The other immediate result from table 2 is the significant
contribution of viscous heating (stirring) to the SAR which
reduces with viscosity. The values in wax indicate that
30–40% of the SAR is generated by viscous heating. The
dynamic effects are complex because they are determined by
hydrodynamic parameters. Reference to table 1 shows that
sample B has the largest hydrodynamic size which accounts
for the larger contribution of stirring to theSARfor this sample.
One clear conclusion that can be drawn form these results is
that measurements made in water based colloids will not be
representative of heating effects in vivo.
Table 2 also shows the measured values of SAR together
with calculated values of hysteresis heating from equation
(11). The results for the samples dispersed in wax can
be compared with the calculated values for hysteresis loss
because in the solid matrix and at a frequency of 111.5 kHz
only hysteresis heating can occur. On first inspection the
agreement between the measured and calculated values is quite
poor. However the integral across the active region of the
particle size distribution is very sensitive to the value of K
used (K = 3.0 × 105 ergs cm−3) which affects the values of
Vp(0) and Vp(H) in equation (11). Depending where these
parameters lie on the particle size distribution large variations
are possible. This problem is due to the extreme sensitivity
of the value of the shape anisotropy constant (Ks) to particle
elongation at low aspect ratios (r). For example a change in r
from 1.8 to 3.0 varies Ks by 70% leading to a reduction in the
SAR value for sample C by a factor 7. Inspection of the TEM
image for this sample shown in figure 2(c) shows that there
is a significant fraction of the particles having aspect ratios of
3.0 or greater with the majority having aspect ratios of greater
than the value of 1.8 used to obtain the data in table 2. Hence
numerical agreement between calculated and measured values
would not be expected. This is particularly true for sample C
with Dm = 15.2 nm because at this size the colloid is unstable
and sedimentation was seen to occur. This accounts for the