Phototactic behavior of self-propelled micrometer-sized oil droplets in a surfactant solution

Article abstract of DOI:10.1039/c6cc09236e

We demonstrate the phototactic behavior of self-propelled micrometer-sized oil droplets in the presence of azobenzene-containing surfactants. These droplets respond sensitively to UV light irradiation due to a variation in the interfacial tension at the droplet surface induced by the molecular conversion of the azobenzene-containing surfactants.

Full text of DOI:10.1039/c6cc09236e

View Article Online
View Journal
ChemComm
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: S. Kaneko, K.
Asakura and T. Banno, Chem. Commun., 2017, DOI: 10.1039/C6CC09236E.
Volume 52 Number
1
4
January 2016 Pages 1–216
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
ChemComm
Chemical Communications
www.rsc.org/chemcomm
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 1359-7345
COMMUNICATION
Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
N@C2n (2n
=
68 and 80). Synthesis of the
3
N@D5h-C80
rsc.li/chemcomm

Please do not adjust margins
ChemComm
Page 1 of 4
View Article Online
DOI: 10.1039/C6CC09236E
Journal Name
COMMUNICATION
Phototactic Behavior of Self-Propelled Micrometer-Sized Oil
droplets in a Surfactant Solution
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
Sho Kaneko, Kouichi Asakura and Taisuke Banno
DOI: 10.1039/x0xx00000x
www.rsc.org/
We demonstrate the phototactic behavior of self-propelled state.
micrometer-sized oil droplets in the presence of azobenzene-
Herein, we report the development of a self-propelling
containing surfactants. These droplets respond sensitively to UV system using oil droplets that exist in a far-from-equilibrium
light irradiation due to a variation in the interfacial tension at the state and whose motion mode can be controlled using
droplet surface induced by the molecular conversion of the irradiation. The self-propelled motion of oil droplets in
azobenzene-containing surfactants.
surfactant solutions is driven by the Marangoni effect based
on heterogeneity in the interfacial tension at the droplet
2
2
Mobile objects including liquids and solids have attracted increased
attention in recent years because of their potential application as
probes or sensors for exploring hazardous areas and as carriers for
surface.
We used a photoresponsive gemini cationic
surfactant containing an azobenzene moiety in the linker
moiety (CnAzo) (Fig. 1). Recently reported studies have used
azobenzene-containing surfactants to control the interfacial
1
-6
transporting or removing compounds.
Among these objects,
2
3, 24
liquid droplets perform simple tasks and their motion is
properties of aqueous solutions
and the morphology of
In addition, an instantaneous change in
the interfacial tension of an oil-water interface induced by the
photoisomerization of azobenzene-containing gemini
surfactants was found to cause reversible demulsification and
7
‒11
2
5, 26
programmable using various external stimuli.
Thus, the liquid
self-assemblies.
droplets can mimic the taxis behavior of living organisms. Their
motion mode can be controlled by the system components and/or
1
2‒14
chemical reactions.
2
7
Despite considerable progresses, precise control over droplet
locomotion by external physical or chemical stimuli still remains a
major challenge in the application of mobile droplets. One highly
promising approach towards achieving this goal is the use of
photoresponsive materials that can change the surface properties
emulsification. Therefore, when CnAzo is added to an
emulsion system containing the surfactant N-hexadecyl-N,N,N-
trimethylammonium bromide (C16TAB), we expect controlled
motion to occur upon irradiation (phototaxis) due to the
triggering of heterogeneity in the droplet-surface interfacial
tension.
1
5‒19
of solids and both water and oil droplets.
For example, Baigl
and co-workers reported a manipulation system comprising a
millimeter-sized oil droplet on a water surface controllable by
First, we investigated the effect of surfactant composition
on the oil droplet dynamics in an emulsion (200 ꢀL) at room
photoisomerization of
a cationic surfactant containing an
temperature (23-25 °C). Details of the synthesis of CnAzo are
described in the Supplementary Information (ESI). n-
Heptyloxybenzaldehyde (HBA) was dispersed into a mixed
surfactant solution composed of 30 mM C16TAB and 10 mM
2
0
azobenzene moiety. Suzuki et al. demonstrated a phototactic
system comprising micrometer-sized underwater oil droplets that
2
1
responded to the photolysis of oil compounds. In both of these
systems, the phototaxis was considered to be induced from an
equilibrium state because the oil droplets only moved when the
emulsion system was irradiated. However, there are no reports on
the taxis of self-propelling oil droplets in a far-from-equilibrium
Fig. 1 Molecular structure of the photoresponsive surfactant
CnAzo and a non-reactive surfactant (C16TAB).
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 2 of 4
View Article Online
DOI: 10.1039/C6CC09236E
COMMUNICATION
Journal Name
C8Azo. This emulsion was packed into a 280
ꢀm-thick for the negative phototaxis of HBA droplets.
1
compartment consisting of two glass slides and a spacer. The
2
H NMR analysis of a D O solution containing 30 mM C16TAB
oil droplets therein were then observed using phase-contrast and 10 mM C8Azo was carried out at room temperature (23-25 °C)
microscopy. In the mixed solution, the HBA droplets exhibited to probe the isomerization kinetics of the azo surfactants induced
2
self-propelled motion for 4 min, and then ceased. When the by the 950 mW/cm UV irradiation. The isomerization ratio of the
2
emulsion was entirely irradiated with 950 mW/cm ultraviolet trans- to cis-isomer of C8Azo was calculated by the shifting of
(UV) light, the self-propelling droplets immediately exhibited benzene methine protons (see Fig. S1, ESI). Figure 3a shows the
negative phototaxis in the opposite direction to the light time-course of the C8Azo isomerization ratio, indicating that
source (Fig. 2a and Movie S1, ESI). The droplet trajectory in Fig. isomerization readily occurred under UV irradiation and the
2a clearly indicates that the direction of droplet movement isomerization ratio exceeded 50% within 1 min. Because the UV
immediately changed in response to the UV light irradiation. irradiation induced a temperature increase (Fig. S2, ESI), a
When the motion speed of oil droplets was relatively fast, oil temperature gradient probably formed in the oil droplets. However,
droplets moved randomly after UV irradiation. In addition, the addition of 60 °C hot water to an emulsion, where the oil
stationary droplets that had ceased self-propelled motion also droplets had ceased any motion, did not cause the droplets to
exhibited the negative phototaxis under UV irradiation and move. We thus concluded that the negative phototaxis of oil
they sustained the directional motion away from the light droplets was induced not by the formation of a temperature
source for 6 s, followed by random motion for 60 s (Fig. 2b and gradient but by the partial photoisomerization of the azo
Movie S2, ESI). The convective flow observed within the surfactants.
mobile droplets was as illustrated in Fig. 2a (see also Movies S1
Next, we measured the variation in interfacial tension
and S2, ESI), and indicated that their locomotion was caused between HBA and an aqueous surfactant solution of 30 mM
8
2
by the Marangoni effect. Similar negative phototaxis was also C16TAB and 10 mM C8Azo upon UV irradiation using the
observed in a mixed solution composed of 30 mM C16TAB and Wilhelmy vertical plate technique with a sandblasted glass
10 mM C12Azo. On the other hand, HBA oil droplets did not plate. The interfacial tension between HBA and 30 mM C16TAB
exhibit any locomotion in solutions containing only 1-10 mM aqueous solution gradually decreased under UV irradiation,
C8Azo even with UV irradiation. In a 30 mM C16TAB solution, whereas that between HBA and an aqueous solution
HBA droplets exhibited self-propelled motion for 7 min, composed of C16TAB and C8Azo increased slightly but
whereas they did not respond to the UV irradiation. These instantaneously after UV irradiation and then gradually
results clearly suggest that the azo surfactants are necessary
decreased over time (Fig. 3b). The mechanism of the interfacial
tension variation can be explained as follows. The area of the
oil-water interface occupied per molecule of the straight trans-
2
2
isomer is larger than that of the bent cis-isomer. Therefore,
the contact area between the water and oil phases increases
Fig.
2 Typical sequential micrographs of HBA droplet
locomotion in response to UV irradiation in a solution of
C16TAB (30 mM) and C8Azo (10 mM) at room temperature
(
23-25 °C). The wavy-lined and block white arrows represent
Fig. 3 (a) Time-course of the C8Azo (10 mM) isomerization ratio in a
random and directional motion, respectively. The dotted white
circles in the micrographs represent the trajectories of mobile
2
D O solution containing C16TAB (30 mM) upon UV irradiation at
room temperature (23-25 °C). (b) Variation in the interfacial tension
between HBA and a mixed aqueous solution composed of 30 mM
C16TAB and 10 mM C8Azo at at room temperature (23-25 °C). The
solid line is present to aid visualization. (c) Schematic
representation of the UV irradiation induced dynamics of surfactant
molecules at the oil-water interface.
oil droplets. Scale bar: 100 ꢀm. (a) Negative phototaxis of self-
propelling droplets induced by UV irradiation from the left-
hand side 2 s after starting observation. (b) Negative
phototaxis of stationary droplets that had ceased self-
propelled motion induced by UV irradiation from the upper
side.
2
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 4
Please dCo h ne omt Ca do j um s mt margins
View Article Online
DOI: 10.1039/C6CC09236E
Journal Name
COMMUNICATION
upon photoisomerization from trans- to cis-C8Azo, inducing the
observed slight instantaneous increase in the interfacial tension.
Because the interfacial tension activity of the cis-isomer is higher
2
7
than that of the trans-isomer, it gradually decreased to reach the
equilibrium value (Fig. 3c).
Furthermore, the relationship between the HBA droplet
dynamics and both the UV intensity and surfactant concentration
were evaluated. The UV irradiation intensity was varied between
2
6
0-950 mW/cm in solution of 30 mM C16TAB and 10 mM C8Azo.
The negative phototaxis of stationary droplets occurred
2
instantaneously when the irradiation was ≥ 60 mW/cm (Table S1,
2
ESI), while at 60 and 95 mW/cm stationary droplets exhibited
negative phototaxis 10 s and 5 s, respectively, after the initiation of
UV irradiation. This shows that photoisomerization of C8Azo takes
longer with a weaker UV light intensity (Table S1, ESI). On the other
hand, self-propelling droplets instantaneously responded to all
tested UV irradiation intensities (Table S1, ESI). The C8Azo
concentration was varied from 1-20 mM in a 30 mM C16TAB
2
solution, and 950 mW/cm UV irradiation was found to induce
negative phototaxis of both self-propelling and stationary droplets
at all C8Azo concentrations (Table S2, ESI). The irradiation time was
Fig. 4 Proposed mechanism for the negative phototaxis of self-
propelling HBA droplets induced by UV irradiation. The wavy-lined
and block white arrows represent random and directional motion,
respectively. σ is the local interfacial tension at the oil droplet
surface.
2
also tested and even 1 s of 950 mW/cm UV irradiation induced a
change in the direction of the self-propelled droplets and
locomotion of stationary droplets (Table S3, ESI). These results
indicate that the oil droplet system containing C16TAB and C8Azo is
sensitive to UV irradiation and the negative phototaxis is induced
regardless of the UV irradiation conditions and surfactant
concentration. Furthermore, we analyzed the response time upon
UV irradiation for self-propelling droplets to change direction and
stationary droplets to start the motion. No significant difference in
the response time was observed between self-propelling and
stationary droplets (Fig. S3a, ESI). In both cases, the droplet speed
immediately after UV irradiation was almost identical and did not
depend on the droplet size (Fig. S3b, ESI). This implies the
instantaneous (within 0.5 s) formation of new flow fields that
induce the negative phototaxis of oil droplets regardless of the
presence or absence of initial flow fields. In addition, several
seconds after UV irradiation a larger droplet tended to be faster,
indicating that this locomotion mechanism are proposed from
fields are formed by the adsorption of surfactant molecules on the
droplet surface from all directions (Fig. 4d). The locomotion is
sustained until the interfacial tension around the oil droplet
equilibrates, at which point the droplet slows down and becomes
stationary. When stationary droplets are irradiated with UV light,
they start the negative phototaxis because of an instantaneous
increase in the interfacial tension on the side of the droplet surface
facing the UV irradiation.
2
Furthermore, under 950 mW/cm UV irradiation, we
investigated the dynamics of n-decane, decanol, heptyloxybenzene,
undecanal and benzaldehyde oil droplets, instead of HBA, in the
mixed surfactant solution of 30 mM C16TAB and 10 mM C8Azo
(
Table S4, ESI). n-Decane, decanol and heptyloxybenzene droplets
did not exhibit self-propelled motion or respond to the UV
irradiation. Even though the oil droplets composed of undecanal
and benzaldehyde, which contain a formyl group, were not self-
propelled they exhibited negative phototaxis in response to the UV
irradiation. We then conducted multiple UV irradiations from three
different directions on the emulsion system comprising
benzaldehyde oil droplets, C8Azo, and C16TAB (Fig. S5 and Movie
S3, ESI). The following discussion of the direction of UV irradiation
and subsequent droplet movement is relative to the images in Fig.
S5. Negative phototaxis of the stationary droplets was first induced
by irradiation from the left-hand side, and a second irradiation from
the right-hand side then changed their direction. A final third
irradiation from the upper side caused the droplets to move
downwards. This sequential control over droplet motion has
potential applications in the targeted delivery of molecules or small
2
8
viewpoints of the flow fields as described by Thutupalli et al.
On the basis of these results, we have proposed a mechanism
for the sensitive negative phototaxis of self-propelling HBA droplets
upon UV irradiation (Fig. 4). The self-propelled motion of oil
droplets is due to the Marangoni effect based on an imbalance in
1
7
the interfacial tension at the droplet surface. The droplets can
take up more surfactants whilst continuing to move (Fig. 4a). When
the self-propelling oil droplets are irradiated with UV light,
photoisomerization of the azo surfactants from the trans- to cis-
isomer readily occurs mainly on the side of the droplet surface
facing the irradiation because HBA does not transmit UV light due
to its strong absorption in the 300‒400 nm UV region (Fig. S4, ESI).
Thus, the interfacial tension at this local area on the droplet surface
increases slightly but instantaneously (Fig. 4b). This is sufficient for
the formation of new flow fields that induce the negative
phototaxis of droplets (Fig. 4c). However, this controlled
locomotion becomes random within several seconds as other flow
7‒11
objects.
2
Under continuous 950 mW/cm UV irradiation in a 30 mM
C16TAB solution in both the presence and absence of C8Azo, we
found that HBA droplets self-propelled and then ceased, and after
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 4 of 4
View Article Online
DOI: 10.1039/C6CC09236E
COMMUNICATION
Journal Name
1
20 s the stationary droplets then locomoted toward the light 10 T. Ban, T. Yamada, A. Aoyama, Y. Takagi and Y. Okano, Soft
Matter, 2012, 8, 3908.
source, i.e. positive phototaxis (Fig. S6a and Movie S4, ESI). Because
initial negative phototaxis also occurred when C8Azo was present,
the locomotion mode of the oil droplets transferred from negative
to positive against the UV irradiation. Under similar UV irradiation
1
1
1 I. Lagzi, S. Soh, P. J. Wesson, K. P. Browne and B. A.
Grzybowski, J. Am. Chem. Soc., 2010, 132, 1198.
2 J. M. P. Gutierrez, T. Hinkley, J. W. Taylor, K. Yanev and L.
Cronin, Nat. Commun., 2014, 5, 5571.
that induced the photoisomerization of C8Azo, HBA was slowly but 13 T. Banno, A. Asami, N. Ueno, H. Kitahata, Y. Koyano, K.
Asakura and T. Toyota, Sci. Rep., 2016,
4 N. J. Suematsu, Y. Mori, T. Amemiya and S. Nakata, J. Phys.
Chem. Lett. 2016, , 3424.
5 J. Berna, D. A. Leigh, M. Lubomska, S. M. Mendoza, E. M.
Perez, P. Rudolf, G. Teobaldi and F. Zerbetto, Nat. Mater.,
6, 31292.
steadily oxidized to n-heptyloxybenzcarboxylic acid (HBCA) at the
minute time-scale (Fig. S6b and S6c, ESI). In addition, the interfacial
tension between HBA and aqueous C16TAB solution was almost
constant over time without the UV irradiation, while it decreased
gradually over time upon UV irradiation (Fig. S6d, ESI). It has been
reported that the surface activities in a complex system composed
of a cationic surfactant and a fatty acid are higher than those in an
1
1
7
2005,
6 K. Ichimura, S.-K. Oh and M. Nakagawa, Science, 2000, 288
624.
7 S.-K. Oh, M. Nakagawa and K. Ichimura, J. Mater. Chem.,
002, 12, 2262.
of the positive phototaxis is not completely understood, we 18 L. Florea, K. Wagner, P. Wagner, G. G. Wallace, F. Benito-
4, 704.
1
1
,
1
29
individual surfactant system. Even though the precise mechanism
2
consider it to be a result of HBCA production at the minute time-
scale mainly at the droplet-side facing the UV irradiation because
HBA droplets do not transmit UV light. This molecular conversion
induces lowering the interfacial tension around the droplets, thus
gradually forming the flow fields that cause the positive phototaxis
of the droplets.
Lopez, D. L. Officer and D. Diamond, Adv. Mater., 2014, 26
339.
9 S. Nakata, T. Miyaji, Y. Matsuda, M. Yoshii and M. Abe,
Langmuir, 2014, 30, 7353.
0 A. Diguet, R. Guillermic, N. Magome, A. Saint-Jalmes, Y.
Chen, K. Yoshikawa and D. Baigl, Angew. Chem., Int. Ed.,
2009, 48, 9281.
,
7
1
2
2
2
1 K. Suzuki and T. Sugawara, ChemPhysChem, 2016, 17, 2300.
2 C. C. Maass, C. Krüger, S. Herminghaus and C. Bahr, Annu.
Rev. Condens. Matter Phys., 2016, 7, 171.
In conclusion, we have demonstrated the phototactic behavior
of micrometer-sized oil droplets in a far-from-equilibrium state
using photoresponsive surfactants. The controlled motion of
2
3 J. Y. Shin and N. L. Abbott, Langmuir, 1999, 15, 4404.
droplets was explained by heterogeneity in the interfacial tension at 24 T. Shang, K. A. Smith and T. A. Hatton, Langmuir, 2003, 19
,
the droplet surface that was induced by the molecular conversion
of system components. The negative phototaxis was very sensitive
and exhibited directional change within 0.5 s. Such sensitivity is
highly effective for applications in the spatial arrangement of
10764.
5 S. Dante, R. Advincula, C. W. Frank and P. Stroeve, Langmuir,
999, 15, 193.
6 I. Kim, J. F. Rabolt and P.Stroeve, Colloids Surf. A, 2000, 171
67.
In addition, 27 Y. Takahashi, K. Fukuyasu, T. Horiuchi, Y. Kondo and P.
2
2
1
,
1
1
7, 18
micrometer-sized objects and also as transporters.
the locomotion mode of oil droplets in the azo surfactant system
transferred from negative to positive against the UV irradiation over
time. Therefore, our emulsion system provides living-organism-
mimetic properties, such as the adaptation to an external stimulus,
Stroeve, Langmuir, 2014, 30, 41.
8 S. Thutupalli, R. Seemann and S. Herminghaus, New J. Phys.,
2
2
2
011, 13, 73021.
9 A. Stocco, D. Carriere, M. Cottat and D. Langevin, Langmuir
010, 26, 10663.
2
3
0‒32
in this case irradiation.
30 S. Kapoor, M. Berghaus, S. Suladze, D. Prumbaum, S.
Grobelny, P. Degen, S. Raunser and R. Winter, Angew.
Chem., Int. Ed., 2014, 53, 8397.
We thank Prof. Taro Toyota (Univ. of Tokyo), Prof. Hiroyuki
Kitahata and Ms. Yuki Koyano (Chiba Univ.) for the fruitful
discussions. This work was supported by a grant-in-aid for Scientific
Research for Young Scientists (B) (No. 16K17504) from the Ministry
of Education, Culture, Sports, Science and Technology of Japan.
3
1 R. K. Kumar, R. L. Harniman, A. J. Patil and S. Mann, Chem.
Sci., 2016, , 5879.
2 M. M. Hanczyc, Life, 2014,
7
3
4, 1038.
Notes and references
1
W. F. Paxton, S. Sundarajan, T. E. Mallouk and A. Sen, Angew.
Chem., Int. Ed., 2006, 45, 5420.
2
3
4
J. Wang and W. Gao, ACS Nano, 2012,
G. Zhao and M. Pumera, Chem. Asian. J., 2012,
G. Zhao, T. H. Seah and M. Pumera, Chem. Eur. J., 2011, 17
2020.
6, 5745.
7, 1994.
,
1
5
6
7
8
9
G. Zhao, S. Sanchez, O. G. Schmidt and M. Pumera, Chem.
Commun., 2012, 48, 10090.
B. Dai, J, Wang, Z. Xiong, X. Zhan, W. Dai, C.-C. Li, S.-P. Feng
and J. Tang, Nat. Nanotechnol., 2016, 11, 1087.
M. M. Hanczyc, T. Toyota, T. Ikegami, N. Packard and T.
Sugawara, J. Am. Chem. Soc. 2007, 129, 9386.
T. H. Seah, G. Zhao and M. Pumera, ChemPlusChem, 2013, 78
395.
J. Čejková, M. Novák, F. Štěpánek and M. M. Hanczyc,
Langmuir, 2014, 30, 11937.
,
4
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins