Such systems can mimic the motion of living cells as similar stimuli are present in nature, and can also result in our fundamental understanding of early origin of life events.The movement of droplets is linked with nonuniform surface tension (ST), resulting in fluid flow, known as the Marangoni effect. [18][19][20] This effect is based on an asymmetric exposure of the droplet surface to a chemical cue, i.e., chemotaxis, whereas ion or pH gradients are most common. [21][22][23] The asymmetry in the droplet/solution interface creates a nonuniform change in the ST. Light is a convenient tool to control the asymmetry and ST of droplets, and accordingly the motion of droplets. To use light as stimuli, there is a need for photochromic molecules next to the droplets, undergoing a photochemical process. To date, the commonly used families of photochromic molecules for mediating dynamic processes, e.g., droplet motion are azobenzenes that undergo trans-cis photoisomerization, [24,25] and spiropyran and its derivatives that overgo photocleavage primary to the proton release, resulting in minute-length reaction timescales. [26][27][28][29][30][31][32] Therefore, a yet-to-beresolved challenge in the use of such photochromic molecules is the associated timescales and especially the reversibility of the dynamic process.Herein, we propose a new approach to chemophototaxis for gaining fast subsecond responses not only to turning on the light, but also for the reversible process of turning the light off, which is based on the use of Brønsted-Lowry photoacids and photobases. This class of organic molecules undergoes a dramatic change in their dissociation equilibrium constant (pKa) upon light excitation. [33] Thus, only in their electronically excited state do photoacids and photobases behave as strong acids and bases. The above-mentioned spiropyrans are also referred to as photoacids since their photocleavage process from spiropyran to merocyanine involves the release of a proton, however, there is a large difference in terms of mechanism and timescales relative to Brønsted-Lowry photoacids. In this context, we hypothesize that the fast excited-state proton transfer (ESPT) of Brønsted-Lowry photoacids and photobases can induce rapid chemical changes on the surface of pH-sensitive droplets and initiate their motion. We show here a variety of different droplet systems involving the use of photoacids and photobases either within the droplet, on its surface, or in solution, whereas we use light to self-propel and to guide the movement of the droplet. Nature demonstrates many examples of response and adaptation to external stimuli. Here, this study focuses on self-propulsion (motion) while presenting several self-propelling droplet systems responsive to pH gradients. Light is used as the gating source to gain reversibility, avoid the formation of chemical wastes, and control the self-propulsion remotely. To achieve light-stimuli ultrafast response, photoacids and photobases are used, capable of donating or capturing a proton, respect...
