Fibre-reinforced microfluidic droplets


Jan. 2018 – Present: Scientist, Marie Curie Fellow. Collaboration with Professor François Gallaire at EPFL, Lausanne, Switzerland.
Title: Microfluidic production and in situ mechanical testing of fibre-reinforced droplets.
Keywords: Microfluidics, contactless micromechanical testing, fluid-structure interactions, encapsulation.

Elastocapillary forces have been shown to be able to induce large deformation in thin elastic structures, and are relevant in small scale configurations such as 3D microfabrication, lung airway collapse and spider capture silk. 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 explore 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, as presented in figure 1.


Figure 1- Schematic of the production and mechanical testing of the fibre-in-drop microsystem. The chip contains 3 zones: (I) a liquid jet (a) production zone, (II) a fibre UV curing and coating (b) area and (III) a micromechanical testing area. The fibre coating occurs while the cured (fibre) and uncured (liquid) PEG-DA travel in the serpentine. After micromanipulating the sample into the capillary trap (c), the mechanical response of the system is recorded under increasing flow rates.

The physico-chemical parameters (diameter, length and softness) of the fibres can be varied, as well as the volume of the droplets carying 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 (see figure 1-(c)). 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.
In figure 2, we present a proof of concept of the fibre-induced reinforcement of the droplet. We compare the deformation under flow of two systems with the same lateral surface (thus experiencing the same drag in the undeformed state): one drop alone (blue points) and one drop loaded with a fibre (green points). We find a strong delay of the break up instability promoted by the presence 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.
The present results open opportunities to form the building blocks of microfluidic elements with high on-off ratio that may be useful for the design of on-chip tunable circuits for bioengineering applications.


Figure 2- Comparison of the deformation under increasing flow rates of a drop alone (blue points and bottom row insets) and of a drop and fibre (green points and top row insets). The black circles on the graph (left) relate to the pictures displayed for optical comparison (right) and the color code describes the mechanical behaviour (orange for reorientation of the system, red for deformation) . The two systems have the same lateral area at rest and the fibre physical parameters are d = 90 ± 3 μm, L = 2100 ± 30 μm, E = 100 ± 30 kPa.


We acknowledge funding from the Marie Sklodowska-Curie Actions Fellowship, project “El_CapiTun” n°750802.

Related publications:

  1. The heavy windlass: Buckling and coiling of an elastic rod inside a liquid drop in the presence of gravity

    H. Elettro, S. Neukirch & A. Antkowiak

    Mechanics Research Communication, 93:58-61 (2018)

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  2. Drop-on-coilable-fibre systems exhibit negative stiffness events and transitions in coiling morphology

    H. Elettro, S. Neukirch, F. Vollrath & A. Antkowiak

    Soft Matter, 13(33):5509-5517 (2017)

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  3. In-drop capillary spooling of spider capture thread inspires hybrid fibers with mixed solid–liquid mechanical properties

    H. Elettro, S. Neukirch, A.Antkowiak & F. Vollrath

    Proceedings of the National Academy of Science (PNAS), 113(22):6143-6147 (2016)

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  4. Elastocapillary coiling of an elastic rod inside a drop

    H. Elettro, P. Grandgeorge, S. Neukirch

    Journal of Elasticity, 127(2):235-247 (2016)

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