Micrometer-sized
hydrophobic polyaniline (PANI) grains were synthesized
via an aqueous chemical oxidative polymerization protocol in the presence
of dopant carrying perfluoroalkyl or alkyl groups. The critical surface
tensions of the PANIs synthesized in the presence of heptadecafluorooctanesulfonic
acid and sodium dodecyl sulfate dopants were lower than that of PANI
synthesized in the absence of dopant, indicating the presence of hydrophobic
dopant on the grain surfaces. The PANI grains could adsorb to air–liquid
interfaces, and aqueous and nonaqueous liquid marbles (LMs) were successfully
fabricated using liquids with surface tensions ranging between 72.8
and 42.9 mN/m. Thermography studies confirmed that the surface temperature
of the LMs increased by near-infrared light irradiation thanks to
the photothermal property of the PANI, and the maximum temperatures
measured for nonaqueous LMs were higher than that measured for aqueous
LM. We demonstrated that transport of the LMs on a planar water surface
can be achieved via Marangoni flow generated by the near-infrared
light-induced temperature gradient. Numerical analyses indicated that
the LMs containing liquids with lower specific heat and thermal conductivity
and higher density showed longer path length per one light irradiation
shot and longer decay time. This is because generated heat could efficiently
transfer from the LMs to the water surface and larger inertial force
could work on the LMs. The LMs could also move over the solid substrate
thanks to their near-spherical shapes. Furthermore, it was also demonstrated
that the inner liquids of the LMs could be released on site by an
external stimulus.
Phototaxis,
which is the directional motion toward or away from
light, is common in nature and inspires development of artificial
light-steered active objects. Most of the light-steered objects developed
so far exhibit either positive or negative phototaxis, and there are
few examples of research on objects that exhibit both positive and
negative phototaxis. Herein, small objects showing both positive and
negative phototaxis on the water surface upon near-infrared (NIR)
light irradiation, with the direction controlled by the position of
light irradiation, are reported. The millimeter-sized tetrahedral
liquid marble containing gelled water coated by one polymer plate
with light-to-heat photothermal characteristic, which adsorbs onto
the bottom of the liquid marble, and three polymer plates with highly
transparent characteristic, which adsorb onto the upper part of the
liquid marble, is utilized as a model small object. Light irradiation
on the front side of the object induces negative phototaxis and that
on the other side induces positive phototaxis, and the motion can
be controlled to 360° arbitrary direction by precise control
of the light irradiation position. Thermographic studies confirm that
the motions are realized through Marangoni flow generated around the
liquid marble, which is induced by position-selective NIR light irradiation.
The object can move centimeter distances, and numerical analysis indicates
that average velocity and acceleration are approximately 12 mm/s and
71 mm/s2, respectively, which are independent of the direction
of motions. The generated force is estimated to be approximately 0.4
μN based on Newton’s equation. Furthermore, functional
cargo can be loaded into the inner phase of the small objects, which
can be delivered and released on demand and endows them with environmental
sensing ability.
Multimode motion of Marangoni propulsion ships on a water surface as per a near-infrared, two-wavelength selective response is achieved for the first time. The ships are rhombus-or propellershaped polyacrylamide or siloxane resin-based gels in which Nd 2 O 3 and Yb 2 O 3 nanoparticles are incorporated separately for photothermal conversion at 808 and 980 nm, respectively. The rhombus geometry is for straight locomotion, and the propeller geometry is for rotation. On/off remote control of the forward and backward locomotion of a rhombus-shaped ship and of the clockwise and counterclockwise rotations of a propeller-shaped ship via irradiation with 808 or 980 nm near-infrared light is demonstrated. The nanoparticles are incorporated into the desired locations of the gels, enabling selective local heating of the gels without focusing the light. The temperature gradient of the ships by local heating, based on a photothermal conversion, generates a Marangoni propulsion force to move the ship in the desired direction.
Solvent-free chemical
oxidative polymerizations of pyrrole and
its derivatives, namely
N
-methylpyrrole and
N
-ethylpyrrole, were conducted by mechanical mixing of monomer
and solid FeCl
3
oxidant under nitrogen atmosphere. Polymerizations
occurred at the surface of the oxidant, and optical and scanning electron
microscopy studies confirmed production of atypical grains with diameters
of a few tens of micrometers. Fourier transform infrared spectroscopy
studies indicated the presence of hydroxy and carbonyl groups which
were introduced during the polymerization due to overoxidation. The
polymer grains were doped with chloride ions, and the chloride ion
dopant could be removed by dedoping using an aqueous solution of sodium
hydroxide, which was confirmed by elemental microanalysis and X-ray
photoelectron spectroscopy studies. Water contact angle measurements
confirmed that the larger the alkyl group on the nitrogen of pyrrole
ring the higher the hydrophobicity and that the contact angles increased
after dedoping in all cases. The grains before and after dedoping
exhibited photothermal properties: the near-infrared laser irradiation
induced a rapid temperature increase to greater than 430 °C.
Furthermore, dedoped poly(
N
-ethylpyrrole) grains
adsorbed to the air–water interface and could work as an effective
liquid marble stabilizer. The resulting liquid marble could move on
a planar water surface due to near-infrared laser-induced Marangoni
flow and could disintegrate by exposure to acid vapor via redoping
of the poly(
N
-ethylpyrrole) grains.
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