**1. Introduction**

Many lab-on-a-chip (LoC) applications have become possible thanks to the ability to control mixing of different droplet contents, which enabled the sequencing of many complex bio-chemical and biological reactions with a high level of control and flexibility over the last decade; see for a review [1–7]. Hence, among all manipulation schemes allowed by droplet-based microfluidics technology [8–16], active merging of microdroplets (AMD) is probably one of the most important. It is generally achieved using a high alternating current (AC) voltage [17], or using a direct current (DC) voltage [18]. Nevertheless, light-driven merging of droplets is a more attractive approach since

light provides not only high temporal and spatial resolutions but also wavelength and intensity tunability [19–21].

Recently, Dunkel et al. [22] showed that active merging of microdroplets can be achieved optically and very selectively using the photolysis process of photolabile surfactants [22]. In the present study, we consider a new strategy based on the photo-isomerization process (Figure 1) of a newly synthesized azobenzene derivative surfactant polymer, whose structure is given in Figure 2 and which is named in this study KryAz600, to achieve an active merging of water-in-oil (W/O) microdroplets using a picosecond (ps) pulsed UV laser.

This study was inspired initially by a previous work carried out by Takahashi et al. [23], who reported that a light-induced destabilization of an overall emulsion based on the photo-isomerization process of azobenzene-derived photosensitive surfactants, through the light induced interfacial activity change of gemini-like azobenzene derived surfactants and the conversion between a higher surface activity *trans* isomer and a lower surface activity *cis* isomer. This resulted in a destabilisation of the overall emulsion without the aim of achieving spatial differentiation and requesting several minutes of irradiation.

Our approach is different and is far from trivial. In fact, the dynamics of the change of azobenzene surfactants surface activity at the microscale, at which new interfaces are produced involve both the change of surface tension driven by the photo-isomerisation process and the diffusion of the new surfactant molecules to the interface, as well as the adsorption on the droplet surface, each partial step adding a typical time and length-scale [24]. Our approach is also different from the recent study reported by Dunkel et al. [22] as the mechanisms lying behind the two merging processes involved in the two studies are completely different. Moreover, photo-isomerization of azobenzene is fully reversible, which makes this new approach particularly suitable for the reuse of the photo-sensitive surfactant, which is generally produced in a small quantity.

**Figure 1.** Light-driven merging process principle. (**a**) Targeting droplets and change of surface activity of the surfactant molecules at the droplet surface under laser irradiation, (**b**) depletion of the surfactant molecules at the the droplet interface, (**c**) merging induced following UV laser irradiation and the *trans* to *cis* photo-conversion, (**d**) targeted droplets after merging.

**Figure 2.** Stable *trans* form (**left**) and metastable *cis* form (**right**) of the synthesized KryAz600 molecule; *trans* to *cis* transition occurs under UV irradiation and back to *trans* occurs under visible light.

#### **2. Materials and Methods**
