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An elastocapillary-enabled self-tunable microfluidic chip

Periodic Reporting for period 1 - El_CapiTun (An elastocapillary-enabled self-tunable microfluidic chip)

Reporting period: 2018-01-01 to 2019-12-31

Capillary forces act at fluid interfaces and can maintain dense objects afloat. Their relative importance is strongest at small scales. For instance, they can collapse elastic microstructures, and has been a major struggle for high quality microfabrication. Recently, this mechanism has been elegantly harnessed to control the self-assembly of complex 3D shapes in microfabricated systems. Spiders also use this trick to enhance the mechanical resilience of their web, by spooling extra fibre into water droplets.
Here we take to opposite point of view, and focus on how solid fibres may increase the resilience of liquid droplets. We fabricate a hybrid fibre-in-drop system in a controlled microfluidic environment by curing a controlled microjet of photopolymer (PEG-DA) liquid precursor into a soft (solid) fibre, which is then coated by the uncured (liquid) solution. The resulting sample is then trapped at an area of locally decreased confinement. The mechanical properties of the hybrid system are then investigated under increasing flow rates: hydrodynamic forces deform the system and may also actuate the deployment of the microfibre from its originally coiled state. Our results show that the effective surface tension of the drop container is increased, as well as their resilience under flow.
These results open opportunities for a better design of liquid microcontainers and tunable microfluidic circuits with high on-off ratio for bioengineering applications.
Elastocapillary forces have been shown to be able to induce large deformation in thin elastic structures. In case of a drop-on-fibre systems, fibres can be spooled within their liquid cavities if the capillary forces overtake the bending resistance of the fibre. These geometrical effects strongly enhance the resilience of the system seen as an elastic structure.
Here we explored the mechanical response of these hybrid systems seen as liquid drops. The production and mechanical testing are performed in a controlled microfluidic environment, all embedded onto a single chip. We produce a PDMS microfluidic chip through conventional soft photolithography that contains three zones: (I) a liquid jet production zone, (II) a fibre curing and coating area and (III) a micromechanical testing area.
After reticulation, the fibre is coated by the uncured (liquid) PEG-DA solution, allowing very good wetting conditions and ease of coiling.
The physico-chemical parameters (diameter, length and softness) of the fibres can be reliably varied, as well as the volume of the droplets carrying the fibres. The sample is then micromanipulated towards an area of the microfluidic chip with locally higher channel thickness, which allows capillary relaxation and trapping at a controlled point. The trap holds the drop as long as the drag from the external flow does not overcome the capillary forces necessary for reconfinement of the drop into the main channel.
We then compare the deformation under flow of drops and fibre-in-drop systems. We find a strong delay of the break up instability promoted by the presence of the fibre, as well as an increase in effective surface tension. These results open opportunities for a better design of liquid microcontainers and tunable microfluidic circuits with high on-off ratio for bioengineering applications.
Traditional elastocapillary interactions have produced an array of complex 3D shapes at the microscale. However, these microstructures are either static or the actuation methods are invasive. Here we develop a contactless method based on hydrodynamic drag to actuate capillary-held microstructures. Based on conventional techniques for microjet production, we fabricated fibre-in-drop hybrid systems in situ. We implemented micromanipulation methods and embedded mechanical testings. Our results show a reinforcement of the drops due to the contribution of the bending rigidity of the fibre. Through the analysis of the drop enveloppe and the local profile of the fibre, we measure the surface and bending energies of the system. We show that the reinforcement mechanism stems from a competition between drag, drop surface and fibre curvature minimizations, leading to a new out-of-equilibrium hydroelastocapillary response.
Schematic of the on-chip production and mechanical testing of fibre-in-drop microsystems
Deformation for increasing flow rates of drops and fibre-in-drops systems.