Optical properties, Molecular Organization and Dynamics of Droplets Interfaces
Droplet microfluidics allows for the production and manipulation of millions of highly monodisperse droplets, each of which can serve as an independent microreactor. This technology offers great prospects in many scientific and industrial fields. Droplets can be produced, analyzed and manipulated at a very high rate, thanks to optical methods.
Many studies have been conducted to improve this technology and its applications, but a thorough understanding of the complex dynamic processes occurring at the interface of droplets and carrier fluid (oil) is still not fully understood.
New optofluidic approaches
During this thesis, we were interested in the development of new optofluidic approaches allowing for a better analysis of dynamics and molecular organization at the droplet interfaces in the microfluidic flow.
In a first study, we were interested in developing a controlled light-driven merging of droplets using a ps UV-laser. This approach is particularly attractive approach since light provides flexibility, wavelength/intensity tunability and high temporal/spatial resolutions. We investigated two different methods: photolysis of photolabile molecules (irreversible process) and photo-isomerization of azobenzene derived molecules (reversible process).
The success of such an approach was far from trivial, since illumination at the microscale induces changes not only in the dynamics of the interfacial tension but triggers also changes in diffusion and absorption of surfactant molecules at the droplets interface, each partial step adding a typical time and length-scale. Analysis of the measured merging time (found at the ms time scale) allowed for the determination of the diffusion coefficient of surfactant molecules around the droplet interface. Another important result was the first experimental demonstration.
Droplet interface in the flow
In a second study, we focused on the real-time detection and analysis of the optical properties of dyes at the droplets interfaces, in order to better understand the building, the dynamics and the molecular organization of the droplet interface in the flow. For this aim, we developed an original broadband highly sensitive detection system, using an off-axis full VIS spectrum - collection, reflection and detection scheme.
Our setup enables to achieve a real-time detection of droplets photo-luminescence over a large spectral range and at the ms timescale and to show for the first time the occurring of a thermally activated hot band anti-stokes shift emission. The later was found to localize mainly at the droplets interfaces.
Based on this original result, we propose that our optofluidic system may serve as a new analysis tool to detect and study soft interfaces without the aid of optical imaging/recording techniques. The observed hot band anti-stokes shift is shown to be suitable for instance to detect and discriminate between flowing droplets and vesicles (or double emulsions) in a real-time and high throughput detection mode.
In the last study, we were particularly interested in the study of mass transport and diffusion of dyes across biomimetic bilayers systems. Two major approaches were addressed, the droplet-interface-bilayer (DIB) and solvent evaporated water/oil/water double emulsions. Both techniques required rigorous design and micro-fabrication characterization. Preliminary results show that such systems may lead to the development of smart applications in soft-bio-mimetic membrane’s design, mass transport and drug carriers studies.