ON-OFF Control of Marangoni Self-propulsion via A Supra-amphiphile Fuel and Switch.

Marangoni self-propulsion refers to motion of liquid or solid driven by a surface tension gradient, and has applications in soft robots/devices, cargo delivery, self-assembly etc. However, two problems remain to be addressed for motion control (e.g., ON-OFF) with conventional surfactants as Marangoni fuel: (1) limited motion lifetime due to saturated interfacial adsorption of surfactants; (2) in- situ motion stop is difficult once Marangoni flows are triggered. Instead of covalent surfactants, supra-amphiphiles with hydrophilic and hydrophobic parts linked noncovalently, hold promise to solve these problems owing to its dynamic and reversible surface activity responsively. Here, we propose a new concept of 'supra-amphiphile fuel and switch' based on the facile synthesis of disodium-4-azobenzene-amino-1,3-benzenedisulfonate (DABS) linked by a Schiff base, which has amphiphilicity for self-propulsion, hydrolyzes timely to avoid saturated adsorption, and provides pH-responsive control over ON-OFF motion. The self-propulsion lifetime is extended by 50-fold with DABS and motion control is achieved. The mechanism is revealed with coupled interface chemistry involving two competitive processes of interfacial adsorption and hydrolysis of DABS based on both experiments and simulation. The concept of 'supra-amphiphile fuel and switch' provides an active solution to prolong and control Marangoni self-propulsive devices for the advance of intelligent material systems.

A Multi-engine Marangoni Rotor with Controlled Motion for Mini-Generator Application.

Marangoni rotors are smart devices that are capable of self-propulsive motions based on the Marangoni effect, namely interfacial flows caused by a gradient of surface tension. Owing to the features of untethered motions and coupled complexity with fluid, these Marangoni devices are attractive for both theoretical study and applications in biomimicking, cargo delivery, energy conversion, etc. However, the controllability of Marangoni motions dependent on concentration gradients remains to be improved, including the motion lifetime, direction, and trajectories. The challenge lies in the flexible loading and adjustments of surfactant fuels. Herein, we design a multi-engine device in a six-arm shape with multiple fuel positions allowing for motion control and propose a strategy of diluting the surfactant fuel to prolong the motion lifetime. The resulting motion lifetime has been extended from 140 to 360 s by 143% compared with conventional surfactant fuels. The motion trajectories could be facilely adjusted by changing both the fuel number and positions, leading to diverse rotation patterns. By integrating with a coil and a magnet, we obtained a system of mini-generators based on the Marangoni rotor. Compared with the single-engine case, the output of the multi-engine rotor was increased by 2 magnitudes owing to increased kinetic energy. The design of the above Marangoni rotor has addressed the problems of concentration-gradient-driven Marangoni devices and enriched their applications in harvesting energy from the environment.

A Hydrolyzable Supra-amphiphile as a Marangoni Self-Propulsion Fuel for Efficient Macroscopic Supramolecular Self-Assembly.

Self-assembly of µm-to-mm components is important for achieving all-scale ordering with requirements of extra energy for motion and interaction of components. Marangoni flows caused by surfactants on water provide appropriate energy but have limited lifetimes because of the inevitable interfacial aggregation and difficult decomposition of aggregated covalent surfactants that inactivate Marangoni effects. Here we have synthesized a supra-amphiphile Marangoni "fuel"-sodium-4-(benzylideneamino) benzenesulfonate (SBBS)-that can be hydrolyzed in a timely manner to a species without surface activity to extend the motion time by 10-fold. The motion was optimized at pH=2 by a fine equilibrium between the releasing and removal of interfacial SBBS, leading to the self-assembly of millimeter-scaled ordered dimers. The underlying mechanism was interpreted by motion analyses and simulation. This strategy provides an active solution to self-assembly at the µm-to-mm scale, as well as interactive ideas between miniaturized chemical robots.

Self-sorting in macroscopic supramolecular self-assembly via additive effects of capillary and magnetic forces.

Supramolecular self-assembly of µm-to-mm sized components is essential to construct complex supramolecular systems. However, the selective assembly to form designated structures at this length scale is challenging because the short-ranged molecular recognition could hardly direct the assembly of macroscopic components. Here we demonstrate a self-sorting mechanism to automatically identify the surface chemistry of µm-to-mm components (A: polycations; B: polyanions) based on the A-B attraction and the A-A repulsion, which is realized by the additivity and the competence between long-ranged magnetic/capillary forces, respectively. Mechanistic studies of the correlation between the magnetic/capillary forces and the interactive distance have revealed the energy landscape of each assembly pattern to support the self-sorting results. By applying this mechanism, the assembly yield of ABA trimers has been increased from 30%~40% under conventional conditions to 100% with self-sorting. Moreover, we have demonstrated rapid and spontaneous self-assembly of advanced chain-like structures with alternate surface chemistry.

A Light-Powered Triboelectric Nanogenerator Based on the Photothermal Marangoni Effect.

The photothermal Marangoni effect enables direct light-to-work conversion, which is significant for realizing the self-propulsion of objects in a noncontact, controllable, and continuous manner. Many promising applications have been demonstrated in micro- and nanomachines, light-driven actuators, cargo transport, and gear transmission. Currently, the related studies about photothermal Marangoni effect-induced self-propulsion, especially rotational motions, remain focused on developing the novel photothermal materials, the structural designs, and the controllable self-propulsion modes. However, extending the related research from the laboratory practice to practical application remains a challenge. Herein, we combined the photothermal Marangoni effect-induced self-propulsion with the triboelectric nanogenerator technology for sunlight intensity determination. Photothermal black silicon, superhydrophobic copper foam with drag-reducing property, and triboelectric polytetrafluoroethylene film were integrated to fabricate a triboelectric nanogenerator. The photothermal-Marangoni-driven triboelectric nanogenerator (PMD-TENG) utilizes the photothermal Marangoni effect-induced self-propulsion to realize the relative motion between the triboelectric layer and the electrode, converting light into electrical signals, with a peak value of 2.35 V. The period of the output electrical signal has an excellent linear relationship with the light intensity. The accessible electrical signal generation strategy proposed here provides a new application for the photothermal Marangoni effect, which could further inspire the practical applications of the self-powered system based on the photothermal Marangoni effect, such as intelligent farming.

Parallel and Precise Macroscopic Supramolecular Assembly through Prolonged Marangoni Motion.

Macroscopic supramolecular assembly (MSA) is a rising concept in supramolecular science, in which building blocks with sizes exceeding 10 µm self-assemble into larger structures. MSA faces the challenge of developing appropriate self-propulsion strategies to improve the motility of the macroscopic building blocks. Although the Marangoni effect is an ideal driving force with random motion paths, excessive aggregation of the surfactant and fast decay of motion remain challenging problems. Hence, a molecular interference strategy to drive the self-assembly over longer times by finely controlling the interfacial adsorption of surfactants using dynamic equilibria is proposed. Surfactant depletion through molecular recognition in the solution to oppose fast interfacial aggregation efficiently facilitates macroscopic motion and assembly. The resulting motility lifetime is extended remarkably from 120 s to 2200 s; with the improved kinetic energy, the assembly probability increases from 20 % to 100 %.