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.
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