In situ microviscosity measurements in complex systems by using molecular rotors. – MicroVISCOTOR

The project consists in several tasks: 1) Calibration of the response of the rotor to the viscosity of the surrounding medium both by steady state and time-resolved spectroscopy. This will be performed by measuring the fluorescence and the decay time of the rotor in fluids of known viscosity. Validation of the methodology will be performed by applying this approach to systems of known behavior.
2) Grafting rotor-functionalized polymer design and preparation. This includes a polymer functionalized with the rotors by copolymerizing rotor-functionalized monomers or creating a rotor-functionalized initiator.
3) Implementation of emulsions in microfluidic devices.
4) Microfluidic device preparation for on-line measurement.
5) Microfluidics device preparation for confined droplet measurements

Overall, the working program presented in this project 1) will allow the design of cost-effective “lab on a chip” devices and sensors; 2) will produce results useful to better implement complex fluids in microfluidic devices; 3) will give insight on fundamental challenges in complex fluids rheology.

We have demonstrated the proof of concept and feasibility of using molecular rotors to perform microviscosity mapping in systems containing complex fluids. During a co-flow of dimethylsulfoxide (DMSO) and glycerol inside a Y-junction microfluidic chip, viscosity mapping was performed along the mixing channel with good spatial resolution and in real time. A calibration curve based on the previously established Forster-Hoffman equation allowed us to retrieve the local viscosity from the DCVJ fluorescence response and perform the microviscosity mapping. The concentration profiles along the microchannel were fitted. This allowed us to calculate the effective interdiffusion coefficient of glycerol as well.

The synthesis of a polymeric material with grafted molecular rotors was considered using a linear block copolymer functionalized with 9-(2-carboxy-2-cyanovinyl) julolidine (CCVJ). CCVJ is chosen because it has a carboxyl group that can be modified while preserving the functionality of a molecular rotor.
CCVJ functionalized macromonomers were synthesized by condensation of cyanoesters and julolidine aldehyde (Figure 2a of the report). Cyanoesters were obtained by esterification of cyanoacetic acid with hydroxyethyl methacrylate (HEMA) induced by N-ethyl-N'-(3-dimethylamino propyl)carbodiimide, while julolidine aldehyde was obtained by formylation of julolidine with phosphorus oxychloride and dimethylformamide.
The CCVJ functionalized macromonomer will be copolymerized with dimethyl acrylamide (DMAA) by reversible addition-fragmentation chain transfer (RAFT). The entire CCVJ polymer chain will be grafted onto the surface of the PEG-DA microfluidic chip by photopolymerization.

The arrival of the postdoc (Florence Gibouin) is scheduled for May 3, 2021. She will work on in situ and online mapping of microviscosity within emulsions and micelles confined in microchannels.

The striking results described in the report pave the way for further studies on 1) the microviscosity of complex systems and 2) the use of viscosity-sensitive polymeric materials in various applications.

Two publications are in progress and it is planned that the PhD student will participate in the ACS 2021 Summer Conference (by videoconference, abstract sent)

Two publications are in progress and it is planned that the PhD student will participate in the ACS 2021 Summer Conference (by videoconference, abstract sent)

Micro rheology is a field in which the study of viscoelasticity of materials serves to consider how their dynamic behavior changes with length scale. Applied to complex fluids, this field is of extreme industrial importance: from paints to foods, from oil recovery to processing of plastics, understanding the flow of complex fluids is essential to a wide range of technologies. The microrheology of complex systems still faces significant challenges: 1) in situ measurements within confined systems; 2) on line measurements in dynamic microsystems, and 3) viscosity mapping at the nanoscale. Microrheology is closely connected to the field of microfluidics, which considers phenomena such as those involved in ink jet printing, 3D printing, microelectrophoresis on a chip, microvalves and the kinetics of protein crystallization. The overlap is thus quite strong and the fluid mechanics of materials in confined geometries is a common area to the two research fields. The dynamics and the microrheological behavior of confined fluids often change dramatically when, for instance, they are confined near a surface. The interfacial characteristics of gas, liquid and solid interfaces all require individually optimized methods for the measurement of the surface microrheology. Even more, traditional mechanical viscometers are not able to measure viscosity of microsystems and, more important, they cannot measure microviscosities in real-time conditions or perform time- and spatially-resolved mapping of the microviscosities in confined systems. MICROVISCOTOR proposes an innovative strategy to develop a cost-effective microfluidics device capable to measure and map on-line and in real-time spatially- and time-resolved microviscosities of fluids by using molecular rotors. The strategy presented here is applied to microfluidic processes with the aim to develop low-cost “lab on a chip” devices or sensors.