Document 4207Q258 V3.00 January 2018
Page 27 of 30
Chirascan Couette Cell LD Accessory User Manual
3.6
Other experimental considerations
The considerations for sample concentration for an LD experiment are the same as those for a circular
dichroism (CD) experiment. The total absorbance, i.e. of the sample and buffer, should be below about 2 AU
at all wavelengths, and the maximum signal-to-noise ratio occurs at an absorbance of about 0.9 AU [1]. But a
typical LD signal is much stronger than a typical CD signal, often by several orders of magnitude, and it is often
possible to obtain good LD data at lower concentrations than would be used for a CD experiment. In the
Couette Cell LD accessory, the absorbance and LD can be measured simultaneously.
There are various points to look out for. The flow should be laminar and circular, which means that each point
in the sample follows a circular path. At high shear rates the flow may be laminar, but not circular. One such
flow regime involves the formation of secondary flows known as Taylor vortices [2, 3], in which each point in
the fluid follows a closed helical path. Normally these vortices are only seen when the inner cylinder rotates,
which is one reason why the Applied Photophysics Couette cell is designed with the outer cylinder as the rotor.
More importantly, at high shear rates the flow regime may become turbulent. The transition from laminar to
turbulent flow is described by the Reynolds number, Re, which is the ratio of inertial to viscous forces.
i.e.
Re =
γ̇
R
2
/
, where
is the density and
is the viscosity of the liquid
The value for the critical Reynolds number for the Couette system is given in some textbooks as greater than
50,000 [4], but this is a value calculated for an ideal system of perfect components with no end effects and a
completely homogenous sample. In reality, like the free energy it represents a maximum value, and for real
systems turbulence is likely to occur at a lower value of Re. However, it has been found for the Applied
Photophysics Couette system that flow is usually stable for aqueous systems up to the maximum shear rate
of 10,200 s
-1
, giving a critical Reynolds number greater than about 1,200.
Another consideration is the effect of shear heating on the sample temperature. The shear energy, with units
of J m
-3
s
-1
, is given by:
E
shear
=
γ̇
2
/2
Under adiabatic conditions, for an aqueous sample with a viscosity of about 1 x 10
-3
Pa s, a heat capacity of
about 4,200 J kg
-1
K
-1
and a density of about 1000 kg m
-3
at 20°C, the temperature rise would be only about
10
-2
°C per second at a shear rate of 10,000 s
-1
. The conditions are of course not perfectly adiabatic, but since
a spectrum usually takes several minutes to acquire, and multiple spectra at each shear rate are normally
taken, without active cooling the temperature increase could be unacceptably high.
