Birefringence around bubbles in elastomers: local mechanical and gas transfer probe

Some materials, such as elastic polymers, become birefringent when mechanically stressed. A collaboration between Liphy and Saint-Gobain studies the visualization of stresses in confined elastomers.

Some materials, such as elastic polymers, become birefringent when mechanically stressed. If these materials are placed between crossed polarizer and analyzer, interference fringes are visible in the stressed areas. This effect, called photoelasticity, can be exploited to visualize non-invasively the stresses existing inside a solid subjected to external forces.

Cloverleaf light interferenceThe idea of this internship is to use photoelasticity to probe the stresses within a soft elastomer confined between two glass plates, such as PDMS used in microfluidics, by seeding it with bubbles (as shown in the figure: "cloverleaf" light interference observed around bubbles in a confined polymer). Using bubbles has many advantages: i) they are easy to make and visualize, ii) one can easily modulate the intensity of the mechanical stresses they exert on the polymer by changing the pressure or the external temperature iii) they can be small enough to make measurements near a solid wall and probe the adhesion between polymer and wall.
More precisely, the objectives of the internship are the following. First, a quantitative link between the mechanical parameters of the bubble (pressure, radius, displacement) and the intensity of the optical birefringence for different temperatures should be established. This part finalizes a work started in the LIPhy laboratory, for which many tools (experimental and data analysis) have already been developed. After this calibration step, we would like to explore the possibility of using the birefringence of the bubbles as a local probe in two situations: on the one hand, when the bubbles are placed more or less close to a wall and, on the other hand, when they swell due to the diffusion of a gas in the polymer. In the first case, the solid/polymer adhesion energy could be estimated by comparing the birefringence of bubbles more or less close to the wall. In the second case, the permeability of the gas in the polymer could be determined by simultaneously measuring the temporal evolution of the birefringence and the bubble radius.

Opened to:

Students in Master 2
Interested students should have a good knowledge of thermodynamics and soft matter or fluid mechanics and optics. A taste for experimentation and instrumental development is highly desirable. The will last a minimum of four months, ideally six months, and may continue into a thesis. The intern will receive an internship stipend of approximately 550 euros per month.

Continuation in PhD possible.
Published on February 10, 2020
Updated on September 29, 2022