Droplets: Moving Droplets in 3D Using Light (Adv. Mater. 35/2018)

Abstract

In article number https://doi.org/10.1002/adma.201801821, David L. Officer and co‐workers break new ground in fluidic chemical transport by guiding organic droplets containing a photoisomerisable material through water with precision and control using light. The droplets can not only be remotely moved from one point to another in a container, but can also be guided to pick up cargo and transfer it to another site to undergo a chemical reaction.

Dual Droplet Functionality: Phototaxis and Photopolymerization

“…Inspired by recent developments in the use of synthetic polymers as wet adhesive materials,(6−9) we have investigated site-specific photopolymerization via a droplet-based approach that can be achieved under water. Using our previous photoactive droplet system containing a commercially available nitro-substituted spiropyran (SP) and 2-hexyldecanoic acid (HDA) (Figure 1), (5) we have now demonstrated droplet-based transport of a photoinitiator to, and mixing with, a N-isopropylacrylamide (NIPAM) monomer droplet, which results in polymerization at a specific location under water. This has led to the discovery that the photoinitiator itself, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO), can be used to induce both droplet movement and photopolymerization.…”

“…Irradiation of the BAPO droplet (365 nm) led to a ∼5 mN m -1 decrease in IFT, consistent with that previously observed for SP. (5) This results from photocleavage of BAPO ( Figure S2a) and presumably subsequent reaction of the resulting radicals with a radical-scavenging species like oxygen. (19) As previously reported for self-propelled droplets and spiropyran photoactive droplets, (5,21) Therefore, it is likely that the photocleavage of BAPO leads to a change in surfactant distribution on the surface of the droplet as a result of the interaction of the photocleavage product with the surfactant inducing an internal droplet fluid flow and creating an IFT gradient that is continuously sustained.…”

“…(5) This results from photocleavage of BAPO ( Figure S2a) and presumably subsequent reaction of the resulting radicals with a radical-scavenging species like oxygen. (19) As previously reported for self-propelled droplets and spiropyran photoactive droplets, (5,21) Therefore, it is likely that the photocleavage of BAPO leads to a change in surfactant distribution on the surface of the droplet as a result of the interaction of the photocleavage product with the surfactant inducing an internal droplet fluid flow and creating an IFT gradient that is continuously sustained. This leads to the external Marangoni flow in the direction of the higher IFT that results in mass transfer along the liquid interface (Marangoni effect), (21−23) propelling the droplet away from the light source, as previously shown for other droplets moving by an analogous chromocapillary effect.…”

“…This leads to the external Marangoni flow in the direction of the higher IFT that results in mass transfer along the liquid interface (Marangoni effect), (21−23) propelling the droplet away from the light source, as previously shown for other droplets moving by an analogous chromocapillary effect. (5,24) While the droplet was moving, a narrow plume emerging from the droplet was visible (Movie S4). This is not surprising given previous observations of plumes from photoactivated droplets and the reported advection of materials from droplets driven by chemical reactions.…”

“…This is not surprising given previous observations of plumes from photoactivated droplets and the reported advection of materials from droplets driven by chemical reactions. (5) To attempt to identify the components of this plume, UV-vis spectra were collected from an irradiated (365 nm) pendant BAPO droplet immersed in water (see Figure S4a). The spectra of the plume ( Figure S4b) with the absorptions at 360 and 280 nm suggested that BAPO was present in the released material.…”