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8.3
Advancing mode
This mode captures a video sequence showing the dynamic interaction
between a liquid and a surface as the liquid spreads over the dry surface
(“wets”) and then retracts from the wet surface (“de-wets”). This interaction,
described as “wetting hysteresis”, is characterized by the highest (“advancing”)
and lowest (“receding”) contact angles. This test is based on a reversible
pump flow. The optional PG Dosing Unit is a programmable dispenser
suitable for this task, but any stand-alone pump with reversible flow can
be used for the test. With a little practice it is even possible to use a regular
handheld syringe to run this test manually.
In some applications it is of interest to characterize the “wetting hysteresis” of
a surface.
The purpose is to determine the “highest” contact angle as the liquid spreads
across the dry surface and the “lowest” contact angle as the liquid retracts
from the wet area on the surface. These two contact angles, referred to as the
advancing/receding contact angles describe the “wetting hysteresis”.
8.3.1 Parameters
Liquid
It has no influence in the results.
Sample Rate
It’s the number of frame per second that the camera will collect.
8.3.2 Automatic measurement using a reversible pump
The dispensing tip must be equipped with a fine capillary tube inserted
into the PTFE tubing (P/N 860309). In addition, the distance between the
dispensing tip and the test surface must be short enough to make sure the
droplet remains in contact with the tip during the test sequence, as the test
otherwise will be terminated.
Click on Measure to start a new test.
Pump out a droplet, which must contact the surface within 30 seconds to
trigger the test.
The pump should now pump out liquid until the droplet has spread
sufficiently across the dry surface (the “advancing” phase). Next the pump
flow should be reversed so the liquid retracts from the wet surface (the
“receding” phase).
The test is automatically aborted when the contact is lost between the tip
and the surface.
8.3.3 Automatic measurement without using a reversible pump
When a reversible pump is not available a liquid droplet can be applied
manually from a syringe with the tubing inserted directly into the drop
applicator. Alternatively, a pendant droplet of suitable size can be pumped
out and the droplet is slowly pushed towards the surface after the test has
been initiated. As the dispensing tip is pushed well into the sessile drop, its
base will expand across the surface creating an advancing contact angle. As
the dispensing tip is slowly retracted from the sessile drop, its volume will
shrink and a receding contact angle will appear on the condition the droplet
base is reduced.
