Molecular Desorption by a Moving Contact Line

The interaction of the contact line with topographical or chemical defects at the nanometer scale sets the macroscopic wetting properties of a liquid on a solid substrate. Based on specific atomic force microscopy (AFM) experiments, we demonstrate that molecules physically sorbed on a surface are removed by a dynamic contact line. The mechanism of molecules desorption is directly determined by the capillary force exerted at the contact line on the molecules. We also emphasize the potential of AFM to clearly decorrelate the effects of topographical and chemical defects and monitor, with a subsecond time resolution, the dynamics of molecules adsorption on a surface.

Figure 1

Scanning electron microscopy images of (a) a NaugaNeedle tip and (b) a silicon tip. (c) Scheme of the setup. Inset: enlargement of the nanomeniscus.

Figure 2

Overlaid force curves obtained during successive dipping cycles (from dark blue to red) of a NaugaNeedle tip ($R\sim 430\mathrm{nm}$) in tetraethylene glycol at $V=50\mu \mathrm{m}/\mathrm{s}$. Inset: evolution of the force measured on a topographical defect of the advancing branch as a function of the number of cycles $N$.

Figure 3

Evolution of the characteristic force of one peak with the number of wetting cycles for a NaugaNeedle tip ($R\sim 430\mathrm{nm}$) in tetraethylene glycol at $V=100\mu \mathrm{m}/\mathrm{s}$. The delay time $t_p$ between series is increased from 1 to 30 s. Inset: relative change of the force $\epsilon$ as a function of the delay time $t_p$ for the advancing and receding branches. The error bars correspond to a 95% confidence interval.

Figure 4

Force curves before and after 1000 partial dipping for two different homemade tips ($R\sim 60\mathrm{nm}$) and $V=50\mu \mathrm{m}/\mathrm{s}$ with glycerol. (a) Only the extremity of the tip is dipped into the liquid. (b) Only the intermediate zone between $h\sim 1\mu \mathrm{m}$ and $h\sim 2\mu \mathrm{m}$ is swept by the contact line. The dotted line indicates the limits of these zones.