Encyclopedia of Smart Materials (Vols 1 and 2) - M. Schwartz (2002) Episode 9 ppt

P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-P-DRV January 18, 2002 21:0 762 PEST CONTROL APPLICATIONS Influence on wild norway rat population Pre treatment Post treatment Treatment 1 0.8 0.6 Index of treated results vs total 0.4 0.2 0 −2642 Days 0 Figure 1. The influence of ultrasonic noise on the Norway rat population. Figure 1 shows the effect of treatment on the Norway rat. Figure 2 shows the effect of the treatment on wild house mice. The influence on both populations is most significant for food consumption. The tracking activity of the wild house mice is not heavily influenced by the ultrasonic effect. The rodents’ hearing was checked before and after the testing. Only rodents that had good hearing were selected for the study. It has been postulated that the rodents might eventually become accustomed to the noise, but this was not the case. There were instances where rodents were not influenced, but this was due to hearing loss. The sound patterns (frequency and amplitude) of four of the pace electronic pest repeller units were measured. 1 0 −20 2 4 6 Days 81012 0.2 0.4 0.6 Index of treated results vs total 0.8 Pre treatment Treatment Post treatment Influence on wild housemice population Figure 2. The influence of ultrasonictreatment on the wild house mice population. The primary source of total sound output was at 40 kHz and above. The sound output dropped slightly at 31.5 kHz. Sound output below 20 kHz was negligible. CAVITATION AS A DESTRUCTOR Piezoceramic elements are commonly used to induce cavitation in fluids in biological applications for scaling instruments, but killing microorganisms is normally done by high-temperature sterilization. The erosive effect of cavitation is what is useful in removing a variety of type of scales. Cavitation is caused when the localized pressure drops below the fluid vapor pressure. This results in cavitating bubbles. The collapse of cavitating bubbles is accompanied by a rapid release of energy. It is the collapse of the cavitating bubbles that is used to destroy microorganisms. It is not clear whether the microorganism population is immediately killed by the bubble collapse, or if the population is just weakened enough to limit its viability. The generation of cavitation is limited to areas fairly close to the pressure/sound source. Cavitation can be applied to a large volume of fluid either by moving the source through the fluid or by moving the fluid past the source. The application described here moves the fluid past the source by pumping the volume through tubing to ensure fairly even exposure of the liquid to the pressure field. This does not sterilize the fluid, but it does eliminate a significant portion of the microorganism population. The biological test results available indicate that cavitation does significantly reduce the population in both water and diesel fuel, butthe effect varies for the typesofmicroor- ganisms tested. The population reduction is of the order of 50%. It is expected that piezoceramically induced cavitation could be used to reduce zebra mussel population in nuclear reactor water intake tubes by interfering with the zebra mussels during an early stage of their development, such as the larval stage. The specific engineering design that follows was based on controlling microbial growth in military marine diesel tanks. These populations are currently controlled by “good housekeeping” of ships’ tanks and by using environmen- tally harmful biocides. If an ultrasonic cavitation system were to be installed on a ship, it would be necessary to include an antinoise system to cancel the ultrasonic sound that creates the cavitation. This would be needed to mini- mize the likelihood that the vessel would be detected by unfriendly ships. Engineering Application/Design The cavitation of a fluid is induced when local pressure drops below its vapor pressure. It involves the release of relatively small amounts of energy (compared to boiling), so that though there is a temperature change in the fluid; it is small (of the order of 1–2 ◦ C, depending on exposure time and volume). One of thewell-known side effects of cavitation is itsero- sive effects on materials. This presents a practical problem P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-P-DRV January 18, 2002 21:0 PEST CONTROL APPLICATIONS 763 Driver electronics Cavitation bubbles Inner tube Working medium Piezoceramic rings Transmission medium Figure 3. Schematic of cavitation concept. in trying to use cavitation. The components used to cause the cavitation need specialconsiderationto survive the erosive environment. A general requirement for pest control is that it is needed for large volumes. Cavitation is a fairly local effect. To apply it to a large liquid volume, the fluid must be brought into a fairly local range. One way of achieving this is a flow-through system. The liquid is pumped through tubesthat are exposed to thecavitating field. Such an arrangement could involve expenditures of significant amounts of power. A flow-through configuration was studied analytically to achieve maximum fluid cavitation at minimum power consumption. The particular system modeled was based on a two-fluid system to avoid the electrode erosion that would be induced by cavitation. Figure 3 shows the con- ceptual arrangement. The fluid immediately adjacent to the electrodes is pressurized to eliminate cavitation. This fluid is used to transmit energy through a thin-walled pipe (stainless steel) into the fluid that contains the microorganism. The analytical model of the system was a piezo- dynamic field modeled by using finite elements. It is based on a finite element formulation of the piezoceramic elements, the physical piping structure, a liquid transmission medium, and the sound pressure field experienced by the microorganism-borne fluid (either water or diesel fuel). The model was then test verified before applying it to a specific design. Finite Element Formulation. The finite element method is an analytic technique for solving general field problems. It offers a number of advantages over competing methods. It can handle arbitrary geometries and both static and dynamic problems. It uses matrix numerical methods for which very efficient and general algorithms have been developed. The special purpose FE formulation developed to handle both the fluid characteristics and the electrical input (as well as the normalstructuralcharacteristics) was based on the principles of the FE method in (2). The code modeled the structural behavior of the elements that represent the piezoelectric components, as outlined in (2, p. 22). The piezoelectric behavior was included using the approach of (3, p. 86). The fluid areas of the model were analyzed using the approach described in (2, p. 540). The degrees of freedom of the model are the group of r nodal displacements of the solid components, r nodal pressures of the fluid components, r nodal electrical potentials of the piezoelectric components, and r the junction voltages of an external electrical circuit connected to the piezoelectric components (this latter capability was not used, though it is included for possible future use). Then, the defining equations of the finite element approach used are [A 2 ]  d 2 w dt 2  + [A 1 ]  dw dt  + [A 0 ]{w}+[A −1 ]  {w}dt + [A −2 ]  {w}dt.dt ={b}, (1) where [A 2 ] =     M 000 SG00 0000 0000     , [A 1 ] =     c 000 0 f 00 00 00 00 00     , [A 0 ] =      K 1 ρ S T E 0 0 H 00 E T 0 −∇ 2 0 00 0C      , [A −1 ] =     0000 0000 0000 000R     , [A −2 ] =     0000 0000 0000 000I     , {b}=        F 0 Q Q N        , {w}=        U P  ν        . P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-P-DRV January 18, 2002 21:0 764 PEST CONTROL APPLICATIONS In these equations, M =  [N s ] T ρ s [N s ]dV s S =  S [N f ] T ρ f [N s ]dS sf G =  [N f ] 1 a 2 [N f ]dV f c =  [N s ] T µ s [N s ]dV s f =  [N f ] T µ f [N f ]dV f K =  [B] T [D][B]dV p E =  [B e ] T [][B e ]dV p H =  [∇N f ] T [∇N f ]dV f I = external circuit inductance C = external circuit capacitance R = external circuit resistance U = solid element nodal displacements P = fluid element nodal pressures V = external circuit voltages F = externally imposed force on solid element nodes Q = externally imposed charges on piezoelectric elements Q N = externally imposed charges on external circuit φ = piezoelectric element nodal potentials a = speed of sound in fluid where [N s ] = shape function matrix for solid elements [ N f ] = shape function matrix for fluid elements [ B ] = shape function derivatives giving strain in solid elements [ B e ] = derivatives of potential shape function in piezoelectric elements ρ = mass density (subscript s for solid, f for fluid) µ = damping (subscript s for solid, f for fluid). The model assumed axisymmetry which was implemented as described in (2, p. 119). The elements describe the cross section of the complete unit from the centerline out, that is, that section which is rotated about the axis of symmetry to sweep out the 3-D geometry of the unit. The elements used were eight-node, isoparametric quadri- laterals, using quadratic shape functions for all fields (2-D solid displacements, fluid pressures, and electrical fields). Third-order Gaussian numerical integration was used for all element integrals. The integrals across volume are done by the usual finite element approach of integrating across each elementindependently, followed by assembling the resulting equations into matrix form, as described in (2, p. 9). Damping was included in the model by adding material damping to the fluid regions, as described in the pre- ceding equations. Based on experimental measurements, enough damping was included to give a resonant amplifica- tion (Q factor) of 5 to 8. Two extreme conditions were used. In the first, damping was distributed across both the transmission and working media. In the second, damping was concentrated in the working medium. The first case corre- sponds most closely to low excitation levels, whereas the second should more closely match high excitations when cavitation is occurring. Then, the energy dissipation will be concentrated in the working medium because of the cavitation. The model is linear. This is expected to give good results up to the point at which cavitation begins. Beyond that point, the response of the system is no longer linear because the fluid behaves effectively less stiff on the negative side of the pressure wave than on the positive side due to the formation of cavitating bubbles. In principle, this effect could be modeled using the nonlinear approaches described in (2, p. 450). This simplification was accepted because the objective was to compare alternative designs, rather than to analyze the behavior in absolute terms. It is assumed that systems that give a greater linear response will also give a greater nonlinear response. This may not be true in unusual cases, and it may not represent the effect of changes in the spatial distribution of the acoustic field in all cases (it would be expected that the “softening” nonlinearity which will occur here would tend to make the energy distribution more uniform in the system, compared to the linear case). Figure 4 shows typical results from the model. These show the pressure distribution across the fluid cross section for 100 volt peak–peak excitation of the piezo rings for various excitation frequencies. It can be seen that the energy in the working medium in all cases is concentrated at the center. At low frequencies, only a single pressure peak occurs. At higher frequencies, when the wavelength of the sound waves in the fluid becomes comparable to the dimensions of the device, two and then three pressure peaks Figure 4. Finite element predictions of cavitating field. P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-P-DRV January 18, 2002 21:0 PEST CONTROL APPLICATIONS 765 Table 2. Finite Element Model Parameters Parameter Material Dimensions Inner tubing Stainless steel tube 1.5 in outer diameter (E = 30E6 psi) 0.012 in wall thickness Piezoceramic rings PZT4 2 in diameter (stack of four) 0.125 in wall thickness 0.5 in height Transmission fluid SAE 10W30 motor oil Density, speed of sound Working fluid Water or diesel fuel Density, speed of sound occur axially along the centerline. These observations are consistent with qualitative results. These results were obtained by suspending an aluminum foil strip in the cavitating field. Because it is known that cavitation erodes aluminum, the distribution and degree of perforation provide an indication of the cavitating intensity. The specific parameters of the model are listed in Table 2. Test Verification of Analytical Model. Modeling a com- bined electrical/piezoelectric/structural/fluid system is complex. A number of approximations and simplifications were made. For this reason, some model correlation was done in advance of prototype development (experimental data taken from breadboard unit). The FE model was done for a four-ring prototype. The experimental testing was done on a three-ring arrangement. There were two type of measurements made for the correlation exercise, the current–voltage relationship and sound pressure measurements. The predicted and measured current versus voltage relationship for the system is shown in Figure 5. Measured values are shown at 22.7 kHz 10 0 10 0 10 1 10 2 10 −1 10 −2 P-P Piezo current (A) P-P Piezo voltage (V) Piezo current vs voltage Measured at 22.7 kHz Model at 26.5 kHz Model at 22.7 kHz Figure 5. Measured and predicted current vs voltage. which gives the peak piezo current. Model values are shown for both this frequency and for 26.5 kHz, which is the frequency at which the model shows peak current. It can be seen that the measured values at low voltages are about 60% of the modeled values. This is mainly due to the four rings in the model versus three in the breadboard. The sound pressure field was measuredusing the Specialty Engineering Associates needle hydrophone, Model SPRH- 2-0500. Figure 6 shows the response of the hydrophone at two different excitatory voltage levels, as captured on a digital storage oscilloscope. Note that the two cases were at slightly different frequencies. These frequencies corre- spond to the peak responses at each excitatory level. That they are different indicates nonlinearity in the model. It can be seen that the hydrophone response waveform is un- symmetrical and has pressure spikes on the positive voltage (low pressure) side. This is an indication of cavitation. It is more prominent at the higher excitatory voltage. The model predicts that the peak pressure in the unit should be 1 kPa per volt of excitation. The transducer output should be 0.25 mV per volt of excitation. The results in Fig. 6 show a 20-mV peak-to-peak response at 130-V peak-to-peak excitation in (a) and 65 mV response at 240 V excitation, or 0.16 mV/V and 0.27 mV/ V, respectively. This agreement is reasonable given the uncertainty of the hydrophone (it was being used somewhat out of its design frequency range). The model predictsthat thepressure should lead the voltage by 10 to 20 ◦ , and it can be seen that this is reasonable, though the experimental measurements do not really allow testing this. Figure 7 shows the pressure distribution measured along the centerline of the device for low voltage excitation (where the nonlinearity of the system does not con- fuse the results), and Fig. 8 shows the pressure distribution measured across the centerline at the midheight of the piezo rings. The hydrophone readings in these figures have been converted to acoustic pressures. The model predictions are also shown. It can beseen thatthe model andmea- sured values show the same trends and the differences are 1–3dB. Design Studies Outer Diameter of Transmission Medium. A design was studied to optimize the outer diameter of the transmission medium on the sound intensity in the working medium. P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-P-DRV January 18, 2002 21:0 766 PEST CONTROL APPLICATIONS 0 −150 150 (a) 100 50 0 −50 50 0 −50 −100 20 40 60 Time (µ sec) Response at 26.8 kHz 80 100 120 0 20406080100120 Piezo excitation (V)Hydrophone output (mV) 100 (b) 50 0 −50 Response at 26.2 kHz 0 20 40 60 80 100 120 −100 20 10 0 −10 −20 0 204060 Time (µ sec) 80 100 120 Piezo excitation (V)Hydrophone output (mV) Figure 6. Hydrophone response at (a) 130 V p–p excitation; (b) 240 V p–p excitation. The integral of acoustic pressure across the volume of the working medium was used as a performance indicator. Two extremes of damping models were used—damping concentrated in the working medium and damping distributed over both working and transmission media. Fig- ure 9 shows the results for both cases (as the integral of pressure vs. the outer diameter, (OD) of the transmission medium. It can be seen that when damping is concentrated in the working medium, the optimum occurs at an OD of 113 mm because the spacing between the outside of the piezo ring and the OD of the transmission medium is about one-half an acoustic wavelength. Such a condition would be expected to result in translating the high acoustic impedance condition at the rigid outer wall to a low acoustic impedance at the ring [see (8), p. 18 for an example]. This low acoustic impedance of the transmission medium Rings Model at 25.0 kHz 13 V P−P Excitation Measured at 23.7 kHz Measured at 26.0 kHz 84 82 80 78 76 74 72 70 68 66 −50 500 Z (mm) Axial pressure distribution on centerline P−P Pressure (dB re 1 Pa) Figure 7. Acoustic pressure distribution along centerline. at the ring is mismatched to that of the ring so that the coupling between the ring and transmission medium is poor at the outside of the ring. Little energy is launched outward from the ring, leaving more to be launched inward to the working medium. The figure also shows that when damping is distributed across both transmission and working media, the optimum occurs at a lower OD. This may be due to the fact that when damping is included in the transmission medium, the increase in transmission medium volume, which occurs as its OD is increased, results in more energy losses in the system, thus biasing the optimum to a smaller diameter. 84 82 80 78 76 P−P Pressure (dB re 1 Pa) 74 72 70 68 66 −10 r/R 1 13 V P-P Excitation Measured at 26.0 kHz (assumed symmetrical) Measured at 23.7 kHz (assumed symmetrical) Model at 26.0 kHz Radial pressure distribution at ring mid-height Figure 8. Acoustic pressure distribution across diameter at ring midheight. P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-P-DRV January 18, 2002 21:0 PEST CONTROL APPLICATIONS 767 0 30 25 20 15 10 5 OD (mm) Effect of outer diameter 10 0 70 75 80 85 90 95 100 105 110 115 120 70 75 80 85 90 95 100 105 110 115 120 2 4 6 8 Integral (PdV) (Pa.m ^3 )Integral (PdV) (Pa.m ^3 ) Distributed damping Prototype design Working fluid only damping Figure 9. F f  0 Power Acoustic νs φ. Electronics Concept. Three electronics concepts were considered, and two were experimentally evaluated: r a function generator to produce a sinusoidal (or other) waveform and a power amplifier to generate a final high-power output signal to be sent through a transformer to the piezo elements in themechanical module r a high-power oscillator r a switching power supply The first approach was used in prototype testing and development. It was not continued in the higher power, high flow-rate evaluation unit because the readily available Switched voltage source 3 - Pole butterworth low-pass filter Coil to produce tuned circuit with piezo Piezo model 1.53 mH L 1 L T 8.49nF 8.49nF C 1 C 2 R T R P C P 100 21.2nF 1.91mH Figure 10. Electronics concept. power amplifiers are limited in power (so would have to be ganged to drive the larger system) and the class A amplifier action used is relatively inefficient, making cooling of the electronics an issue. The high-power oscillator was not developed because of concerns of achieving high power without instability problems. The switching power supply was used for designing the evaluation unit. It is in line with current methods of driving high-power motors using pulse-width modulation (PWM). Digital circuitry is used to generate square wave- forms. These may be duty-cycle modulated and are used to switch power MOSFET transistors on and off rapidly so that the average voltage presented to the equipment as a result of the variable duty-cycle appears sinusoidal. Such an approach is efficient because the transistors are always completely on or completely off (except during short switching transients), and they dissipate little power in either of these states. In our case, the output frequencies are too high for true PWM, but square waves can be gen- erated at these frequencies and filtered to eliminate higher harmonics. Figure 10 shows an electronic filtering concept evaluated by analysis. A high voltage supply that has positive and negative polarity and a 33% duty cycle is switched on and off. The fundamentalfrequency of thesource is 25kHz. This is followed by a three-pole low-pass filter that has a cutoff at 62.5 kHz. The output from this filter feeds a tuned circuit that represents the piezo rings (21.2-nF capacitance and a 100-ohm resistor to simulate a system Q of 3) in series with an inductance chosen to tune the circuit to the 25 kHz fundamental. This makes the driven system of this tuned circuit appear resistive at the fundamental frequency and so matches the low-pass filter’s output impedance expectation. Note that no transformer is shown, though by adding a transformer between the filter and the piezo, lower voltages would exist in the left-hand P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-P-DRV January 18, 2002 21:0 768 PEST CONTROL APPLICATIONS 10 1 10 2 10 0 10 −2 10 1 10 2 10 1 10 2 Freq (kHz) 10 0 10 −1 Voltage across piezo (V)Power spectral density (arbitrary units) 10 −2 10 −3 Spectral content for 33% duty cycle +/− square wave PWM Frequency response (voltage across piezo for 1 V PWM input) PWM input voltage Piezo voltage Figure 11. Frequency response function of electronics concept. side of the circuit which would probably ease component choice. Figure 11 shows the calculated frequencyresponsefunc- tion. It also shows the spectral content of the voltage out of the switched power supply and into the piezo. The output from the switched power supply it is assumed, is both positive and negative in the 33% duty cycle and has switching transients 25% as long asthe on-time, that is, 1.67 µs. Sum- ming all power above the fundamental to 250 kHz gives a total harmonic distortion figure of 71% for the switched power supply output that has this waveform, but only 4% for the voltage across the piezo. A breadboard of this system was built and tested. It was felt that the advantages of the switching amplifier concept outweighed its disadvantages for a production application. A commercial supplier (Instruments Inc. of San Diego CA) was found. Implementation Issues. The thin walled stainless steel tube that contains fluid-borne microorganisms was de- signed to be as thin as possible to maximum the pressure transmitted through to the fluid. The thickness is limited by the pressure in the transmission medium. The thin walled tube is fairly close to buckling under the pressure of the transmission medium. In the prototype system, there was no pressuresensor to ensure that the pressure of the transmission medium was maintained between 30–100 psi. The small temperature change (1–2 ◦ C) that results from the excitation of the system causes the pressure to vary. The temperature change is kept to this low level by pumping the working fluid continuously past the transmission medium. During biological evaluation of the prototype system, the pressure did drift above 100 psi. After completing of prototype testing, the system was dismantled, and it was discovered that the tubing had buckled. The evaluation unit which was built as a follow-on to the prototype includes both a temperature and pressure sensor as part of the design. This ensures that the system will shut down before the critical pressure is exceeded. In an early version of the evaluative design (which contained 16 piezo rings, rather than the original four), the stainless steel tubing did buckle because the unsupported length of the tubing had more thandoubled.Modifications ofthe tubing boundary conditions weremadeto ensure that buckling did not occur but at the same time maintained as thin a profile as possible to maximize the energy transfer to the microorganism-borne fluid. Another significant issue that arose during early testing of the evaluative system relates to the importance of tolerancing the rings themselves. After short runs of the 16-ring stack system, failures in the rings occurred. They were failing mechanically—breaking into two pieces. The initiation of the crack seemed to be associated with a burn mark on the ring. It was postulated that the set of rings being used was not sufficiently well toleranced for roundness. The system was rebuilt using rings of improved tolerance (proved by Sensor Technologies of Collingwood, Ontario). There have been no ring failures since the system was rebuilt. The original electronic driveforthesystem was based on square wave input switching. When this was implemented, switching noise was feeding back to the input, causing noise spikes that were outside the acceptable range of the microprocessor. To eliminate this problem, the signal generator was rebuilt to use sine wave excitation. Figure 12 shows a drawing of the cavitation portion of the system. The elements of the figure are as listed in Table 3. Effectiveness of Cavitation in Destroying Microorganisms The effectiveness of using a cavitation field to destroy microorganisms was measured for two types of fluid hosts (water and diesel fuel) (9) and three types of microorganisms: r Serratia marcescens r Pseudomonas aeruginosa r Saccharomyces cerevisiae (yeast) The fitted results are shown in Fig. 13, plotted as a function of exposure timeto the cavitation field. Regression analysis was used to fit the data to the following equation: log  Irradiated Control  = (Slope ×Time) + const. (2) These test results were for microorganisms exposed to cavitation while the working medium was moving (being pumped) through the cavitation field. Earlier test results were performed while the medium was static during P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-P-DRV January 18, 2002 21:0 PEST CONTROL APPLICATIONS 769 11 12 13 14 15 16 17 18 I 10 9 8 7 6 5 4 3 2 1 Figure 12. Cavitation unit—16 ring. exposure to the cavitation field. The cavitation effect was more pronounced on the moving population than on the static population. It was hypothesized that the motion en- sured improved distribution of the microorganisms in the cavitation field. There were two different strains of Pseudonomas aeruginosa used in the study. Tests in water were done using ATCC 10145. A strain of Pseudonomas aeruginosa was isolated from a sample of marine diesel fuel. This strain would not survive at elevated temperatures (37 ◦ C) where the ATCC 10145 thrived. Table 3. Parts of Cavitation Unit Drawing Label Part 1 Lower sealing flange 2 Hydraulic O-ring 3 Lower flange 4 Hydraulic O-ring 5 Body 6 Body assembly rods 7 Flow-through tubing 8 Supporting ring 9 Hydraulic O-ring 10 Hydraulic O-ring 11 Upper flange 12 Upper supporting ring 13 Hydraulic O-ring 14 PZT ring, 2.0 in OD 15 Middle PZT supporting ring 16 PZT Assembly rods 17 Self-locking nuts 18 Lower PZT supporting ring 10 0.001 0 5 10 15 20 0.01 0.1 1 Treated/control Exposure time(s) Flow through testing Saccharomyces (yeast) Pseuds in water Serratia in water Pseud in diesel Serratia in diesel Pseud 'isolate' in diesel Figure 13. Biological test results. The results werebased on a flow-through testing system that involved recirculating the population to obtain the re- quired exposure time. Figure 14 shows a schematic of the experimental facility. The contaminated working fluid was recirculated during testing. This eliminated the need for disposal of large volumes of contaminated fluid. The recirculating effect underestimates the effectiveness of the method because the population is being gradually reduced for each pass through the cavitation field. It had been postulated that the pumping action itself might influence the microorganism population, but that effect was studied and found insignificant on either the Serratia marcescens or the Pseudomonas aeruginosa. There did seem to be a small effect on the yeast results. An attempt was made to predict the kill efficiency of a single pass of the population through the cavitation field. Kill efficiency e is the ratio of microorganisms per unit volume of fluid killed in one pass to microorganisms present in an untreated unit volume of fluid. 6 UDM experimental facility 1 8 7 5 4 3 2 1 − Cavitator 2 − Tank for treated water 3 − Tank for contaminated water 4 − Control valves 5 − Pump 6 − Power supply 7 − Hydraulic cylinder 8 − Screw Figure 14. Schematic of flow-through experimental facility. P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-P-DRV January 18, 2002 21:0 770 PHOTOCHROMIC AND PHOTO-THERMO-REFRACTIVE GLASSES NOTATION C o = initial concentration (microorganism’s/litre) C n = concentration after n passes through cavitation field e = kill efficiency n = number of times sample passed through cavitation field V = volume of cavitation field X = holding tank volume C n C o =  X − e × V X  n (3) When this equation is applied to the yeast test data obtained, the resulting kill efficiency is 0.49. When it is applied to the test results for Pseudomonas aeruginosa in diesel fuel, the resulting kill efficiency is 0.45. These results were based on an exposure time of 3.15 seconds in the cavitation field. BIBLIOGRAPHY 1. A.D. Ashton. Laboratory Evaluation of Ultrasonic Devices: Weitech Electronics, 2. O.C. Zienkiewicx, The Finite Element Method. McGraw-Hill, NY, 1977. 3. K. Ragulskis, R. Bansevicius, R. Barauskas, and G. Kulvietis, Vibromotors for Precision Microrobots. Hemisphere, NY, 1988. 4. Modern Piezoelectric Ceramics, Morgan Matroc Vernitron Division, Bedford, OH, 1988. 5. J.R. Frederick, Ultrasonic Engineering. Wiley, NY, 1965. 6. S.S. Save, A.B. Pandit, and J.B. Joshi, Chem. Eng. J. 55 B67– B72 (1994). 7. A.J. Chapman, Heat Transfer. Macmillan, NY, 1967. 8. G.L. Gooberman, Ultrasonics: Theory and Application. Hart P, NY, 1969. 9. S. Draisey. Ultrasonic Destruction of Microorganisms in Ship- board Fuels: Biology Report. Canadian National Defence Re- port, DREA CR 98/426. PHOTOCHROMIC AND PHOTO-THERMO-REFRACTIVE GLASSES L.B. GLEBOV University of Central Florida Orlando, FL INTRODUCTION Inorganic glasses are the main transparent material, which people have long used for observation (windows in buildings, windshields in cars, eyeglasses, prisms and lenses in optical instruments), light delivery (light bulbs, projectors, lasers, optical fibers), and fine arts (crockery, bijouterie, jewelry). The ability of glasses to change coloration after exposure to sunshine was well known since the last century. A new era in glass application was started in 1949 by S.D. Stookey’s publication (12) in which recording a permanent photographic image in silicate glass was described. This two-step process of exposure to UV radiation and thermal developmentthatresulted in a crystalline phase precipitation in the exposed areas was similar to the classical photographic process. As a result of inten- sive research during a long period of time, a great number of different photosensitive glasses were developed, which have found very wide application in different branches of industry and personal use. When exposed to optical radiation, these glasses (andglassceramics) change their optical properties (absorption, refraction, or scattering) instantly or after thermal development, permanently or transiently. Among the great variety of photosensitive glasses, we em- phasize only the two most widely used types. The largest commercial application was obtained for so-called “photochromic glasses,” which exhibit reversible coloration after exposure to UV or visible light and can vary their absorption depending on the illumination level. Glasses that contained small concentrations of microcrystals of silver and copper halides, proposed by Armistead and Stookey in 1965 became the most widely used for reversible coloration (13). A peculiarity of these materials is that they are produced by glassmaking technology whereas the photochromic processes occur in microcrystals distributed in the glass matrix. Several hundred original papers were dedicated to different aspects of heterogeneous photochromic glasses in those years. The vast bibliography and detailed descriptions of these heterogeneous photochromic glasses were collected in books (3,4), and therefore we will not include a list of original publications in this article. Another type of photosensitive glass, which is beginning its application in optics and photonics right now, is “photo- thermorefractive (PTR)” glass. If this glass is exposed to UV radiation followed by heat treatment, it varies in refractive index. Aphase hologram in thevolume of this glass was recorded in 1990 by Glebov and coauthors (5). The fea- ture of this process is that homogeneous glass is exposed to light and a microcrystalline phase is produced in the volume of the glass matrix by a thermodevelopment process. No books have been written on this subject. The main results concerning phase hologram recording in glasses can be found in a few original papers (5–7) and a survey (8). Similar processes of photoionization followed by ther- moinduced crystallization were studied for single- and full- color photography in polychromatic glasses, as described in (1, 9–12). Thus, these references can also be used for learning the basic physical phenomena that result from irradiation and development of PTR glasses. Some basic data concerning intrinsic absorption, electronic excitation, and nonlinear photoionization in multicomponent glasses can be found in (13,14). PHYSICAL PRINCIPLES OF PHOTOSENSITIVITY IN GLASSES Photosensitivity is the variation in glass properties from exposure to optical radiation. 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P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB0 9 1- P-DRV January 18 , 2002 21: 0 774 PHOTOCHROMIC AND PHOTO-THERMO-REFRACTIVE. the left-hand P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB0 9 1- P-DRV January 18 , 2002 21: 0 768 PEST CONTROL APPLICATIONS 10 1 10 2 10 0 10 −2 10 1 10 2 10 1 10 2 Freq (kHz) 10 0 10 1 Voltage. during P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB0 9 1- P-DRV January 18 , 2002 21: 0 PEST CONTROL APPLICATIONS 7 69 11 12 13 14 15 16 17 18 I 10 9 8 7 6 5 4 3 2 1 Figure 12 . Cavitation unit 16 ring. exposure

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