An elastocapillary-enabled self-tunable microfluidic chip

Project Information

El_CapiTun

Grant agreement ID: 750802

DOI

10.3030/750802

Project closed

EC signature date 20 March 2017

Start date 1 January 2018

End date 31 December 2019

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 175 419,60

EU contribution

€ 175 419,60

Coordinated by

ECOLE POLYTECHNIQUE FEDERALE DE LAUSANNE
Switzerland

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

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

Summary of the context and overall objectives of the project

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.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

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.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

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.