Various molecules are known to form self-assembled monolayers (SAMs) on the surface of liquids. We present a simple method of investigating the kinetics of such SAM formation on sessile drops of various liquids such as mercury, water and fluorocarbon. To measure the surface tension of the drops we used an optical tensiometer that calculates the surface tension from the axisymmetric drop shape and the Young-Laplace relation. In addition, we estimated the SAM surface coverage fraction from the surface tension measured by other techniques. With this methodology we were able to optically detect concentrations as low as tenths of ppb increments of SAM molecules in solution and to compare the kinetics of SAM formation measured as a function of molecule concentration or chain length. The analysis is performed in detail for the case of alkanethiols on mercury and then shown to be more general by investigating the case of SAM formation of stearic acid on a water droplet in hexadecane and of perfluorooctanol on a Fluorinert FC-40 droplet in ethanol.

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...

Plants are protected from the elements by a complex hierarchical epicuticular wax layer which has inspired the creation of super-hydrophobic and self-cleaning surfaces. Although many studies have been conducted on different plant wax systems to determine the mechanisms of water repulsion hardly any have studied the recovery of the epicuticular wax layer. In the current study the wetting properties and crystallographic nature of the wax surface of Brassica oleracea var. italica (broccoli) has been studied, as well as the time-dependent recovery of the surface after mechanical damage. It was found that the surface of the broccoli leaves is not only super-repulsive and self-cleaning in regards to water but also in regards to glycerol and formamide, both of which have considerably lower surface tension values. Furthermore, it was shown that the surface properties do indeed recover after damage and that this recovery is multi-stepped and strongly dependent on the recovery of the roughness of the surface.

Size effects and structural modifications in amorphous TiO2 films deposited by atomic layer deposition (ALD) were investigated. As with the previously investigated ALD-deposited Al2O3 system we found that the film’s...

The dynamic control of pH-responsive systems is at the heart of many natural and artificial processes. Here, we use photoacids, molecules that dissociate only in their excited state and transfer their proton to nearby proton acceptors, for the dynamic control of processes. A problem arises when there is a need to protonate highly acidic acceptors. We solve this problem using super photoacids that have an excited-state pK a of −8, thus enabling them to protonate very weak proton acceptors. The process that we target is the light-triggered self-propulsion of droplets, initiated by an excited-state proton transfer (ESPT) from a super photoacid donor to a surfactant acceptor situated on the surface of the droplet with a pK a of ∼0. We first confirm using steady-state and time-resolved spectroscopy that a super photoacid can undergo ESPT to the acidic surfactant, whereas a “regular” photoacid cannot. Next, we show self-propulsion of the droplet upon irradiating the solvated super photoacid. We further confirm the protonation of the surfactant on the surface of the droplet using transient surface tension measurements. Our system is the first example of the application of super photoacids to control dynamic processes and opens new possibilities in the field of light-triggered dynamic systems.