 5 to 10% lower than the pressure node. 
Figure 1: Sketch of the acoustic resonant cavity. The cavity height 
is 200 µm leading to an acoustic resonant frequency fac = 3.65 MHz. 
The cavity upper wall is directly the quarter-wave layer of a 
packaged transducer, embedded in PDMS walls.  When the 
ultrasounds are turned on the micro-rods gather at the equilibrium 
focusing plane and form a swarming cluster. The equilibrium 
focusing plane is lower than the nodal plane because of the gravity. 
The distance from the nodal plane is larger for high-density micro-
rods, like gold micro-rods. In micro-gravity, the micro-rods are 
expected to gather in the nodal plane. 
Thus, we carried out a micro-gravity experiments campaign, 
to test the influence of the local vertical acceleration, and thus 
the effect of the vertical aggregating position on the behavior 
of our objects. 
Figure 2 presents a striking feature of our findings. An 
aggregate of gold nano-cylinders is formed under a 3.65 MHz 
acoustic field, and its behavior during the 1g and 2g phases of 
the experiment is comparable to what we observe in the 
laboratory. However, as soon as we start the microgravity 
phase, its appearance changes. It becomes darker, which we 
interpreted as a reorientation of the nano-rods. Indeed, in 
every acoustic propulsion of these objects we know of, the 
long axis of the rods is always parallel to the observation 
plane, which leads to the visualization of bright nanorods. 
Since we mostly perform reflection observation experiments, 
these objects always appear brighter than the background. 
However, if one considers that the rod long axis is now 
perpendicular to the observation plane, then the effective are 
reflecting the light becomes much smaller, leading to a darker 
object.  
Focusing planes 
PDMS 
hůƚƌĂƐŽŶŝĐ
ƚƌĂŶƐĚƵĐĞƌ 
Glass Reflector 
DŝĐƌŽƐĐŽƉĞ
ŽďũĞĐƚŝǀĞ 
walls 
 
ܨݎܽ݀ሬሬሬሬሬሬԦ 
ܨݖሬሬሬԦ 
ܨݎܽ݀ሬሬሬሬሬሬԦ 
ܨݖሬሬሬԦ 
0 g 
 1 g 
 
26th European Low Gravity Research Association Biennial Symposium and General Assembly
14th International Conference on "Two-Phase Systems for Space and Ground Applications”
European Space Agency Topical Teams meetings
Granada (Spain), September 24-27, 201926th European Low Gravity Research Association Biennial Symposium and General Assembly
14th International Conference on "Two-Phase Systems for Space and Ground Applications”
European Space Agency Topical Teams meetings
Granada (Spain), September 24-27, 2019
Figure 2: Visualizations of a large aggregate of self-propelled 
micro-rods in acoustic levitation in micro-gravity (a), 1 g (b) and 
hyper-gravity (c). The large circles observed in the aggregates are 15 
micrometers polystyrene particles that were used to find the 
equilibrium positions of aggregates before the experiment, and that 
were not completely washed by the buffer. (d) In standard gravity 
conditions, the micro-rods are self-propelled by the ultrasounds once 
they have reached the acoustic levitation plane. Here, micro 
cylinders of pure gold (1µm length, ~200 nm radius) are propelled 
in a 3.6 MHz acoustic field, with an acoustic energy density of 25 
J.m-3. Tracks denoting their trajectories over a few seconds are
plotted in color, propulsion velocities typically reach ~30 µm.s-1.
A contribution of the transversal acoustic radiation force 
component? 
Why should these objects change orientation when the 
vertical acceleration changes? An interesting additional 
observation, as denoted before, is that in every visualization 
we know of, the rods present their long axis parallel to the 
aggregation plane. Up to now, this observation has been taken 
for granted. Our experiments show that this is in fact not 
trivial, and can be explained only if they are forced to do so 
by the vertical component of the acoustic radiation force. As 
the vertical component of the acoustic force vanishes in the 
nodal plane of the resonator (Whitworth et al 1991, Tuziuti et 
al 1999), only the transverse one remains, which will then 
force the rods to reorient themselves along the observation 
axis. 
The vertical position of aggregation for these high-density 
objects seems then to have a role to play in their acoustic 
response, a hypothesis that has not been much considered 
before. It also means that the orientation and propulsion is 
linked to the transverse component of the acoustic field, 
which is a hypothesis currently under investigation. 
Conclusion 
We compared laboratory and microgravity experiments, to 
correlate between the vertical position of the aggregate and 
the propulsion speed of the swimmers. We observed a 
different behavior of the cylinders in each case, which leads 
to reconsidering the classic hypothesis used to study self-
acoustophoresis. This result suggests that the transverse 
component of the acoustic force plays an important role in the 
propulsion mechanism of these objects. 
Acknowledgements 
We would like to thank the foundation Bettencourt and the doctoral school 
FIRE for funding Gabriel Dumy’s PhD. 
References 
W. Wang, L.A. Castro, M. Hoyos, T.E. Mallouk, Autonomous
motion of metallic microrods propelled by ultrasound, ACS
Nano, 6, 7, 6122 (2012).
S. Ahmed, W. Wang, L. Bai, D.T. Gentekos, M. Hoyos, T.E. 
Mallouk, Density and Shape Effects in the Acoustic
Propulsion of Bimetallic Nanorod Motors, ACS Nano 10, 4,
4763 (2016).
G. Whitworth, M.A. Grundy, W.T. Coakley, Transport and
harvesting of suspended particles using modulated ultrasound,
Ultrasonics, 29, 439 (1991).
T. Tuziuti, T. Kozuka, H. Mitome, Measurement of
distribution of acoustic radiation force perpendicular to sound
beam axis, Japanese journal of applied physics, 38, 3297
(1999).


Acoustic propulsion of metallic nano-cylinders: contribution of the local vertical acceleration from micro to hyper gravity

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Acoustic propulsion of metallic nano-cylinders: contribution of the local vertical acceleration from micro to hyper gravity

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