ON–OFF Control of Marangoni Self‐propulsion via A Supra‐amphiphile Fuel and Switch

Abstract

Marangoni self‐propulsion refers to motions of liquid or solids 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.

Marangoni Droplets of Dextran in PEG Solution and Its Motile Change Due to Coil–Globule Transition of Coexisting DNA

“…Ces et al reported that v / r follows as (similar equations are shown in refs and − in ref ) v r = ∂ C p ∂ x ∂ γ ∂ C p 1 2 η o + 3 η i where C p is the PEG concentration outside the droplet, x is the position of the droplet center of gravity, γ is the interfacial tension between the dextran-rich phase and the PEG rich phase, and η o (or η i ) is the solution viscosity outside (or inside) the droplet. In our experimental system, Reynolds number Re is estimated to be ≪1 (for example, Re = ρ vr /η o ∼ 10 –6 by using ρ = 1.01 × 10 3 kg/m 3 , v = 1 μm/s, r = 10 μm, η o = 10 mPa s) so that the contributions of Marangoni convection and drag forces to the velocity should balance immediately.…”

“…This alternation of droplet motile velocity via DNA conformational change will contribute to applied research using droplets as soft robots. 44 In addition, combining this system with Marangoni convections driven by chemical reactions, 45−47 concentration gradients of surfactants, 48,49 and local heating, 50,51 more complex and longterm movements of motile droplets will be expected. Incorporation of various biopolymers should result in shearinduced segregation 52 and also the alignments of stiff polymers such as collagen fibrils by the flow.…”