In the drying process of polymer solution droplets, we propose an experimental procedure for visualizing the solute concentration profile by combining the fluorescent microscopy with the lateral profile observation. We have conducted a dynamical observation of the transport process of the solute polymer toward the edge that causes the "coffee stain phenomenon". We have found that the polymer concentration increases sharply near the edge, while it remains almost constant in the central region until the last stage of drying. The method is useful to understand the dynamical process that occurs near the contact line.

We studied how the addition of surfactants alters the drying and film formation processes of polymer solution droplets with contact lines strongly fixed by bank structures. We found that even if the amount of surfactant is quite small, it drastically changes the final profile of the polymer film from a ringlike profile to a flat profile. This property is observed commonly, irrespective of the polymer concentration, droplet volume, and type of solvent. We conjecture that the inhomogeneous distribution of the surfactant caused by the outward capillary flow induces the Marangoni flow directed toward the center of the droplet, which suppresses the outward flow. The present phenomenon implies an effective method for controlling the profile of the polymer film in inkjet printing technologies.

International audienceWe studied the dynamics of water sessile droplets advancing on hydrophobic and visco-elasticpoly(styrene-butadiene-styrene)(SBS)–paraffin gel substrates at various inflation rates. During theadvancing process, the droplet contact line exhibits three different regimes of motions. When thecontact line advances at a high velocity, it moves continuously with a constant contact angle. As thecontact line slows down, it starts a stick-slip motion: the contact line is pinned at a certain position andthen suddenly slips forward. With further decrease of the velocity, the contact line stops the stick-slipmotion and continuously advances again. The observed threshold values for the transitions of thecontact line motions (continuous–stick-slip–continuous) indicate that the rheology of the gel drasticallyaffects the dynamics of liquid on its surface. We suggest that on visco-elastic gels, the moving contactline exhibits both aspects of wetting on elastic solids and wetting on viscous liquids depending on thecharacteristic frequency of the gel surface deformation. At an intermediate regime, the stick-slip motionof the contact line appears. We also propose a simple geometrical model in the stick-slip regime whichallows us to relate the jumps of the droplet radius to the jumps of the apparent contact angl