As is common in everyday life experience, evaporation of a drop of non-pure liquid on an inert substrate leads to the formation of a stain. Its morphology appears to be extremely variable with the parameters of the evaporation and can thereby take the shape of a sombrero, a spiral, dendrites, an eye, an igloo, etc.

In the most common cases, during the evaporation of a drop of coffee, blood, milk or ink for example, the final stain is systematically denser at the periphery than at the center. This phenomenon may be detrimental, for example during the preparation of ultra-clean micro-electronic surfaces or inkjet printing. However, it has also been applied with success to numerous situations, like medical diagnoses from bloodstains, DNA manipulation, or manufacturing of transparent conductive films.

We have shown that this coffee-stain effect, ubiquitous in everyday life as much as in nature and industry, can occur with pure water as well, provided the drop is deposited on a soluble substrate, for example a salt or sugar crystal. In these conditions, the peripheral deposit, stemming from the dissolved substrate, has the specificity of taking the original shape of a hollow rim. We have shown that this morphology was a consequence of a capillary centrifugal flow inside the drop. This phenomenon is especially striking, given that in the symmetrical case of evaporation of a drop of salty water on an inert substrate, the final deposit is completely unstructured.

The PhD student will investigate experimentally the evaporation of drops on a soluble substrate in two novel configurations. First, he/she will study the evaporation of a drop of colloidal suspension on a salt substrate. Preliminary experiments have shown that new unexpected deposit morphologies tend to appear, probably originating in the diffusio-phoresis experienced by the colloids in the concentration gradients of dissolved salt. The PhD student will benefit from the expertise of our research group regarding colloids, and will study the role of their nature, size, concentration, and surface charge on the spatial organization of the final deposit of the salt-colloid mixture.

Alongside this project, the PhD student will use the micro-fabrication platform of our institute to etch salt crystals, in order to constrain the shape of the drop and provoke the growth of new forms of deposit. Indeed, the crystalline growth-wetting coupling is a particularly fertile configuration for generating solids with novel and controlled shapes, as has been shown for example in the manufacturing of semi-conductor nanowires and carbon nanotubes.

The formation of the deposit is the consequence of numerous interacting mechanisms, among which evaporation and convection. To contribute to simplify the phenomenon, we suppress the effects of gravity, i.e., the convection of the liquid inside the drop and the convection of the vapor around the drop, in carrying out the majority of our experiments in the microgravity environment of the parabolic flights of the A310 Airbus of Novespace. For this, we have at our disposal an instrumented device already operational for the flights, built on a Novespace assembly. The PhD student will participate in several parabolic flight campaigns during the thesis, including the campaign preparation, the flights, and the data analysis.

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For more Information about the topics and the co-financial partner (found by the lab!); contact Directeur de thèse - jean.colombani@univ-lyon1.fr

Then, prepare a resume, a recent transcript and a reference letter from your M2 supervisor/ engineering school director and you will be ready to apply online  before March 13th, 2026 Midnight Paris time!