Self-Propulsion of Droplets via Light-Stimuli Rapid Control of Their Surface Tension

Abstract

Biology demonstrates many examples of response and adaptation to external stimuli, and here we focus on self-propulsion (motion) while presenting several self-propelling droplet systems responsive to pH gradients. We use light as the gating source to gain reversibility, avoid the formation of chemical wastes, and control the self-propulsion remotely. To achieve light-stimuli ultra-fast response, we use photoacids and photobases, capable of donating or capturing a proton, respectively, in their excited-state. We control the movement and directionality of the droplet’s self-propulsion by introducing the photo-acid/base either in bulk solution, on the surface of the droplet, or inside the droplet. We show that proton transfer between the photo-acid/base and the droplet results in a rapid change in the droplet’s surface tension, which induces the self?propulsion movement. The high versatility of our systems together with a record-breaking ultra?fast response to light makes them highly attractive for the design of various controlled cargo?carrier systems.<br>

Sol-gel transition programmed self-propulsion of chitosan hydrogel

“…Dissipative systems self-organize to various spatiotemporal structures and produce autonomous activities. 1 Active material moves on its own by consuming chemical, [2][3][4][5] electromagnetic, 6 electrical energy, [7][8][9] or light [10][11][12] . Various mechanisms such as Marangoni effect, 13,14 electroosmosis, 9 bubble ejection, 15,16 dielectrophoresis 7 have been proposed to direct the propulsion activities.…”

Switching of photocurrent polarity in electrochemical cells with light via an excited state proton transfer mechanism

“…This ability of Brønsted PAs and PBs has been used before to dynamically control dynamic processes that are dependent on the protonation or deprotonation of molecules, such as self-assembly processes, enzymatic activities, self-propulsion, and protonic transport. [22][23][24][25][26][27][28][29] It is worth mentioning that other types of molecules are also termed PAs, which are not Brønsted type, but rather a photochromic molecule that undergoes a light-induced structural change in which a proton is released, with the spiropyran-merocyanine system being the common representative of photochromic PAs.…”