Wetting at High Temperature - Drops and Bubbles in Contact with Solid Surfaces - page 303

B. Wetting Typologies
Three main wetting categories can be recognised, in general: (a) Non-reactive (or
adsorptive) wetting, (b) Dissolutive wetting, (c) Reactive wetting.
1. Non-reactive (or Adsorptive) Wetting
Non-reactive wetting is a modality that only seldom can be invoked to describe
completely high temperature processes, and is verified if certain conditions are met.
Indeed, if the interplay of the three interfacial tensions allows a low contact angle
to be established (as computed by Eq. (1)), and if, at the same time, no dissolution
nor reactions occur between the solid and the liquid phases, the liquid spreads over
the solid which retains its original plane geometry. This happens, nearly always,
with low temperature systems, but also at high temperatures in the following cases:
(a) the solid and the liquid phases are mutually immiscible, (b) the liquid phase is
in chemical equilibrium with the solid (i.e., it is saturated of the solid elements),
(c) the temperature is not high enough to allow for a sufficiently fast diffusion of
atoms from the solid into the liquid phase.
It may also happen that the spreading process, i.e., the displacement as a function
of time of the triple line, is fast if compared to dissolution processes, so that the
liquid front moves fast on a flat, unmodified surface [37]. In this case the friction
at the triple line is the main factor governing the kinetics of the process. The study
of the kinetics of spreading is a very complex subject, because the laws governing
this phenomenon depend closely on the real physical phenomena occurring at the
solid-liquid-vapour interfaces, so that different laws can be applied to the different
possible cases which may happen in a single experiment. A systematic overview of
the kinetics of spreading is outside the scope of this paper; however, the interested
reader is referred to recent specialized papers [38-43].
Another process, which can be classified in the non-reactive category, is the
wetting 'assisted' by the adsorption, at the solid-liquid interface, of active metal
elements. This possible process is especially relevant when dealing with liquid-
metal/oxides systems, as the one found in metal/ceramic and ceramic/ceramic braz-
ing processes. It is usual, in these cases, to add some transition metal elements
(such as Ti, Zr, but also oxygen can play the same role) which migrate to the solid-
liquid interface adsorbing on it and, eventually, reacting with it. There is a great
deal of discussion on whether the wetting process is governed only by the adsorp-
tion process or whether the reaction(s) involving the 'active' element supply the
full driving force for spreading [19, 44]. The study of the kinetics of spreading
from the very first instant of solid-liquid contact can offer a means to understand
which phenomenon is prevalent in the specific process. However, especially at high
temperatures, the experimental part is very difficult, even if high-speed recording is
today at hand easily. What is still lacking, is the possibility to have access to an ' in-
situ ' analysis of the interface, which is usually studied after the drop solidification,
thus when the wetting process is finished and a large number of chemical reactions
and/or precipitation phenomena have occurred.