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

Abstract

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...

Application of Super Photoacids in Controlling Dynamic Processes: Light-Triggering the Self-Propulsion of Oil Droplets

“…Figure shows 5 out of 15 trajectories that have been used to calculate the averaged velocity and confirm the direction of the droplet’s self-propulsion. In line with our previous work, also here, the averaged intensity can be attenuated by the concentration of the photoacid (Figure S1).…”

“…As discussed above, our working hypothesis is based on the ESPT from solvated QCy9 to SDBS (Figure a, for molecular schemes of the molecules) that is situated on the surface of a nitrobenzene droplet. The asymmetric light gradient between the two sides of the droplet will lead to an asymmetric SDBS protonation and, consequently, an asymmetric change in the ST, eventually resulting in the droplet self-propulsion away from the light (Figure b, and further discussion below) …”

“…Previously, we have used oil droplets as a convenient model for an artificial system capable of stimuli-responsiveness and motion. While there are many examples where chemical or physical signals, such as a change in pH or light, were used as a stimulus for a droplet’s self-propulsion, − we were the first to use Brønsted photoacids to control their motion.…”

“…Brønsted photoacids are organic molecules that undergo a dramatic drop of p K a in their electronically excited state. − Accordingly, such photoacids can be considered as a transient source of proton donors only upon their light excitation. In our previous study, we showed that following light excitation of photoacids, an excited-state proton transfer (ESPT) took place between the photoacids and proton acceptors situated on the surface of the droplet. Accordingly, the application of a gradient light intensity in our system resulted in an asymmetric change of the protonation state of groups on the surface of the droplet, which in turn creates a nonuniform surface tension (ST), and the formation of Marangoni flows, which initiates the droplet’s self-propulsion. − …”

“…While the use of photoacids in propelling droplet systems has several advantages, such as a fast response time to the light stimuli, fast reversibility, absence of chemical waste, and the control of the number of released protons from the photoacid simply by changing light intensity, it is limited by the excited-state p K a (p K a *) of the photoacid. For instance, in our previous reported system, we used droplets decorated with carboxylated moieties as well as pyranine and 2-naphthol-6-sulfonate (2N6S) photoacids, having an excited-state p K a (p K a *) in the range of 0.4–1.7. Thus, an ESPT between the excited photoacid to the carboxylate (having a p K a larger than 4) is possible.…”