Influence of Substrate Conductivity on Circulation Reversal in Evaporating Drops

Overview

• [1] Ristenpart, W., Kim, P., Domingues, C., Wan, J, & Stone, H. Physical Review Letters. 99, 234502-1 - 234502-4 (2007).
• Keywords: Evaporation, Marangoni Effect, Contact Angle, Thermal Conduction, Surface Tension

Summary

Figure 1 from [1].

In this paper, Ristenpart et. al. study the flow patterns inside liquid drops on flat surfaces. Evaporation from a drop's surface leads to a thermal gradient and, thus, a gradient in surface tension. Displaying the Marangoni effect, fluid in a drop flows from regions with low surface tension to regions with high surface tension. The authors set out to answer: What is the flow pattern and direction? Using both theoretical and experimental approaches, the authors determine conditions which lead to two different flow directions.

Figure 1 is a diagram of a drop with important parameters labeled. $k_L$ and $k_S$ are the thermal conductivities of the liquid and the substrate respectively. $\theta_c$ labels the contact angle.

Figure 4 from [1].

The authors determine that there are two qualitatively different flow patterns. Both patterns have the same streamlines which are shown in figure 4 (a). However, the direction of the flow depends on the contact angle and the ratio of thermal conductivities $k_R=\frac{k_S}{k_L}$.

The researchers determined theoretically that for $k_R$ above two, the fluid flows radially inward across the free surface of the drop and radially outward over the surface of the substrate. Below $k_R$ of 1.45, the fluid flows in the opposite direction. In between $k_R$ of 1.45 and 2, the flow direction depends on the contact angle. Under all flow conditions, particles gather in the center of the drop (stagnation points of the Marangoni flow) leaving interesting patterns after the drop evaporates completely.

Ristenpart et. al. determine that $k_R^{crit}$ depends on the critical angle as follows:

$k_{R}^{crit}=tan(\theta_{c})cot\left(\frac{\theta_c}{2}+\frac{\theta_c^2}{\pi}\right)$.

The researchers check this equation by observing the flow in evaporating drops of four different fluids: methanol, ethanol, isopropanol, and chloroform. By observing suspended particles in the fluids, the scientists make qualitative observations of flow direction which coincide with the predictions of the $k_{R}^{crit}$-dependent flow.

Soft Matter Details

Experimental Techniques:

The scientists place the liquid drops on a clear substrate (PDMS) through which they can observe suspended particles with a microscope. The direct method of observing the direction of flow is limited to clear substrates. Looking at the flow from above might disturb the natural evaporation rate. The authors suspend polystyrene particles in three of their test fluids and silica particles in the forth (because polystyrene is not soluble in the forth fluid?).

Theoretical Techniques:

The researchers are interested in flow and particle deposition. Particularly, the flow near the contact line is of interest. This is similar to the case of a receding or advancing contact angle. Determining the flow right near the triple point where liquid, substrate, and air meet is very difficult. Maybe some of the methods in this paper could be used to study receding and advancing contact lines if they have not been already.

Critical Value:

Soft matter is full of critical values which signify the border between two qualitatively different regimes. In this case, the critical value separates flow directions completely reverse! The authors look at four fluids with different contact angles and thermal conductivities. An interesting experiment would be to dynamically vary either the contact angle or the thermal conductivity to see if you can observe the transition from flow in one direction to flow in the opposite direction. Also interesting would be to observe drops very close to $k_{R}^{crit}$.

Open Questions: At the end of their paper, the authors suggest that their methods could be used to analyze the influence of surfactant gradients on Marangoni flow in drops- particularly in water and biological systems.

This article is listed under charged substrates, but the conductivity here is actually thermal conductivity, not electrical conductivity. Using a charged substrate and charged particulates would add complexity to the system, but may also allow greater control over the deposition of particulates.