Controllable Positive/Negative Phototaxis of Millimeter-Sized Objects with Sensing Function

Abstract

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.

Liquid Marble as an Amphibious Carrier for the Controlled Delivery and Release of Substances

“…It is also possible to induce the motion of LMs by creating a vertical deflection of the water surface around the LM (Figure c) . Furthermore, it has been shown that a surface tension gradient formed at the water surface around LMs causes LMs to move due to the resulting Marangoni flow (Figure d–f). ,− To date, several methods used to move LMs based on Marangoni flow have been developed: (1) evaporating alcohol from the inner liquid of the LM through its capsule wall (Figure d), (2) dissolving a photoresponsive reagent in the supporting liquid phase below the LM and changing the surface tension of the light-irradiated part of the liquid surface (Figure c), and (3) stabilizing the LM with particles with photothermal ability (the ability to convert light into heat) and then irradiating the LM with light (Figure e,f). − The third method is promising technology for motion manipulation because light irradiation is highly controllable in terms of timing and direction and can be easily conducted without the need for special equipment and reagents. Specific examples for motion control based on light-induced Marangoni flow are shown below.…”

“…The use of inexhaustible sunlight as a light source should facilitate the development of an environmentally adaptive and clean delivery system. More recently, an LM that moves in the opposite direction of light irradiation (positive phototaxis) has also been developed by coating the upper surfaces of LMs with light-transmitting particles (Figure f) …”

“…(f) Negative and positive phototaxis of an LM on a planar air–water surface driven by NIR laser irradiation. Reproduced with permission from ref . Copyright 2021 American Chemical Society.…”

“…More recently, an LM that moves in the opposite direction of light irradiation (positive phototaxis) has also been developed by coating the upper surfaces of LMs with lighttransmitting particles (Figure 3f). 21 The above-mentioned sliding motion of LMs on the water surface can be expressed by Newton's equation of motion (eq 2). 12 = m x t w (d /d ) 2 2…”

Nonspherical Epoxy Resin Polymer Particles Synthesized via Solvent-Free Polyaddition Reactions

“…Liquid droplets stabilized in air by adsorption of hydrophobic solid particles on the droplet surface are called liquid marbles (LMs). − Since the LMs are covered by solid particles, the liquid can be treated as a soft solid. Recently, LMs have attracted increasing interest because of their potential for use as miniature reactors, ,− sensors, − microcentrifuges, microfluidics, collision-based computing devices, and material carriers. − Various solid particles, including lycopodium, silica particles, polytetrafluoroethylene (PTFE) particles, and polystyrene particles, have been used as stabilizers for LMs. Most of the stabilizers are spherical or have ill-defined shapes, and their ill-defined aggregates are often adsorbed on the droplet surfaces as LM stabilizers.…”