(186bf) The Drying of Particle-Laden Sessile Drops on Solid Substrate

It has been well established that when a small drop of dilute colloidal suspension is dried on a wetted solid substrate, colloidal particles are accumulated at the contact line and a typical drying pattern so called ?coffee ring' remains. The reason for the formation of such a pattern is the contact line pinning due to the particles suspended in the liquid and the movement of particles and liquid together toward the rim to replenish the liquid that evaporates much faster near the contact line [1] (This is called the Deegan flow). The previous studies on drop drying were mostly on the drying characteristics of very dilute suspension of colloidal particles dispersed in Newtonian liquid. In this study we investigated the drying of non-colloidal particle-laden fluids on glass substrates to understand the drying characteristics of particle-laden fluids in more detail

The liquid we examined was distilled water. Glass substrates with controlled wettability were prepared by adsorbing a self-assembled monolayer of octadecyltrichlorosilane (OTS) followed by the partial oxidization of CH3 at the end of OTS to COOH [2]. Depending on the exposure time to the UV-Ozone plasma, the contact angle of air-water interface varied from 10° to 110°. Depending on the wettability of the surface the wetted diameter of the drop varied from 1 ? 2 mm. We used several different kinds of spherical particles: 50nm alumina, 1 ? 3µm silica particles, 4 ? 8µm polymethylmetacrylate beads and 9 ? 13µm glass beads. We also prepared polystyrene (PS) particles of 500nm, 2, 4 and 5.7µm in diameter. The PS particles have almost uniform diameters. In suspending solid particles no surfactants were added to examine the purely hydrodynamic effect. The particle loading was 1%.

As expected the drying pattern of dilute colloidal suspension shows the typical coffee ring. However, in the case of large particles of 9 ? 13µm, no such pattern was observed and the particles move toward the center. There was a tendency of stronger inward movements for larger particles. The movement should not be caused by the depinning of the contact line since the wetted area and the final drying residue are the same. The aggregation or Marangoni effects were also excluded.

We mixed two different sized particles of 1 ? 3µm silica and 9 ? 13µm glass particles, dried on the solid surface and found that the smaller particles still moved to the contact line while the larger particles tended to move toward the center. The mixture of 50nm and 9 ? 13µm glass particles showed an almost uniform distribution. It was observed that the particles larger than 4µm partially protruded from the air-liquid surface and interacted with each other. It implies that there should be a competition between the force causing the outer movement and the capillary force inducing the inward movement. In the case of large particles, the flow inside the drop induced by the surface pulling appears to be much stronger than the Deegan flow.

It has been reported that the partial protruding or floatation of particles is possible due to the surface tension force [3]. Kralchevsky and [4] derived the force between two floated spheres of the same radius trapped at the air-liquid interface. Based on their theory it has been found that the particle movement toward the center will become important for particles larger than 5µm. We have found that this prediction is consistent with the experiments on the particles with different sizes. As the particle size becomes larger, the outward movement of particles becomes weaker and finally there is a strong inward movement (9 ? 13µm) as described earlier. The series of experiments suggest that the surface force due to the partial protruding of large particles plays a significant role in the movement of particles while being dried.

On less hydrophilic surfaces the drying patterns of water based suspension were different from those on hydrophilic surfaces. When the contact angle is 65°, as the drying process continues the contact line retreats and the particles accumulated near the contact line are dragged by the retreating contact line. The final drying pattern becomes smaller and the border becomes irregular.

References

[1] Deegan, R.D.; Bakajin, O.; Dupont, T.F.; Hueber, G.; nagel, S.R.; Witten, T.A. Nature 1997, 389, 827-829.

[2] Son, Y.S.; Kim, C.; Yang, D.H.; Ahn, D.J. Langmuir 2008, 24, 2900-2907.

[3] Rapacchietta A.V.; Newman, A.W. J. Coll. Interface Sci. 1977, 59, 555-567.

[4] Kralchevsky, P.A.; Nagayama, K. Adv. Coll. Interface Sci. 2000, 85, 145-192.