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Digital Optofluidics for Remote Actuation of Liquids

Final Report Summary - DIOPTRA (Digital Optofluidics for Remote Actuation of Liquids)

Summary description of project context and objectives

The general objective of the project DIOPTRA, as included in Annex I of the Grant Agreement, was the development of an inclusive optofluidic toolbox for the manipulation of both organic and aqueous liquids on liquid and solid substrates, respectively. In order to accomplish that, fundamental research on the physicochemical aspects of two qualitatively different actuation mechanisms was proposed. These mechanisms were the chromocapillary effect, based on the optically induced Marangoni flows in liquids containing photosensitive surfactants and the photothermal wetting effect, based on the use of thermoresponsive polymers. After obtaining the necessary fundamental knowledge, it was proposed to subsequently exploit these mechanisms for the accomplishment of simple and complex microfluidic operations.
For the reporting period in question (Months 1-24), regarding the chromocapillary effect in particular, the following tasks were proposed: i) the investigation of the influence of geometrical, physicochemical and irradiation parameters, ii) the development of an appropriate optical setup to achieve structured illumination patterns and iii) the application of such complex light patterns to perform optofluidic operations. Concerning the photothermal wetting effect, the following steps were suggested: i) the development and the optimization of responsive polymer-based hybrid surfaces that would exhibit the photothermal wetting effect, ii) the demonstration of basic light-driven operations using water drops on the developed surfaces and iii) the employment of the responsive surfaces for the execution of complex optofluidic tasks.

Executive summary

The work performed during the whole duration of DIOPTRA (01/04/2014-31/03/2016) was broadly concerned with the optofluidic actuation of liquids for controlled wetting and tunable colloidal patterning applications, and it can be mainly divided in three parts; a fourth part describing work indirectly related to DIOPTRA is briefly described in this report as well. In the first part of the research reported here, I demonstrated the successful development of thermosensitive polymer-based hybrid surfaces and their employment for the realization of light-controlled wettability alterations. Although light-driven water drop motion has not been achieved yet, our findings are expected to be the fundamental basis for the accomplishment of the above-mentioned goal in the near future. In addition, the light-controlled spreading of aqueous drops demonstrated in this study, might find practical applications in fluidic applications (i.e. the controlled coalescence of adjacent droplets) and drop-based optofluidic platforms.
In the second part of the DIOPTRA research, the control of particle deposition from evaporating colloidal dispersion drops containing surfactants, with and without light irradiation, was explored. Initially, we systematically investigated the influence of surfactants in the so-called Coffee-Ring Effect which occurs in evaporating particle-laden drops. Next, based on our gathered knowledge regarding the decisive role of surfactants in modulating the dry pattern morphology by controlling particle-interface interactions, we designed simple photoresponsive dispersions consisting of anionic colloids and a photosensitive cationic surfactant. In such systems, the particle-surfactant interactions were reversibly and finely tuned by light, modulating in turn the particle-LG interface interactions and finally the deposition patterns. Looking forward, we anticipate stimuli-responsive stickiness to be a starting point for the development of general strategies to tailor particle assembly at ultimately all kinds of interfaces or in bulk in a non-invasive, highly flexible, and straightforward fashion. This could provide a new and exceptionally promising optofluidic solution to address the challenge of programmable colloidal assemblies. A detailed description of the above mentioned findings can be found in our recent publications (Angew. Chem. Int. Ed. Engl. 2014, 53, 14077, Langmuir 2015, 31, 4113, ChemPhysChem 2015, 16, 2726).
In the third part of the DIOPTRA research, we first constructed a simple, LED-based optical setup for the creation of simple and complex structured light patterns. We then implemented these irradiation patterns for the development of a new light-guided patterning technique, the evaporative Optical Marangoni Assembly (eOMA). This novel flow-based technique was successfully employed to organize particles on solid substrates from evaporating sessile drops containing photosurfactants, regardless of particle size or surface chemistry. This strategy is remarkable for its simplicity and versatility, making it suitable not only for model colloidal suspensions but also for complex, real-world formulations. Undoubtedly, eOMA represents a new and promising optofluidic tool for the manipulation of complex fluids and the potential fabrication of new functional materials and devices. An analytical presentation of our findings can be found in our recent publication (Nano Lett 2016, 16, 644) and a paper to appear soon (Anyfantakis et al., in preparation).