Biomedical Engineering Reference
In-Depth Information
Figure 4.39
Experimental results for the dynamic contact angle versus the capillary number (dots)
and Tanner relation (continuous line).
or
1
3
ACa
θ θ
-
»
(4.58)
d
s
2
θ
s
Note that the capillary number is signed. Equation (4.58) shows that
q
d
-
q
s
is of
the sign of
Ca
; the values of the advancing and receding contact angles are then
given by
ACa
1
3
θ θ
»
+
a
s
2
θ
s
(4.59)
ACa
1
3
θ θ
»
-
d
s
2
θ
s
confirming the experimental observation that an advancing contact angle is larger
than the static contact angle and a receding contact angle is smaller than the static
contact angle (Figure 4.40).
4.3.4 Hysteresis of the Static Contact Angle
Young's law predicts the value of the static contact angle as a function of the sur-
face energy of the different materials (liquid plug, surrounding liquid and solid
Figure 4.40
Sketch of advancing, static, and receding contact angles. The advancing contact angle
is larger than the static contact angle, which is in turn larger than the receding contact angle.
