camera and strobe. Thus, each image of the disc is acquired at the same orientation;
a video generated from these still frames shows liquid moving about these appar-
ently still discs. A typical configuration of a spin stand can be seen in Fig.5.7.
Images acquired using a spin stand can be seen in Fig.5.24.
The most critical aspect for good operation of a spin stand is the correct
synchronisation of the motor and strobe. This usually incorporates two steps;
generating a signal from the motor as the spindle passes through a particular angular
location and filtering this signal before it reaches the camera/strobe.
5.1.1 Signal from the Motor
A stepper or servo-motor should be chosen which has sufficiently high a maximum
speed to overcome capillary effects and pump liquids about the disc. Typically
centrifugal microfluidics operates using a range of 5–30 Hz but, for some applica-
tions, such as blood processing, spin rates of above 60 Hz may be beneficial. The
resulting centrifugal force is also influenced by the radial location of the liquid
(volume) of the disc. Similarly, a motor should be chosen which can permit high
acceleration / deceleration of the disc as this is critical for some applications such as
centrifugo-pneumatic siphon valves [ 27 ].
The motor should also be chosen which can generate a digital signal once per
rotation. If a motor is otherwise suitable, an encoder can be purchased separately
and fitted to the motor. Alternatively, an optical sensor can be manufactured using a
low-cost LED and photodiode detector [ 28 ] (Fig.5.8).
Fig. 5.7 By inserting a piece of dissolvable film between two channels it is possible to create a
pressurised channel between the front of the advancing liquid and the dissolvable film, as the
frequency of rotation is increased the advancing liquid reduces the volume of air within the
channel thereby creating a counteracting pressure gradient that restricts the flow of liquid
(Image (a)). Only when the frequency of rotations increased to the burst frequency can the liquid
advance enough to come in contact with the dissolvable film (Image (b)). At this point the film
dissolves the pressure is released, thereby releasing the total volume of liquid to pass towards the
reaction chamber (Image (c) and (d))
5 The Centrifugal Microfluidic: Lab-on-a-Disc Platform 127