W. Richard Bowen and Nidal Hilal 4

9.6 MESOSCALE ExPERIMENTAL STUdIES 267
sphere and the thickness of the fluid layer are such that only the underside
of the colloid is wetted. In frame 01, meniscus effects can be seen,
when the surfaces are separated, a microscale liquid bridge is formed,
which persists at large separations. The liquid bridge thins until the filament
fails between frames 07 and 08, leaving residual liquid on both the
probe and the lower surface. The evolution of the filament profile closely
resembles the macroscale CaBER results shown in Figure 9.7.
Figure 9.13 shows the behaviour of a thin liquid film during the ‘highspeed’
separation of the surfaces. In this instance, the high rates of separation
and the high stress invoked in the fluid initially resist separation of the surfaces.
Ultimately, the confined liquid layer appears to yield spontaneously
(frame 02), forming a residual filament (initially 10-�m long and 1.3-�m in
diameter created at an apparent rate � 5000 �m s�1 . The liquid filament then
thins and breaks, residual liquid can be observed in frame 04.
The adaptation of an AFM to act as a microrheometer has several
potential benefits. In conventional rheometry, the generation of high
rates of deformation is difficult, particularly so in extensional flow techniques
where the defining strain rate is approximated by the relationship
�ε � . U/h. The uniaxial rate of extension of cylindrical filaments between
separating surfaces is equal to the ratio of the separation velocity U and
the instantaneous length of the filament h.
FIguRE 9.13 The high-speed separation of surfaces bridged by a thin film of silicon oil
(� � 12 Pa s). Frame interval is 2 m s.