CEA-LITEN is developing an original process called Boostream to manipulate, assemble and connect micro- or nano-particles of various origins, sizes, shapes and functions [1]. This process uses the upper surface of a liquid film flowing down a ramp to assemble particles in a manner that is close to the horizontal situation of a Langmuir -Blodgett film constructions. The strong potentials of our new process rely on its advantages: (i) collective handling of micro- or nano-sized particles, (ii) safe handling of powder materials, (iii) numerous potential industrial applications.

The process uses a laminar water film on an inclined plane slope flowing down to a horizontal reservoir. Particles are deposited on the top of the water/air interface and gradually accumulate on the upper surface of the reservoir; once the reservoir surface is filled by single-layer film of particles, this film continues to form on the feeding inclined surface. Thus, a film composed of a single layer of particles is formed (typical size ranging from 280 nm to 10 µm) and can be transferred on any kind of solid support, forming a micro- or nano-structured surface on it and allowing new physical, chemical or optical functions [2], [3].

The film down-flow configuration exhibits two hydraulic jumps, the first being upstream to the border of the particle film, the second located at the connection of the slope and the reservoir surface. The second jump is a classical one and not worthy to us. The first jump is more unusual; it results from the fluid flow accommodation to the change of the film upper surface boundary condition: upstream, the film has a free surface; downstream, the closed-packed particles result in a mixed (or Robin) boundary condition, with vanishing tangential velocity components, whereas the film thickness remains free.

It is worth managing this hydraulic jump to control the film construction process, especially when non-isotropic particles are used, such as micro-fibers. Indeed, unlike spherical particles, micro-fibers are far more sensitive to flow singularities, such as a hydraulic jump that tends to induce recirculating surface patterns upstream of them and lets the particles rotate on the free surface, close to the film border.

In order to master our process, the fluid flow has been modeled and experimentally characterized by optical means, such as with the moiré technic that consists in observing the reflection of a succession of periodic black-and-red fringes on the liquid surface mirror. The fringe images are deformed when reflected by the curved liquid surface associated with the hydraulic jump, the fringe deformation being proportional to the local slope of the surface. This original experimental setup allowed us to get the surface profile in the jump region and to measure it along with the main process parameters (liquid flow rate, slope angle, temperature sensitive fluid properties such as dynamic viscosity or surface tension, particle sizes). 

Our oral presentation will depict the experimental setup and its modeling, the different experimental characterization techniques used and will focus on the way the hydraulic jump relies on the process parameters. Some of the applications of our process are also to be presented.

[1] O. Delléa et al., Control methods in microspheres precision assembly for colloidal lithography, Proc. IPAS Conf., Chamonix, France, 2014.

[2] O. Shavdina, O. Delléa, et al., Large Area Fabrication of Periodic TiO2 Nanopillars Using Microsphere Photolithography on a Photopatternable Sol-Gel Film, Langmuir, 31 (28), pp. 7877-7884, 2015.

[3] O. Shavdina, « Micro-nanostructuration de surface par renforcement local du flux électromagnétique », thèse, Université Jean Monnet, soutenue le 20 Décembre 2016.