Temperature Dependence of Water Contact Angle on Teflon AF1600

Abstract

In this work, we investigate the change of contact angle (CA) of a water droplet during evaporation on a Teflon AF1600 surface in the temperature range between 20 and 80 °C under standard laboratory conditions. An almost constant initial CA and a significant increase of the stabilized CA have been observed. The results reveal a temperature-dependent CA change, mainly due to water adsorption on the solid surface. Soaking experiments indicate that besides adsorption, a temperature-independent friction-like force contributes to the pinning of triple-line and therefore to the CA change. We propose an adsorption coverage parameter and a friction-like force to describe the CA change. Furthermore, we describe a reproducible process to produce smooth and homogeneous Teflon AF1600 thin films, minimizing the influence of roughness and local heterogeneity on the CA.

Figure 1

(a) Optical micrograph of a Teflon AF1600 surface: pinholes are observed after the third baking step with 330 °C. (b) A typical AFM image of Teflon AF1600 surface fabricated by two baking temperatures. The mean root square roughness is 0.82 nm.

Figure 2

Illustration of the experimental setup. (A) Front view, (B) top view, and (C) front view of the Al cover with glass windows and a small opening with 2 mm diameter at the top for inserting dosing needle. A Peltier element under the Al plate, two temperature sensors, and a temperature controller guarantee a homogeneous temperature contribution. For water droplet evaporation process, the Al cover (black dash line in A) is removed to achieve atmospheric humidity and allowing droplet evaporation. For soaking experiments to investigate the effect of water adsorption on WCA, the groove integrated in the Al base plate is filled with water to create an atmosphere of saturated humidity (blue dash line in A and details seen in B), and the Al cover is placed to maintain the saturated humidity inside the chamber.

Figure 3

WCA, contact base diameter, and droplet volume evolution due to droplet evaporation over time at (a) T = 20 °C, (b) T = 40 °C, (c) T = 60 °C, (d) T = 80 °C. The solid lines represent the fitted curves in the CCR and CCA regime. In the CCR regime, the contact base area stays constant and the WCA decreases. In the CCA regime, however, the contact base area decreases and the WCA stays constant. Representative droplet images of (e) initial WCA θ_init at T = 20 °C, (f) stabilized WCA θ_sta at T = 20 °C, (g) initial WCA θ_init at T = 80 °C, (H) stabilized WCA θ_sta at T = 80 °C.

Figure 4

Measurement results for θ_init and θ_sta of DI-water on Teflon AF1600 over temperature.

Figure 5

Force equilibrium at the triple-line. Interface tensions γ_la, γ_sl, γ_sa and a friction-like force F_f contribute to the force equilibrium.

Figure 6

Results of effective solid surface tension change γ_la·Δcos θ from evaporation experiments, the solid–liquid interface tension difference Δγ_sl(θ_sta, θ_init) calculated from Gibbs adsorption equation, the friction-like force F_f-max evaluated from soaking and evaporation experiments. All results are plotted against temperatures.