Jérémie Teisseire

Self-Replicating Cracks: A Collaborative Fracture Mode in Thin Films

Straight cracks are observed in thin coatings under residual tensile stress, resulting into the classical network pattern observed in china crockery, old paintings, or dry mud. Here, we present a novel fracture mechanism where delamination and propagation occur simultaneously, leading to the spontaneous self-replication of an initial template. Surprisingly, this mechanism is active below the standard critical tensile load for channel cracks and selects a robust interaction length scale on the order of 30 times the film thickness. Depending on triggering mechanisms, crescent alleys, spirals, or long bands are generated over a wide range of experimental parameters. We describe with a simple physical model, the selection of the fracture path and provide a configuration diagram displaying the different failure modes.

Confinement and flow dynamics in thin polymer films for nanoimprint lithography

In nanoimprint lithography (NIL) viscous flow in polymeric thin films is the primary mechanism for the generation and the relaxation of the structures. Here we quantify the impact of confinement on the flow rate. Pattern relaxation experiments were carried out above the glass transition temperature as a function of film thickness. The results are adequately fitted by a simple expression for the flow rate valid at all confinements. This expression, based on Newtonian viscosity, should be of use in NIL process design and for the measurement of the rheological properties of confined polymers.

Elastic instability and contact angles on hydrophobic surfaces with periodic textures

Surface textures are used to impart advanced wetting properties to surfaces. However understanding the surface response in relation to the nature of the texture is still a challenge. Here we have measured advancing and receding contact angles on model hydrophobic surfaces with cylindrical pillars as a function of the pillar spacing. We show that the dependances of both advancing and receding contact angles upon spacing are well accounted for by a simple model of the instability of the triple line, following the line elasticity theory by Joanny and de Gennes (J. Chem. Phys. 81 (1984) 552). This result demonstrates the prominent role of the triple line elasticity in determining the wetting properties of textured surfaces.

Self-organized ordered silver nanoparticle arrays obtained by solid state dewetting

Spontaneous dewetting of a silver layer on a templated silica substrate is proposed as a promising low-cost process to produce self-organized metallic nanostructures. Periodic gratings with inverted pyramid pattern and periods ranging from 200 to 1000 nm are fabricated by nanoimprint on a sol-gel silica layer. A silver layer is then deposited on the templated substrate by magnetron sputtering and annealed to form an array of well-organized islands by solid-state dewetting. The resulting islands are shown to have reduced diameter and size dispersion compared to arrays obtained in the same conditions on flat substrates. The density of defects in the periodic array is determined as a function of silver layer thickness and is lower than 10% in optimal conditions. Optical transmission spectra of periodic arrays are measured, showing extinction peaks that can be related to plasmon resonance. This resonance can be tuned by adjusting the period and particle diameter.

A novel technique for simultaneous velocity and interface profile measurements on micro-structured surfaces

We present a novel approach which allows simultaneous measurement of the velocity field and the interface profile close to a composite liquid–gas and solid–gas interface. The proposed scheme is the method of choice for the characterization of those flows where the velocity field is highly dependent on the actual shape and position assumed by liquid–gas and liquid–solid interfaces. The new method is based on the digital processing of microscopy images of a flow seeded with fluorescent passive tracers. The relative position and the shape of both liquid–gas and liquid–solid interfaces can be determined with a resolution of few tens of nanometers. The results for the liquid–solid interfaces are also compared to an additional detection method we devised to accurately determine the absolute position of the solid walls.

Superhydrophobic silica surfaces: fabrication and stability

We report a simple method to make hybrid or pure silica micropatterns at the surface of a substrate based on the combination of sol–gel process and nano-imprint lithography. The silica patterns can be easily designed during the photolithographic step and functionalized with a vapor phase deposition of fluorosilane molecules to obtain superhydrophobic surfaces. Benefiting from the properties of silica, our superhydrophobic patterns can withstand elevated temperatures and show interesting optical properties. These surfaces can be used for thermal transfer applications or microfluidic devices for example to limit noise in fluorescence measurements for biological applications. In connection to the fabrication of superhydrophobic surfaces, the organization of patterns (period of grating) and height of patterns were tested, and the stability of the Cassie–Baxter state studied. The transition can be described on a wide range of tested parameters by the sliding threshold where the control of side wall angle of patterns and chemistry of surface is essential.

Finite size effects on textured surfaces: recovering contact angles from vagarious drop edges.

A clue to understand wetting hysteresis on superhydrophobic surfaces is the relation between receding contact angle and surface textures. When the surface textures are large, there is a significant distribution of local contact angles around the drop. As seen from the cross section, the apparent contact angle oscillates as the triple line recedes. Our experiments demonstrate that the origin of these oscillations is a finite size effect. Combining side and bottom views of the drop, we take into account the 3D conformation of the surface near the edge to evaluate an intrinsic contact angle from the oscillations of the apparent contact angle. We find that for drops receding on axisymmetric textures the intrinsic receding contact angle is the minimum value of the oscillation while for a square lattice it is the maximum.

Controlled angular redirection of light via nanoimprinted disordered gratings

Enhanced control of diffraction through transparent substrates is achieved via disordered gratings in a silica sol-gel film. Tailoring the degree of disorder allows tuning of the diffractive behavior from discrete orders into broad distributions over large angular range. Gratings of optical quality are formed by silica sol-gel nanoimprint lithography and an optical setup for the measurement of continuous diffraction patterns is presented. Sound agreement is found between measurements and simulation, validating both the approach for redirection of light and the fabrication process. The disordered gratings are presented in the context of improved interior daylighting and may furthermore be suited to a wide variety of applications where controlled angular redirection of light is desired.

Enhancing absorption in a thin film photovoltaic system with periodic nanostructures obtained by low-cost techniques

Light trapping structures are a promising approach to increase light absorption in ultrathin absorbers of interest for photovoltaic applications. To be integrated within real systems, their fabrication approach need to be simple and scalable, insuring that the gain in efficiency is worth the extra manufacturing steps. Here, we optimize through simulation using a RCWA code, a variety of 1D and 2D structures that can be fabricated with such simple and scalable techniques. In particular, we investigate the effect 1D and 2D gratings fabricated by Nanoimprint Lithography, onto Bragg mirrors that could be made using simple liquid process approaches. The optimized structures exhibit a significant gain in absorption with almost twice as much absorption in the wavelength range 0.5 to 1 micron, compared to a bare absorbing film of GaAs chosen as our reference. We also identify the various phenomena behind the absorption peaks observed and study the angular dependency of absorption with these structures.