Patterned surfaces in the drying of films composed of water, polymer, and alcohol

A study of the complex drying dynamics of polymeric mixtures with optical microscopy and gravimetric measurement is presented. Droplet formation is observed, followed by a collapse that leads to the residual craters in the dried film. The process is followed in situ under well-defined temperature and hygrometric conditions to determine the origin and nature of these droplets and craters. The drying process is usually completed within 1 h. The observations are explained using a simple diffusion model based on experimental results collected from mass and optical measurements as well as Raman confocal microspectrometry. Although the specific polymeric mixtures used here are of interest to the cosmetic industry, the general conclusions reached can apply to other polymeric aqueous solutions with applications to commercial and artistic painting.

Figure 1

Typical binocular microscope images of the films during drying on a G200 glass substrate: (a) Surface of the film center when droplets appear at $t=18\min$; (b) final dried film at $t=50\min$ with craters left by the droplets. The three snapshots (c)–(e) at the bottom show a closeup (illustrated schematically above) of the drying region that exhibits a coalescence of two droplets between $t=18$ and $t=23\min$, respectively, and the final craters with the trace of the two initial droplets.

Figure 2

Evolution with time of the relative film height $h\left(t\right)/h\left(0\right)$ for an aqueous film made with a 3$\%$ initial mass fraction of polymer and with different XOH initial mass fractions red (circle): $w_{\mathrm{XOH}}=0.4$; yellow (square): 0.6; magenta (pentagon): 0.8; green (star): 1.0; cyan (cross): 1.2; blue (continuous line): 1.8; and black (dashed line): 2.4$\%$. The relative humidity and temperature are fixed at $42\pm 5\%$ and $20\pm 2{}^{\circ }\mathrm{C}$, respectively.

Figure 3

Confocal Raman microspectroscopy of a droplet: The Raman peaks are only those corresponding to top signal: the polymer and bottom signal: the substrate. Bottom of the picture: the confocal microspectroscopy image shows only the presence of the polymer above the substrate.

Figure 4

Raman microspectroscopy of the crater left after the drying of the same droplet analyzed in Fig. 3. In that case, the XOH signal is detected in the crater in addition to the polymer one. The substrate signal is not shown for clarity but is identical to that of Fig. 3.

Figure 5

Optical microscope images of the drying of an XOH-water drop. (a) First droplets appear near the contact line; (b) coalescence of droplets and deposition of XOH around the droplets due to water evaporation. Upper inset: a pure water drop shows no droplet formation during evaporation. Lower inset: closeup showing the XOH shell around the water droplet.

Figure 6

Time of droplet formation as a function of $w_{\mathrm{XOH}}$. The circles: data are in good agreement with the solid line: diffusion model as opposed to a dashed line: simple dilution model. The squares show the time of formation calculated with the diffusion model but using the measured evaporation rate for each case.

Figure 7

The XOH mass fraction profiles near the interface for different times ($t=0$, 13, 26, 39, and 52 s for increasing curves, respectively). The profiles are scaled vertically by the film height $h\left(t\right)$ so that the interface is always located at $z/h\left(t\right)=1$.

Figure 8

Schematic evolution of the droplets to craters. (a) Formation of droplets in the film very close to the free surface. The dashed line indicates the initial level of the film; (b) growth of the drops by coalescence; (c) leveling of the droplets at the surface; (d) formation of the craters.