In this study, the drying of particle-laden sessile drops of non-Newtonian fluid was investigated experimentally and compared with the drying of particle-laden Newtonian drops. We prepared aqueous solutions of 0.1% polyethylene oxide (PEO) and 62.5 – 1000ppm xanthan gum (XG). The PEO solution shows virtually the same viscosity with shear rate while the xanthan gum solutions show severe shear thinning viscosity. As the particle we used glass spheres of 9 – 13 µ in diameter. The drying of droplet of approximately one nanoliter was done on glass substrates with controlled wettability prepared by adsorbing a self-assembled monolayer of octadecyltrichlorosilane (OTS) followed by the partial oxidization of CH3 at the end of OTS to COOH [1].
In the case a water drop the non-colloidal particles move toward the center contrary to the outward movement of colloidal particles as reported by Deegan et al.[2] It appears that the particles trapped at the air-liquid interface strongly affect the flow and the movement of particles. The drying characteristics of particle-laden drops of PEO solution were different from the case of water. When a drop containing particles is spread on the solid surface, the glass beads are well distributed. As drying begins the beads move toward the center until when one half of the fluid is dried. Then some of the beads begin to move outward rapidly in a time scale of 10s (The total drying time is about 300s.). In the final drying pattern, the particles are almost uniformly distributed. It is estimated that the surface of the drying drop has much higher polymer concentration than the average polymer concentration and the viscous force due to the Deegan flow overcomes the surface force that holds the particles at the center. The elastic effect cannot be important, since the shear rate is approximately in the range of 0.001 - 0.01s-1 except at the final stage of drying.
In the case of a 1000ppm XG solution, particles move toward the contact line. Since the drying occurs under very low shear conditions, the shear thinning property of xanthan solution that shows for many decades of shear rate cannot be responsible directly because the Deegan flow occurs only under low shear rate regimes. Rather, the high viscosity of the solution at low shear rate appears to cause the movement. Since the average shear rate has an order of 10-3 s-1 and the shear rate at the position of 1 diameter away from the contact line is estimated to be 0.3s-1, the viscosity of 1000ppm XG solution is 200 times larger than the water viscosity, rendering the drag force much larger than the drag caused by water while keeping almost the same magnitudes in the surface tension force and drying rate because the low concentration of xanthan cannot change the vapor pressure. In XG solutions of lower concentrations, the outward movement becomes weaker. In 125ppm solutions, the particle distribution becomes almost uniform and then the particles barely move outward in 62.5ppm solutions. The series of experiments with xanthan solutions confirm that the large shear stress even at low shear rate causes the outward motion.
The present experimental results imply that, by controlling the drag force and the surface tension force, we can control the drying pattern of sessile drops of particle-laden fluids on solid surfaces. This can be done by adding a small amount of macromolecules which bring high viscosity change with a negligible change in vapor pressure not to lower the evaporation rate. Rodlike molecules such as xanthan gum will be more effective than flexible polymers.
[1] Son, Y.S.; Kim, C.; Yang, D.H.; Ahn, D.J. Langmuir 2008, 24, 2900-2907
[2] Deegan, R.D.; Bakajin, O.; Dupont, T.F.; Hueber, G.; nagel, S.R.; Witten, T.A. Nature 1997, 389, 827-829