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.
For further improvements to the reliability and the white light quality of phosphor‐converted light‐emitting diodes (LEDs), it is imperative to understand how the compositional, optical, and thermal properties of the materials constituting the color‐conversion elements (CCEs) affect their respective thermal loads. By means of a combined optical and thermal simulation procedure, a comprehensive discussion is given on the underlying coherences of the absorption profile of the blue LED light, the phosphor concentration, the quantum efficiency of the phosphor, and the thermal conductivities of the CCEs. Some general strategies of material composition and design are deduced in order to minimize the thermal load of the CCEs, which is a prerequisite for correlated color temperature maintenance and long‐term material reliability of phosphor‐converted white LEDs.
In the growing application field of electrowetting (EW), reliable control of the wetting behavior by an applied voltage is required over a wide temperature range. Despite the rising interest of EW, only few data are reported in the literature on the EW behavior as a function of temperature. In this paper, we investigate the quasi-static EW response on one of the most widely used hydrophobic materials, Teflon AF1600, in a temperature range from 25 to 70$$\,^{\circ }\mathrm{C}$$
∘
C
. The contact angle versus voltage is analyzed to illustrate the EW behavior. The results are in good agreement with the friction-adsorption model, which explains the contact angle (CA) hysteresis by a temperature-independent friction-like force and a temperature-dependent contribution of liquid adsorption onto a dielectric surface. The EW-CAs show a small asymmetry with respect to the polarity of the applied voltage, which might be due to the temporary and reversible charge trapping on the dielectric layer. The results underline that the different effects of the temperature-independent friction force and the temperature-dependent adsorption need to be taken into account to predict and control the CA in any EW-based application scenario.
Graphical abstract
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