Contact Angle Measurements over Macro, Micro and Nano Dimensioned Systems - Drops and Bubbles in Contact with Solid Surfaces - page 19

H. Microscopic and Nanoscopic Contact Angles
The brief and necessarily incomplete remind to the main Theories that nowadays
hold in the wetting science community has been here reported to highlight the
nontrivial issues that affect 'usual' contact angle measurements. When moving
down to micro and nano dimensioned liquid-solid systems experimental evalua-
tions become more 'unusual'. This fact comes directly from two important issues:
(1) At smaller dimensions defects, whatever are, change their status in the impor-
tance rank gained in the scenario of contact angle measurements, moving from
simple disturbing tasks to fundamental problems.
(2) Experimental results appear often difficult to be framed in some classic theoret-
ical schemes. Today it is in fact possible to directly and experimentally evaluate
the influences of what were simply known as boundary conditions , greatly re-
ducing the room available for generic theoretical speculations.
Microfluidics studies finalized to MEMS, Lab-on-chip and thin films definitely
indicated, as first achievement, that contact angles are not size-independent and
scale factors are tremendously important in wetting science [146-158].
A further goal has to become aware that a strict application of the YE, that for-
mally states that a liquid may wet ( θ< 90 deg) or not wet ( θ> 90 deg) a given
solid surface, at smaller dimensions has to be wisely applied. At micro and nano
scale in fact drops and thin films of the same liquid are often found coexisting at
the same time over the solid substrate. Both these facts formally hardly fit with pure
YE predictions and therefore they arose some fundamental theoretical issues con-
cerning the effectiveness of portability of YE to micro and nano dimensions. Indeed
these problems are not new. They were already known in their importance since the
very beginning of the wetting science. For a quite long time they have been left
somehow aside, mainly due to the macroscopic efficacy of the visual approach and
also because of the presumption that the macroscopic wetting knowledge would
also fit micro and nano systems. The contributes of the last decades unequivocally
demonstrated that a correct application of the YE is far to be a simple task.
Now it is possible by the experimental point of view to reproduce the ideal solid-
liquid systems that YE statements prescribes. In fact at nano and micro dimensions
gravity effects can be effectively considered of secondary importance if not com-
pletely negligible while artificial solid supports may be considered perfect. More-
over the liquid-solid systems put under observation have a sensible tendency to
achieve in a very fast way a stable thermodynamic condition. So for example, even-
tual Marangoni effects due to surface tension gradients [159] are greatly reduced
while on the other hand volatility phenomena due to the specific vapor tension of
liquids become very important. Besides the appearance of some specific technical
problems it can be affirmed that small-sized experimental activities greatly helped
to reduce several indetermination causes since long time affecting wetting issues.
A great number of papers concerning the microscopic behavior of solid-liquid sys-