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Cite this: RSC Adv., 2019, 9, 40523

Directional and velocity control of active droplets
using a rigid-frame†
Masato Yamada,*a Hiroki Shigemune,b

Shingo Maedab and Hideyuki Sawadaa

This paper introduces a novel directional control method of self-propelled oil droplets. Oil droplets
locomote spontaneously with surfactant action. This self-propulsion is caused by Marangoni convection
within the oil droplet due to diﬀerences in the surfactant concentration at the droplet surface. Recent
studies have reported that self-propelled oil droplets change their locomotion style depending on theirshapes. We conﬁrm that spherical oil droplets move randomly, including straight motion, bending
motion, and rotation. In particular, we discover that boomerang-shaped oil droplets exhibit only straightmotion. In this study, we introduce an exoskeleton for the directional and velocity control of oil droplets.
A droplet shaped as a boomerang by an exoskeleton locomotes in the direction from a concave region
Received 25th September 2019
Accepted 28th November 2019to a convex region. Through experimental studies, we found that the stability of the velocity andlocomotion direction depended on the boomerang shape. Self-propelled oil droplets with exoskeletons

DOI: 10.1039/c9ra07789h

were then applied to a transporting robot driven only by the energy obtained from chemical reactions.

rsc.li/rsc-advances

We demonstrate the robot pushes and transports an object ﬂoating on water.

Introduction
Living things locomote by utilizing energy derived from chemical reactions and by employing unique mechanisms
combining hard and so materials. For example, humans
actuate muscles using chemical energy obtained from thedecomposing reaction of adenosine triphosphate (ATP)1–5 andrealize dynamic and dexterous so muscle movement restrictedby the hard skeleton and tendons.6–10 Recently, the use ofchemical reactions in locomotion has been extended torobotics.11,12 In particular, Maeda et al. developed a self-walking
gel robot driven by an oscillatory Belousov–Zhabotinsky (BZ)
reaction,11 while Wehner et al. developed an octopus-like,autonomous robot based on an oscillating microuidic
network actuated by pressure from decomposing reaction ofhydrogen peroxide.12 These electronics-less robots have highautonomy because the materials and reactions in the robot act
as both the energy source and the controller. However, themobility of these robots is still severely limited. For example, themobility of the self-walking gel robot depends on its external
environment, while the octopus like robot is xed on a oor.
We have focused on the mechanisms combining hard andso materials that are unique to biological organisms,13–16 andaDepartment of Applied Physics, School of Advanced Science and Engineering, Waseda
University, 3-4-1 Okubo, Shinjuku-ku, Tokyo, 169-8555, Japan. E-mail: yamada@sawada.phys.waseda.ac.jpbDepartment of Engineering Science and Mechanics, School of Engineering, Shibaura
Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo, 135-8548, Japan† Electronic supplementary
10.1039/c9ra07789h

information(ESI)

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DOI:

have developed a new control method by combining chemical
reactions with these mechanisms. In this research, we examine
self-propelled oil droplets,17–30 and apply an exoskeleton toa self-propelled droplet for the controllable droplet locomotion.
Additionally, we exploit the controllability of the exoskeleton todevelop a milli-size transporter on water. In the proposedsystem, the oil droplets fulll the function of the actuator
together with the exoskeleton.
Self-propelled oil droplets locomote in water due to the eﬀectof solutal Marangoni convection.31–34 Fig. 1 shows the propulsion mechanism for a droplet in water, together with thechemical formula of oleic acid. As shown in the formula, theoleic acid is a surfactant, which has a hydrophilic group anda hydrophobic group. When an oil droplet is placed into an
aqueous surfactant solution, the surfactant adsorbs to the oil
droplet. A gradient in the concentration of the surfactant isgenerated at the interface between the droplet and the aqueous
solution. Diﬀerences in the concentration gradient lead todiﬀerences in the surface tension of the droplet, which causes
Marangoni convection to occur inside the droplet body. Then,an anhydride of the surfactant, which exists inside the oil
droplet, is hydrolyzed at the interface. The adsorption anddesorption of oleic acid is locally active in the surface, since thedroplet is locomoting in the direction from the concave region
to the convex region. The adsorption of oleic acid is active at the
tip of the convex region of the self-propelled oil droplet, on theother hand the desorption of oleic acid is active at the tip of theconcave region, by considering the moving direction. As
a result, the greater gradient of the concentration of oleic acidoccurs at the boundary of the convex region in the boomerang-

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Paperexoskeleton to the so active matter. The hard exoskeleton
alters the concentration gradient of the surfactant at the interface by providing boundary conditions for the oil droplet. We
further exploit the proposed chemical robot by utilizing theactive matter to transport objects oating on water.

Preliminary experiment
Random motion of self-propelled oil droplet

Fig. 1 Self-propulsion mechanism and deformation of oil droplets.
The surfactant adheres to the surface of the oil droplet. Marangoni
convection occurrs internally due to gradients of surfactant concentration (the darker red areas have higher surface tension). Finally, thedroplets transform into the boomerang shape and performed stablestraight locomotion.

shaped oil droplets, and a gentle concentration gradient occurs
in the concave regions (please refer to Fig. S1†). Therefore, thisadsorption and desorption of oleic acid maintain the convection inside the droplets. Finally, stable ow occurs in a certain
direction at the droplet–aqueous solution interface, and the oil
droplet is propelled because of this ow.17 We have conrmed
this phenomenon by a simple experiment for visualizing theinternal ow using small colloidal particles. Since the internal
ow in a round-shaped oil droplet initially occurs randomly, thepropulsion direction is stochastic. In addition, we investigated
the induction time until the oil droplet starts to move underdiﬀerent basic conditions to support the mechanism. Table 1
summarizes the measurement results of the induction time inthree basic conditions.
In our previous research, motion control was developed fora self-propelled oil droplet by utilizing ow channels.35 The
locomotion of the droplet was controlled by changing thecourse of the ow channels, but the droplets propelledrandomly in open systems. In this study, we propose a method
for controlling the motion of oil droplets by tting a hard

As described above, the direction of internal ow in the droplet
will occur randomly; thus, the direction of the propulsion will
also be random in some cases. We conducted a preliminary
experiment to conrm the randomness in the locomotion ofself-propelled oil droplets using a simple experimental set up
with both oil and aqueous phases. Oleic anhydride and nitrobenzene were mixed to make the oil phase, and the aqueous
phase was prepared by mixing oleic acid (surfactant) with anaqueous solution adjusted to pH 12 with sodium hydroxide. A
7 cm diameter dish was lled with 3.5 mL of the preparedaqueous solution. The oil was dropped in the center of the dishwith a micropipette in volumes ranging from 1 mL to 10 mL in 1
mL increments. The experiment was repeated seven times foreach volume. The movement of the droplet was recorded forapproximately 50 s, and we used movie analysis soware(Kinovea) to measure the movement of the oil droplet. Fig. 2(a)and (b) show the self-propulsion of the 2 mL and 8 mL oil
droplets. Note that the 2 mL droplet did not split (Fig. 2(a));
however, the 8 mL droplet split into three separate droplets, andeach droplet locomoted in diﬀerent directions (Fig. 2(b)). As
expected, the oil droplets stochastically repeated various
movements such as straight locomotion, bending motion,
rotation, stop, and division. The movements ceased when theanhydride of the surfactant inside the oil droplet wascompletely hydrolyzed.
Locomotion of self-propelled oil droplet without exoskeleton
To examine the fundamental behavior of the droplets, we
further analyzed the movement and velocity of self-propelled
droplets in the experiment described above. We extracted theaverage velocity of the droplet during the rst 3 seconds ofstable self-propulsion, as shown in Fig. 3. In the case of an
example shown in Fig. 3, the velocity of the poured oil droplet

The relationship among three basic conditions of aqueous
phase about induction time

Table 1

pH

Induction time [s]

11.0
11.5
12.0

7.6
6.2
3.7

40524 | RSC Adv., 2019, 9, 40523–40530

Fig. 2 Locomotion of (a) 2 mL and (b) 8 mL self-propelled droplets
without rigid-frame. Large amount of oil droplets break up into small
pieces.

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droplet demonstrated a velocity of 2.15 mm s1. The velocity ofthe oil droplets decreased as the oil droplet volume increased.
An oil droplet with large volume has a large surface area, andthe driving force becomes greater. Simultaneously, the oil
droplet spreads in the direction that is perpendicular to thelocomoting direction as the volume increases, which results in
the increase of viscous resistance. Since the inuence of viscous
resistance becomes large, the velocity of the oil droplet
decreases as the volume increases. The eﬀects of the volume
and the viscous resistance are considered to works as counterbalance with each other.
Fig. 3 The graph shows the velocity of a self-propelled droplet. We
selected the stable period for 3 seconds while locomoting to determine the average velocity.

converged aer approximately 4 seconds. This is because it
takes time for the internal convection of the oil droplets tostabilize aer the ow eld is generated inside the oil droplets.
Since the time to stabilize the locomotion varies depending on
the samples, the time to start extracting the velocity is changedin the experiments.
Fig. 4 shows a comparison of the droplet average velocity andsplits. The 1 mL and 2 mL oil droplets did not split in the aqueous
solution. The 3 mL to 8 mL oil droplets split into smaller dropletsin several attempts, and the 9 mL and 10 mL oil droplets split in
all attempts. To conrm this behavior, the 10 mL oil droplet wastested a total of 24 times, and the oil droplets split in all theattempts (ESI Table S1† summarizes these data). When thedroplet was split we selected the largest fraction from thedivided droplets for the analysis.
The 1 mL droplet demonstrated an average velocity of
3.76 mm s1 over seven attempts, whereas the largest 10 mL oil

Fig. 4 Comparison of the average velocity for self-propelled oil
droplets of diﬀerent volumes (1–10 mL). We conducted 7 experiments
for each volume. When the oil droplets split, the trials were excluded
from the velocity data. The numbers in the circles indicate the numberof times the oil droplets split in 7 experiments. The 1 and 2 mL droplets
did not split, some of droplets between 3 and 8 mL split, and all of the 9and 10 mL droplets split.

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Proposed method
Directional control mechanism of self-propelled oil droplet
When the oil droplet showed straight motion, we observed
a boomerang shape, and the direction of motion was parallel tothe line from the convex region to the concave region, as shownin Fig. 1. This fact has been widely observed and simulated
numerically.36–41 Nagai et al. proposed that the concentration
gradients of pentanol (alcohol) occur at interfaces between
alcohol droplets and the low pentanol concentration aqueous
phase.41 The concentration gradient of pentanol is diﬀerent in
the convex and concave regions of the boomerang shape. These
diﬀerences in concentration gradients lead to diﬀerences ininterfacial tension. Thus, the droplets locomote straightly in the
direction from the concave to convex region while maintaining
their boomerang shape. Referring to the interfacial tension
diﬀerence between the convex region and the concave region
theorized by Nagai et al., we extend the theory to the locomotionof a pentanol oil droplet by considering the similar interfacial
tension diﬀerence.
In the oil droplet utilized in our experimental environment,
oleic acid (surfactant) is generated from oleic anhydride at theoil–water interface by a hydrolysis reaction. Oleic acid (surfactant) is amphiphilic similar to pentanol (alcohol), and theconcentration of oleic anhydride inside the droplets is 50 times
higher than that of oleic acid in the aqueous phase in our
droplet system. Therefore, we assume that the dynamics of oleic
acid (surfactant) is the identical to that of pentanol (alcohol),and the concentration gradients of oleic acid occur at the oil–water interfaces similar to the case of pentanol. Fig. S1† shows
the concentration gradients of the surfactant at the oil–waterinterfaces in the oil droplet system. The absolute value of theconcentration gradients is higher at the convex region than thatat the concave region in boomerang shape of the oil droplet.41,42
By considering the concentration gradients, the interfacial
tension diﬀerence between the convex and concave regions in
the boomerang shape Dg is given as


dc
dc
Dg ¼ gconvex  gconcave ¼ a | |convex |  | |concave |
(1)
dr
dr
where gconvex is the interfacial tension at the convex region,
gconcave is interfacial tension at the concave region, a isconstant, c is the surfactant (oleic acid) concentration, r is therelative position. Because the absolute value of the concentration gradients is higher at the convex region than that at the

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concave region in the boomerang shape of the oil droplet


dc
dc
| |convex | . | |concave | ; the interfacial tension diﬀerence Dg
drdris positive (Dg > 0). This interfacial tension diﬀerence generates
a pair of convection as shown in Fig. 1. Then, oil droplets
continue to locomote in the direction from the concave toconvex region by maintaining their boomerang shape. Herein,
we utilize this theory of locomotion to propose a novel method
for installing a boomerang-shaped exoskeleton on selfpropelled oil droplets in order to ensure straight motion.
Directional control of the self-propelled oil droplet can be
maintained by using the exoskeleton to preserve the droplet's
shape. The exoskeleton further prevents large volume oil
droplets from dividing into smaller droplets. Larger oil droplet
volumes exert larger driving forces that are advantageous in
robotics applications.
Locomotion of self-propelled oil droplet with exoskeleton
Next, we conducted an experiment with exoskeletons (ESI Movie
1†). To determine the optimal boomerang shape for theexoskeletons, we prepared 10 exoskeletons by cutting diﬀerent
internal shapes using 0.1 mm-thick overhead projector (OHP,
polyester) lm (ESI Fig. S2† shows the schematic diagram ofthese exoskeletons, and Table S2† summarizes the details of the
10 types of exoskeletons). In ve exoskeletons (I–V), the convex
regions of the boomerang shape have diﬀerent curvatures(Fig. 5(a)). In the other ve exoskeletons (VI–X), the concave
regions of boomerang shape have diﬀerent protrusions(Fig. 5(b)). The size was adjusted so that the area of theexoskeleton was xed at 37.7 mm2. The outer shape of all
exoskeletons was made circular to ensure that the viscous
resistance was uniform for every exoskeleton.
Similarly, to the experiment without exoskeletons, we placed
3.5 mL of the aqueous solution in a 7 cm dish. We oated theFig. 5 (a) Exoskeletons with diﬀerent convex curvatures. (b) Exoskeletons with diﬀerent concave protrusions. Exoskeleton II andexoskeleton VI have the same boomerang shape. (c) A self-propelled
oil droplet ﬁtted with the exoskeleton locomotes in the direction fromthe concave region toward the convex region. The upper right ﬁgureshows a schematic cross-sectional diagram of the experiment.

40526 | RSC Adv., 2019, 9, 40523–40530

Paper

exoskeleton on the aqueous solution, and 10 mL of the oil was
dropped inside the exoskeleton. We observed the oil droplet
velocity over seven attempts, and only the trials satisfying thefollowing conditions were measured and recorded: (a) selfpropelled oil droplets completely lled the interior of theexoskeleton, and (b) oil droplets locomoted from the center of
the Petri dish to the wall. Fig. 5(c) shows an example locomotion
trajectory of the droplet with exoskeleton X, which has a convex
region with 0.0306 mm1 curvature and a concave region with
0.302 mm protrusion (see ESI Fig. S3† for exoskeleton details).
As described in eqn (1), surface tension diﬀerences occur
between the convex and concave regions in the boomerangshaped exoskeleton. Therefore, self-propelled droplets withexoskeletons should locomote in the direction from the concave
region to the convex region. The 10 mL oil droplet withexoskeleton X showed an average velocity of 2.69 mm s1 overseven attempts. In other words, the 10 mL droplets with an
exoskeleton were 0.54 mm s1 faster than those without.
Moreover, splitting of the droplet was suppressed, which has
the advantage of producing higher power for utilization asa robot. Thus, the internal shape of the exoskeleton preserves
the shape of the droplet and reduces uidic resistance.

Result and discussion
Evaluation on velocity
In the experiments, we analyzed the velocity change of selfpropelled oil droplets tted with exoskeletons. As a result ofapplying conditions (a) and (b), exoskeleton I and exoskeleton V
were excluded from the results because the oil droplets did notll the exoskeleton due to their surface tension. Fig. 6(a) and (b)show the average velocity of oil droplets tted with eightremaining exoskeletons. These exoskeletons were classied into
two groups: one group of three types having diﬀerent curvaturesof the convex region (Fig. 6(a)), and the other group of ve types
having diﬀerent protrusions of the concave region (Fig. 6(b)).
The error bars of the graph show the standard deviation overseven experiments. Droplets tted with exoskeletons IV and X
demonstrated the largest velocity in each category.
We compared exoskeleton IV and exoskeleton X to further
investigate the exoskeleton design. Fig. 7(a) shows the velocityof the droplets tted with exoskeleton IV, and the averagevelocity was 2.61 mm s1, which was the largest velocity amongexoskeletons I–V. Fig. 7(b) shows the velocity of the dropletstted with exoskeleton X, and the average velocity was
2.69 mm s1, which was the highest velocity among exoskeletons VI–X. As shown in Fig. 7(a) and (b), the variation in thevelocity was quite diﬀerent between exoskeletons IV and X.
The standard deviation of the average velocity was
0.193 mm s1 and 0.357 mm s1 for exoskeletons IV and X,
respectively. The stability of exoskeleton IV was also clearlyshown in the obtained graphs.
We developed a mathematical model to calculate the driving
force. An object in a viscous uid follows the following equationof motion:

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with the exoskeletons (I–X) through 7 experiments. (a) I–V. (b) VI–X. Oil
droplets in exoskeletons I and V did not ﬁll the interiors of theexoskeletons. Hence, these data were excluded from the result. The
error bars in the graph represent the standard deviation in 7
experiments.

mdv¼ F  gvdt

(3)

where h, a, hvi are the viscosity of the uid, the radius of the oil
droplet, and the average velocity of the object, respectively. The
oil droplets deformed by exoskeletons have a radius of 3 mm,
and the viscosity of the aqueous solution was 0.92 mPa s. Thus,
the driving force of an oil droplet deformed by exoskeleton IV
was found to be 136 nN while that of exoskeleton X is found to
be 140 nN.
Evaluation on directional control
We further analyze the stability in angular motion for exoskeletons IV and X. Self-propelled oil droplets without the exoskeletons locomote randomly, and do not show stable straight
locomotion. Therefore, we do not compare the stability of theangular change during the locomotion of the self-propelled oil

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exoskeletons IV and X. The velocity stability diﬀered for these
exoskeletons. The average velocity and the standard deviations were2.61 mm s1  0.193 mm s1 in case of exoskeleton IV (a), and 2.69 mms1  0.357 mm s1 in case of exoskeleton X (b). The oil droplets withthe exoskeleton IV have higher velocity stability than the oil dropletswith the exoskeleton X.

(2)

where m is the mass of the oil droplet, F is the driving force ofthe self-propelled oil droplet, and g is the friction coeﬃcient.
When the oil droplet was assumed to have a spherical shapeand zero acceleration, the driving force F can be derived by eqn(2):
F ¼ 6phahvi

Fig. 7 All 7 results showing the velocity change of droplets ﬁtted withdroplet without an exoskeleton. The calculation of the angular
direction of a droplet with an exoskeleton is shown in Fig. 8(a).
Using the same procedure as that for measuring the velocitychange, we focused on 3 seconds of stable droplet motion andcalculated the change in the angle between each image frame.
Fig. 9(a) and (b) show the direction of locomotion of the two
self-propelled oil droplets tted with exoskeletons IV and X,
respectively. We found a remarkable diﬀerence in the traveling
direction aer 3 seconds. The standard deviation in the traveling direction aer 3 seconds was 24.6 for exoskeleton IV and
13.2 for exoskeleton X. From these results, the oil droplet tted
with exoskeleton X showed more stable directional motion thanexoskeleton IV.
Our experimental verication of the stability of exoskeletons
IV and X showed higher velocity stability in exoskeleton IV andhigher directional stability in exoskeleton X. Exoskeleton IV hasa more circular interior shape than exoskeleton X. If the shapeinside the exoskeleton is close to a circle (spherical), the oil
droplet tends to have more random behavior. However, theresistance from the aqueous solution during self-propulsion
tends to be constant from all directions for more sphericalshapes. Therefore, the oil droplets tted with exoskeleton IV

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Fig. 8 The method for analyzing direction of locomotion. The oil
droplet position was determined for each moving image frame, andthe change in the angle between frames was derived. The value of thedirection of locomotion was deﬁned to be positive if the angle
changed in the clockwise direction and negative if the direction wascounterclockwise.

have small variations in average velocity, and those tted withexoskeleton X have small variations in the traveling direction (asdescribed in ESI Fig. S4†).

Demonstration as a transporter
We then utilized the droplets with exoskeletons to demonstratea waterborne transport robot driven by chemical reaction. We
designed a beetle-shaped exoskeleton to push an object oating
on water. In order to transport an object precisely, the directional control is more important than velocity. In addition, thediﬀerences of driving force are small among diﬀerent shapes ofthe boomerang shape. Therefore, we developed a beetle-shaped
transport robot based on exoskeleton X and demonstrated anexperimental drug delivery system using the exoskeleton. We
used colored vegetable oil to represent object A. The waterborne
transport robot successfully transported the object 6.26 mm in
10 s at an average velocity of 0.61 mm s1, as shown in Fig. 10(a)and (b) (ESI Movie 2†). The biocompatibility of the proposed oil
droplets is low, however we consider that the biocompatibility
can be improved by wrapping oil droplets with a gel lm.22

Fig. 9 Direction of locomotion of the self-propelled oil droplets ﬁtted
with exoskeleton IV (a) and X (b) over 3 s, respectively. The direction of
locomotion was the summation of angular changes with each moving
frame.

Oil droplet motion
To observe the oil droplet motion, 3.5 mL the aqueous phasesolution was poured into a Petri dish with a 7 cm diameter

Experimental setup
Materials
The oil phase was prepared as follows. First, 141 mL oleic
anhydride (Sigma-Aldrich) was mixed with 252 mL nitrobenzene(Kanto Chemical) using a magnetic stirrer (AS ONE, RS-6AN).
We used between 1–10 mL oil in each experiment.
The aqueous phase was prepared as follows. First, 100 mL ofpure water was basied to pH 12 using sodium hydroxide(Kanto Chemical). Next, 317 mL oleic acid (Wako) was mixed
into the aqueous solution.

40528 | RSC Adv., 2019, 9, 40523–40530

Fig. 10 (a) Waterborne transport robot. The robot (beetle-shaped
exoskeleton + self-propelled droplet) carried object A (red-dyed
vegetable oil) 6.26 mm in 10 s. (b) Displacement and velocity of object
A with respect to time. Object A was carried at an average velocity of
0.61 mm s1 by the waterborne transport robot.

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(SANSHO, 75  21) using a micropipette (TGK, FPN-1000). Oil
droplets were dropped onto the Petri dish from directly above
using a micropipette (TGK, FPN-20).
Preparing the exoskeleton
The shapes of the exoskeletons were designed with illustrator(Adobe Creative Cloud). The OHP with the 0.1 mm thickness
lm was cut with a cutting plotter (GRAPHTEC, T407A378426)
and double bonded with superglue (ALTECO, 1712F).
Exoskeleton experiments
The exoskeleton oats on the prepared aqueous solution in thecenter of the Petri dish (SANSHO, 75  21). For each experiment, 10 mL of the prepared oil was dropped inside theexoskeleton from directly above with the micropipette (TGK,
FPN-1000). The droplets were measured using a single-lensreex camera (CANON, EOS 70D) and analyzed by Kinovea.
(Temperature: 299 K, humidity: 65%.) The viscosity of theaqueous solution was measured with a viscometer (AND, SV-10).
Waterborne transport robot evaluation
Object A was prepared as follows. Vegetable oil (Nisshin) was
dyed using red ink (Teranishi Chemical Industry). The beetleshaped exoskeleton was prepared as follows. The OHP lmwith the 0.1 mm thickness was cut into a beetle shape and thendouble bonded with superglue. The boomerang shape wasformed based on exoskeleton X. The tip was dyed with purple
ink (ZEBRA) to make the transport clear. For the transport
experiment, 10 mL colored vegetable oil was oated at the centerof a Petri dish, and the beetle-shaped exoskeleton was oatedon the aqueous phase. Then, a 10 mL self-propelled oil droplet
was dropped on the boomerang region, and the transportation
experiment was executed. The waterborne object transportation
was measured with a video camera (SONY, HDR-CX675) andanalyzed by Kinovea.

Conclusions
We have realized the novel directional control of a selfpropelled oil droplet using an exoskeleton. The exoskeleton
was utilized to change the shape of the self-propelled oil droplets in order to produce certain motion behavior. Oil droplets
deformed into the boomerang shape locomoted rectilinearly inthe direction from the concave region to the convex region.
Finally, oil droplets with a more circular boomerang shape hadsmaller directional stability and greater stability in velocity. We
further demonstrated the use of self-propelled oil droplets tted
with an exoskeleton as waterborne transport robots. Because
the locomotion of the oil droplet is based on the internalconvection sustained by chemical reactions, we do not have tosupply external energy, and this robot design can contribute to
the development of chemical robotics. The demonstrated
robots can operate in severe conditions such as that caused bya high pH aqueous solution, as the hydrolysis of oleic anhydrideis active under high pH. Self-propelled oil droplets withexoskeletons also have similar benets to the locomotion style

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RSC Advancesof living things that combine the mechanisms of so and hardmaterials. If the exoskeleton is changed to deformable material,
the locomotion direction of the self-propelled droplet could bemanipulated and controlled more freely. In these ways, dropletswith exoskeletons combine chemical robotics with biologicalmechanisms, which could lead to the creation of robots withmore subtle and dexterous movements specic to living things.

Conﬂicts of interest
There are no conicts to declare.

Acknowledgements
M. Y. performed all experiments and wrote the manuscript. H.
S. setup the experiment and wrote the manuscript. S. M. and H.
S supervised the project. All authors discussed the result. This
paper is a part of the outcome of research performed undera Waseda University Grant for Special Research Projects (Project
number: 2019C-249). We all thank JSPS for their support underGrants-in-Aid for Scientic Research on Innovative Areas(Research in a proposed research area) 18H05473, and underGrant-in-Aid for Challenging Exploratory Research 19K21950.

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