Experimental Rig

The experimental rig is used to test the ability of the particle separator fabricated using a printed PZT transducer to separate clear fluid from the particle/fluid mixture. Depending on the operating conditions used in experiment, varying degrees of particle separation are observed and are an indication of the true behaviour of particles within the device.

Apparatus

The device is designed to operate in a flow-through mode, therefore for the purposes of experiment a convenient means of connecting pumps and other fluid network elements is facilitated using a manifold. Figure 6.4 shows the separator device mounted on an aluminium manifold which contains ports aligned with the inlet and outlet ducts in the silicon. These ports lead to flexible tube connectors which allows pumps to be easily connected to the appropriate duct. The total dead volume of the system is approximately 1ml.

To control the flow rates through the device ducts, peristaltic pumps are used which are able to deliver low flow rates, but typically issue a pulsed flow. The pumps chosen (Watson Marlow 403U/L) are designed to minimise this fluid pulsing effect. Figure 6.5 illustrates schematically the experimental rig and shows that a pump is connected to each outlet duct, corresponding to flow ratesQ1andQ2.

The schematic also shows other parameters used to ensure the correct setting of operating condi- tions, notably the drive frequency of the transducerf and voltage amplitudeVtacross the trans- ducer. These parameters are set using a function generator (TTi TG1304 programmable function generator) feeding to a 50dB amplifier (EIN Model 240L RF power amplifier) then to the trans- ducer. To measure the transducer voltageVt, an oscilloscope (Hameg) is connected in parallel with the transducer.

Figure 6.4: Photograph of micro-engineered particle separator mounted on aluminium manifold.

Figure 6.5: Schematic of experimental rig showing flow rates and transducer drive operating pa- rameters.

To prepare the fluid/particle inlet sample, a balance (Mettler) is used to measure a quantity of fluid into which a pipette is used to dispense a particle concentrate. This ensures that the concentration of samples used for each experimental run are consistent. The particles used in experiment are 1µm latex particles (Polysciences Inc.).

The particle concentration of the outlets after each experimental run is measured using a turbidity sensor (Honeywell) giving valuesC1andC2. To account for particles caught within the fluid net- work, the inlet concentrationC0 is calculated using weighted values ofC1andC2. The turbidity sensor is not connected directly to the fluid network which would facilitate a continuous measure- ment, but instead measures concentration on fluid samples. This is necessary as the dead volume of the turbidity sensor housing is large, which at the low flow rates used in experiments would allow diffusion and settling of particles to take place, affecting concentration measurements. Such effects are otherwise not a significant problem in the experimental rig as fluid connections and tube diameters are kept to a minimum, ensuring higher flow velocities and minimum sedimenta- tion of particles. Reduced dead volume also reduces experimental time and the time taken for the fluid/particle sample to pass through the fluid network.

Control and Measurement of Parameters

Various parameters are controlled and recorded during the experimental process and include the following:

Input voltage,Vin,Vt: The peak-peak voltage input from the signal generator to the amplifier, Vin, is selected as an operating condition and relates to the acoustic energy density within the device and therefore the radiation force. The corresponding transducer voltage Vt is occasionally noted, used principally to check the predicted voltage characteristics.

Frequency,f: The frequency of the input signal to the transducer is again controlled via the signal generator. The resonant frequency of the system is determined by first using a com- bination of measured and simulated impedance data then, more accurately, when connected to the experimental rig by adjusting the frequency until a voltage minimum across the trans- ducer is found (impedance minimum).

Flow rates,Qt,Q1,Q2: For the peristaltic pumps used in the experiments, rotational speed of the pump is selected via a key pad. The flow rate that each pump delivers is proportional to pump speed, therefore, calibration of the pumps provides a simple linear correlation between

flow rate and pump speed, achieved by measuring the time to deliver a known volume of fluid. Calibration is also made for a series of different tube bore diameters, where small diameter tubes deliver small volume flow rates and vice versa. The flow rates are repeatable within measurement error, therefore these calibration results are used throughout the entire experiment. Pumps are connected to the outlet ducts of the device, therefore the pump flow rates correspond to outlet flow ratesQ1andQ2with the sum corresponding to the total flow rate seen within the main channel of the device,Qtotal=Qt=Q1+Q2.

Concentration,C0,C1,C2: The turbidity sensor is used to provide particle concentration mea- surements of the inletC0and outletsC1andC2. The turbidity sensor detects a combination of reflected and transmitted light on a fluid sample and relays a voltage signal directly to a pc. Calibration between the data displayed on the pc and particle concentration is required. For latex particles supplied in a diluted form, this is achieved using a haemocytometer to determine particle numbers within a known fluid volume together with a turbidity reading, and making a series of measurements for a range of sample concentrations. Alternatively, for initially dry particles such as yeast, a sample of particles are weighed to which a known quantity of fluid is added and measurements made for further dilution of the sample. Tur- bidity sensor calibration curves are shown on figure 6.6 where it can be seen that calibration is required for each particle type and size used in experiments. Also, it is noted that the relationship between concentration and the turbidity reading is highly non-linear.

For each data point recorded in the following section, the device is operated using the appropriate voltage and flow rate conditions and at least 5ml of sample is collected from each outlet. This provides enough fluid to enable the turbidity sensor to make a repeatable measurement and is drawn into the sensor using a syringe. Flushing of the sensor is then required to ensure that no particles remain adhered to the sensor surfaces and affecting subsequent measurements.