TN310e / SK 1/2
Effect of drop volume on static contact angles
Technical Note 310e
by
Spyridon Kranias, KRSS GmbH, France
Instruments
Drop Shape Analysis System DSA100
Abstract
Measurement of contact angles of sessile drops deposited on surfaces is a powerful analysis tool
that is widely used in several industry sectors, from cosmetics to semiconductors, coatings and food
industry, wettability studies, surface treatments control, adhesion performance prediction, etc.
The purpose of this short note is to treat one recurrent question users have when measuring static
contact angle : how big a drop size should I use?. Indeed, the drop volume is important as we
will see, in order to be sure that no size effect is influencing measured contact angles.
Surface and Contact Angle of Sessile drop is reflecting the interactions between
Drops the surface and the liquid. Measured contact
angles can then be used in conjunction with
When a drop is deposited onto a solid surface theories published in scientific literature to
there are three interfaces involved : solid/air, determine the surface free energy of solid,
solid/liquid and liquid/air. Once created, the characterizing solid surfaces regarding
drop will then instantly try to equilibrate the wettability or adhesion properties.
system by minimizing its energy at interfaces.
This is mainly achieved by surface The use of a goniometer allows precise
minimization between liquid and other determination of the contact angle value of a
phases. Practically, this state is reached when liquid on a surface along the triple-phase line,
forces acting on the triple line solid/liquid/air since the image analysis is done on a
are equilibrated. Young was the first in 1805 projection of the sessile drop.
to propose a description of the phenomena, Drop size study
by projecting the forces at the three-phase
contact line : Generally, when creating a drop at the tip of
a needle, the volume used is of the order of a
cos = SG SL couple of microliters. Then, in practice the
drop and the sample surface are brought in
contact. Once the drop touches the surface,
the needle and the surface are separated in
SG order to leave the drop reach equilibrium on
the surface. Depending on the affinity of a
SL liquid on a particular surface, the drop will
spread and wet the solid accordingly.
We can then say that the lower the contact In case the drop volume is too large, gravity
angle is, the more is the liquid wetting the force is higher than the capillary force that
surface. Equilibrium geometry of the liquid
KRSS GmbH Borsteler Chaussee 85-99a 22453 Hamburg
Tel.: +49 (40) 51 44 01 - 0 Fax: +49 (40) 51 44 01 - 98 eMail: info@kruss.de http://www.kruss.de
TN310e / SK 2/2
retains the liquid at the needle tip. Then the
drop falls onto the surface, leading to an Volume Drop radius Contact angle
additional kinetic energy injected to the (microliters) (mm) ()
system. This energy will of course have an
1 1.0 54.3
impact on the drop shape. It will lead to a
larger contact surface between drop and 3 1.5 54.5
solid, and thus stimulate an increased 5 1.75 53.9
apparent wetting effect, not representing the
10 2.3 53.8
true physical solid/liquid interactions of the
system.
As can be seen the drop radius is always
Let us now consider what happens when we
below the capillary length defined above, and
change liquid volumes for sessile drops.
same contact angles values are found
Experimentally it is observed that the drops
whatever the drop size.
shape changes from spherical shape to a
more flat one. This is due to gravity effect. To illustrate these results, below are shown
two pictures. The first represents a drop of
There is a length scale defined by the so-
1 microliter and the second a drop of
called capillary length -1, above which
10 microliters.
gravity cannot be neglected anymore
compared to capillary effects :
1 = [Equation 1]
g
This is bases on equilibrating Laplace pressure
/-1 and hydrostatic pressure g-1, at a
depth -1 inside a liquid of density. Summary
Calculating the -1 value for water gives a Contact angle measurements are becoming
value 2.7 mm. more and more widely used in several
Below a drop radius of 2.7 mm for water the industrial fields, either for control purposes or
dominant forces are capillary terms and R&D activity, like in coating applications,
gravity effects can be neglected. semiconductors, polymer development to
biological research.
In the following table contact angle values
measured using the Young-Laplace fitting Many users wonder what maximum drop size
method are displayed. Sessile drops of 1, 3, 5 they should use for static contact angle
measurements. We recall a contact angle is
-1 the expression of resulting molecular
interactions at interfaces between three
SG phases (liquid, air and solid) and gravity
effects must be avoided.
Based on the analysis above, we conclude
SL that for water the use of a drop volume of 1
up to 10 microliters has no influence on static
contact angle values.
and 10 microliters of water deposited on a For other liquids the optimum volume range
microscope glass plate were used. Mean varies with liquid density (see equation 1).
values given in the table were obtained using
at least three drops of each volume.
KRSS GmbH Borsteler Chaussee 85-99a 22453 Hamburg
Tel.: +49 (40) 51 44 01 - 0 Fax: +49 (40) 51 44 01 - 98 eMail: info@kruss.de http://www.kruss.de
