Influence of Particle Shape on Bending Rigidity of Colloidal Monolayer Membranes and Particle Deposition during Droplet Evaporation in Confined Geometries

We investigate the influence of particle shape on the bending rigidity of colloidal monolayer membranes (CMMs) and on evaporative processes associated with these membranes. Aqueous suspensions of colloidal particles are confined between glass plates and allowed to evaporate. Confinement creates ribbonlike air-water interfaces and facilitates measurement and characterization of CMM geometry during drying. Interestingly, interfacial buckling events occur during evaporation. Extension of the description of buckled elastic membranes to our quasi-2D geometry enables the determination of the ratio of CMM bending rigidity to its Young’s modulus. Bending rigidity increases with increasing particle anisotropy, and particle deposition during evaporation is strongly affected by membrane elastic properties. During drying, spheres are deposited heterogeneously, but ellipsoids are not. Apparently, increased bending rigidity reduces contact line bending and pinning and induces uniform deposition of ellipsoids. Surprisingly, suspensions of spheres doped with a small number of ellipsoids are also deposited uniformly.

Figure 1

(a) Cartoon depicting droplet evaporating in a confined geometry. The particle-populated air-water interface and three-phase contact lines are labeled. (b), (c) Examples of buckling events for confined drops containing anisotropic particles with $\alpha =1.2$ and 1.5 [(b),(c) respectively]. (d) Rim width, $d$ (solid line), is defined here in a magnified image of a buckled region as the interface full-width $25\mu \mathrm{m}$ from the vertex of the bent air-water interface (see dashed line). (e) $d$ is plotted versus the square root of the drop radius, $r$. (f) Ratio of the bending rigidity, $\kappa$, to the Young’s modulus, $E$, is plotted versus $\alpha$. (g) $\kappa$ versus $E$, where $E$ comes from previously reported measurements and calculations [18]. The line represents the best power law fit. (h) $\kappa$ versus $\alpha$.

Figure 2

Image of the final deposition of particles with major-minor diameter aspect ratio $\alpha =1.0,1.2,1.5,2.5,3.5$ [(a)–(e), respectively]. (f) The area fraction covered by particles after evaporation is complete, $f$, for suspensions of particles as a function of their aspect ratio $\alpha$.

Figure 3

(a) Image of a pinned region of the air-water interface ($\alpha =1.0$). When the pinned section does not “snap” off, it leaves behind a channel. (b) At a later time [$\sim 100\mathrm{s}$ after (c)], the channel extends, and more particles flow into it, producing a very heterogeneous deposition. (c) Image of the final deposition of particles with major-minor diameter aspect ratio $\alpha =1.0$. The box indicates the deposit left behind by the event depicted in (a) and (b). (d) Image of a colloidal monolayer near the three-phase contact line in a drop containing ellipsoids ($\alpha =3.5$). The three-phase contact line is labeled with a dashed line on the left side of the image. Particles are adsorbed on the air-water interface, forming a monolayer, as evidenced by the fact that particles become more out of focus from left to right as the air-water interface curves. A cartoon (f) shows a side view of the experimental image. (e) The fraction of area covered by particles, $f$, for suspensions of 200 nm diameter spheres doped with different amounts of ellipsoids, represented by the ellipsoid volume fraction, ${\varphi }_E$.