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Liquid-solid phase transition detection with acoustic plate mode sensors: Application to icing of surfaces
693
Sensors and Actuators, A21 -A23 ( 1990) 693 -699
Liquid-Solid Phase Transition Detection with Acoustic Plate Mode Sensors: Application to Icing of Surfaces R C HUGHES, S J MARTIN, G C FRYE and A J RJCCO Sandta NatIonal Laboratones, Albuquerque, NM 87185 (rr S A )
Abstract
An acoustic plate mode (APM) device operatmg at 158 MHz has been used to momtor hqmdto-solid phase transitions Shear-honzontal plate modes propagating m a quartz plate probe the phase of a medium m contact wth the plate When hqmd contacts the device, VISCOUS damping of the APM IS the dominant means of energy transfer, with the resultmg APM attenuation servmg as a measure of hqmd viscosity When the hqmd freezes, acoustic energy IS mechanically coupled from the plate into the adjacent solid, leading to a dramatic increase m APM propagation loss Because effective coupling of acoustic energy from the plate into the adJacent medmm depends on contmulty of shear displacement across the solid/ solid interface, the magmtude of the propagation loss IS an mdlcafion of the intimacy of contact between the solid and the APM surface Possible applications of ths sensor include a monitor for au-plane wmg icmg
Introduction
We have recently demonstrated that acoustic wave devices utdlzmg shear honzontal (SH) acoustic plate modes (APMs) can be extremely useful m prohng the sohd/hqmd interface [l-6] Momtormg perturbations of the propagation charactenstlcs of the SH-APM allows the measurement of hqmd vlscoslty [4], mass density of mterfaclal films [ 1,6], density of chemical species bound at the interface [l--3], and electrical properties of solution [5] As deplcted m Fig 1, SH-APMs are excited and detected by mterdlgtal transducers patterned on one face of a piezoelectnc quartz plate Apphcation of an altematmg electrical potential to one of the transducers imposes an alternating strain field m the plezoelectnc plate, which launches a mechanical wave mto the bulk of the quartz plate Multiple reflection of this shear wave by the upper and lower faces leads to wave confinement so that 0924-4247/90/%3 50
the plate forms an acoustic wavegmde The superposltlon of reflected waves results m an SH plate mode characterized by smusoldal vanahon of shear displacement across the plate, with maxima at the upper and lower surfaces [1] When the APM reaches the second transducer, it generates an electrical signal which conveys mformatlon about the attenuation and phase delay expenenced by the acoustic mode as it traversed the plate Because both surfaces of the APM device substrate undergo equal displacement as the wave propagates, either face can be used to probe a contacting medmm An SH plate mode has displacement parallel to the surface of the device and perpendicular to the &rection of mode propagation When hqmd contacts the quartz plate, this shear dsplacement entrains a thm layer of hqmd, causing VISCOUS damping of the APM When the meclmm contactmg the device surface undergoes a phase change from hqmd to solid, its behavior changes from VISCOUSto elastic Consequently, the APM no longer entrains a thm layer adjacent to the plate, instead radlatmg shear waves mto the bulk of the contacting sohd The radiated power “leaks away” from the propagatmg APM, causing substantial attenuation The loss to the solid phase IS
Teflon Cell
Llquld I” Contact wth Surface
I
I
I
Input Transducer
I
Pleroelscfrlc ouortz
Plats
output
\ ACOUSllC Plate Made
Transducer
Rg 1 Schematic of acoustic plate mode (APM) sensor for detectIon of hqmd-sohd phase transitions Interd@al transducers on ST-cut quartz exate and detect the shear-honxontal AF’M wluch probes the medium contactmg the sensor 0
Elsevler Sequola/Prmted
m The Netherlands
694
related to how well the acoustic impedance of the adJacent solid matches that of the quartz plate Because SH plate modes suffer mmor attenuatlon when a liquid contacts the device surface, but undergo substantial attenuation with a solid m contact, they are very well suited to detectmg liquid-solid phase changes In this paper we report the observation of such phase transitions using SH-APM devices, m particular the freezmg and melting of water and galhum on the APM device surface The capablhty to detect freezmg could be extremely useful m the momtormg of lcmg conditions, such as occur on airplane wings, and for mvestlgatmg the physical chemistry of hqmd-solid phase transitions m general
of tixed frequency and amphtude was input to the device whde momtonng changes m the output signal amplitude A Hewlett-Packard 8656A syntheslzed source provided an RF signal at 158 MHz while a Hewlett-Packard 8508A vector voltmeter monitored the signal level from the output transducer The difference m mput and output signal levels IS a measure of the insertion loss, which includes both transduction and propagation losses Data from the vector voltmeter and RTD were recorded as a function of time by computer (Hewlett-Packard 9816) This mstrumentatlon makes It possible to consistently momtor the large changes m propagation loss which occur upon freezmg
Experiment
Results
The APM device used m this study 1s Identical to those used to measure the vlscoslty of liquids [ 1,4] Interdlgtal transducers were defined photohthographlcally on an ST-cut quartz plate from 200-nm-thick Au-on-Cr metalhzatlon Transducers were composed of 50 finger-pairs each, Hrlth a perlodlclty of 32 pm, this penodlaty, together with an APM propagation velocity of approxlmately 5100 m/s, results m SH-APM excitation being most efficient at 158 MHz The unmetalhzed side of the device was lapped to obtain a thickness of 165 pm, then pohshed to an optical fimsh The device was mounted m a 25 5 mm x 12 7 mm gold-plated-steel flatpack wth a 20 5 mm x 3 7 mm openmg to allow hqmd to contact the unelectroded side of the device The unelectroded face of the device was bonded (m the regon surrounding the acoustic path) to the openmg m the flatpack using a bead of elastomer (room temperature vulcamzmg &cone rubber) Electrical contact was made between transducer bonding pads and flatpack feedthroughs by 25pm-diameter gold leads attached wth an ultrasonic bonder The flatpack was mounted m a brass test fixture contammg impedance-matching networks As illustrated m Fig 1, hqmd was held m contact wth the sensing surface by an open Teflon cell sealed by compression to the metal flatpack About one rmlhhter of hqmd was placed m the cell, water was doubly dlstllled and Ga was 99 999% pure A mlmature platmum RTD (resatance temperature device, & = 100 Q) was freely suspended m the liquid to measure the temperature Controlled, slow temperature ramps were obtained by placing the entire assembly m a vanable temperature environmental chamber To measure changes m APM propagation loss ansmg when the contacting liquid freezes, a signal
The results of a liquid-to-solid phase change usmg distilled water are &splayed m Fig 2 Insertion loss is plotted as a function of water temperature, which was ramped from 20 “C down to - 8 “C and back to 12 “C Though not shown m Fig 2, the mcrease m insertion loss when water was mltlally added to the dry APM device surface at 25 “C amounts to about 5 dB over the 0 75-cm path length between input and output transducers The sequence of events whch occurs durmg the phase changes of Fig 2 are numbered as follows (1) As the water cools, its vlscoslty increases, causing gradually higher viscous attenuation The water supercools to about -8 “C before freezmg begms (2) As nucleation of the first ice crystals occurs, the temperature of the water/ice nuxture humps immediately to 0 “C, there is little increase m insertion loss at this pomt
66 64 s p
62
g
60
0
56
z t:
56
x =
64 62 I -15
-10
-5
0 Temperature
5
10
16
("Cl
Rg 2 Loss vs temperature for dlstrlled water as temperature cycles from 20°C to -8°C and back to 12 “C The tme sequence IS m&cated by arrows and the nrcled numbers A fill descnption of the state of the system correspondmg to each number is gwcn m the text
695
-10
-5
0
5 10 15 Temperature (Z)
20
25
Fig 3 Loss vs temperature vvlth dtstdled water contactrng APM devxq as shown 1x1Rg 2, except that ethylene glycol was added to the sold ILXat -8 “C The amount of ethylene glycol added was sutkcnt to depress the freenng point to -5°C
(3) The two-phase water/Ice rmxture continues to freeze at constant temperature (0 “C), w&h a steady increase m attenuatton accompanymg the phase change (4) When all the water has turned to lee, the temperature of the KZ decreases toward the temperature of the su~o~dlng environmental chamber, -8°C (5) The chamber t~rn~rat~e IS ramped up to 20 “C, causmg the tee to melt The lee/water nuxture remains at 0 “C as meltmg proceeds, mth the APM device ln~~lon loss retra~ng the path followed durmg freezmg (6) When the ice has completely melted, the water tem~rat~e mcreases, resultmg m a decrease m the VISCOUS attenuatton To further test the versatility of the APM sensor for measurmg phenomena associated w&h phase transttions, a second freezmg expenment was carned out, agam startmg with d&died water In t&s case, ethylene glycol (the myor constituent of antifreeze) was added after the ICX was fully formed and had cooled to - 8 “C These data are dlsplayed m Fig 3 m a manner simdar to that of Fig 2 (I) Water IS added to the dry surface of the device, resulting m about 5 dB Increased loss due to the VISCOUS coupling between the shear wave and the hqmd (2) Decreasing temperature causes an increase m the vlscoslty (and about 2 dB further mcrease m loss) of the water as it enters the supercooled regme (3) The supercooled water begms to freeze, but the m&al crystalhzatlon produces only a small increase in loss (4) The loss increases as the two-phase system freezes at 0 “C
(5) After all the water has frozen, temperature decreases below 0 “C (6) When the temperature of the ice reaches - 8 “C, sufficient ethylene glycol 1s added to the cell to make a 15% solution by we&t (7) The solution melts at - 5 “C (over a SOmm penod) In this case the loss does not retrace the path It followed durmg freezmg the meltmg pomt has been depressed by the addltlon of the ethylene glycol (8) In comparison to the begmmng of this run, when the water was pure, the melted solutmn causes increased msetion loss, consistent wth the higher viscosity of the ethylene glycol/water nuxture [4] To correlate the magmtude of viscous damping of the APM with bqmd nscoslty, a serves of glycerol/water nurtures havmg a range of V~SCOSLties between 1 and 62 cP were used With hqmd contactmg the entrre unelectroded side of the APM wave path, mcludmg the transducer regtons, changes m loss AL (m dB) due to changes m hqmd shear vrscos&yAq (m Parse) are found to be gven by AL = 25(~)]~
(1)
The dependence of viscous loss on Mscoslty ~11 be discussed below m ~nn~~on w&h the Increase m loss oceumng m Figs 2 and 3 durmg the coohng of hqmd water The nature of the so~d/~lId mterface which forms when a 1lqmd-~hd phase change occurs on the APM device surface was mvestigated by momtonng the freezmg of gallium metal Shown m Ftg 4 1s the varration m loss measured as Ga sohdilfies on the APM sensor The dashed hne mdicates the loss obtamed wrth a free (unperturbed) deuce surface Placmg hqmd Ga (meltmg pomt 29 8 “C) on the device surface resulted m
+
Bonded sold
Go
34 %
5 lo g .Q $4 t
32 30 28 26 24 22 I
------
-
x,Free surface 5
10
16
20
26
i
Time, mln Rg 4 Insertion loss of APh4 sensor as Ga sohdsfies on the surface The Ga delanates spontaneously shortly after sohd&ation, causing loss to return to near the unperturbed (free surface) value
696
approximately 5 dB of viscous loss Sohdlficatlon of the Ga lmtlally contnbuted an additional 13 dB of loss After about two minutes, however, the loss unexpectedly dropped preclpltously, to near the free-surface value This was apparently due to delarnmatlon of the solid Ga from the quartz, at the conclusion of this expenment, the piece of solid Ga was found to be completely free on the quartz device surface
(4
Discussion
The results shown above indicate that APM devices are very sensltlve to hqmd-solid phase transitions Llqmd water contactmg the device contnbutes 5-7 dB of loss, depending on liquid temperature, ice contacting the device contnbutes an additional 11- 12 5 dB of loss The loss values thus provide unambiguous and reproducible mformation about whether a medmm contacting the device surface IS entirely liquid or entirely solid, temperature measurement alone can be amblguous, due to the posslblhty of supercooling The all-hqmd and all-solid phases are dlstmctly separable, with viscous properties determmmg the energy transfer to the hqmd, while elastic properties dictate loss to the solid phase, these two sltuatlons are readily modeled Because the specific structure of the two-phase nuxture dunng freezing 1s unknown, the response of the APM sensor to the mixed hqmd/sohd phase 1s not readily modeled Loss with an Adjacent Lzqurd Phase With a hqmd m contact mth the APM devtce, loss 1s determined by the hquld viscosity The surface components of the plate modes generate a damped shear wave m the hqmd which decays exponentially with distance from the surface, as shown m Fig 5 (a) The decay length at 158 MHz for hqmd water at 20 “C 1s about 50 nm [4] Thus, events occurnng only microns away, such as the freezing of the water at the top of the cell, are not detected by the APM device This 1s supported by the data of Figs 2 and 3, which show that when the RTD probe detects the onset of freezing, there 1s little accompanying change m the APM sensor output This indicates that crystallization probably did not begm on the APM device surface The increase m loss measured durmg coohng can be attnbuted to an increase m the shear vlscoslty of the water The coohng data of Fig 2 are reproduced as the solid points (0) m Fig 6 The known dependence of water shear vlscoslty on temperature [7] allows calculation, using eqn (l), of the viscous loss expected durmg cooling
@I Rg 5 Shear displacement at the APM surface contactmg a mednnn (a) Llqmd water m contact with the APM device results m entramment of a thm bqmd layer, leadmg to approxnnately 5 dB of loss over a 7 5 mm acoust~ path length (b) Bmdmg of ice to the surface of the APM devrce results m ra&ahon of acoustic energy out of the plate and 11 - 12 5 dB of loss
Tmpsrature
td*g
C)
Fig 6 Insertion loss measured durmg the coolmg of hqmd water, pnor to freezmg, taken from the data of Fig 2 (0) The sohd hne shows the loss calculated from eqn 1 due to changes m water vlscoslty with temperature
This loss, indicated by the sohd line ( -) 6, agrees reasonably with that measured cooling (0)
m Fig durmg
Loss with an Adjacent Sobd Phase While the viscosity of an adJaCXnt hqmd determines propagation loss, with an adJacent solid it is the elastic properties which dictate loss If the solid 1s sufliclently well-bonded to the APM device, a non-slip boundary condition unll exist at the solid/solid interface In this case, shear motion of the device substrate will couple shear motion into the adJacent solid (Fig 5(b)), leading to the
697
radiation of acoustic energy into the latter and greatly increasing propagation loss How mtimate the contact between the two sohds must be IS not fully understood, but we have found that slmply placing a solid, such as a piece of glass, on the APM device surface does not produce any measurable propagation loss The sohdlficahon of Ga on the APM device surface provides further insight into the effects of the nature of the sohd/sohd contact on propagatlon loss It should be pointed out that the observed spontaneous delammation of the Ga from the quartz surface IS qmte reasonable m light of the large contra&on ( -3 1%) whch accompames Its sohdlficatlon [8] The sudden decrease m loss observed (Fig 4) suggests that bondmg between the Ga and the quartz IS transitory, 1e , the solid lmtlally bonds sufficiently well to ensure a non-&p boundary, but then suddenly debonds, allowmg boundary shppage and an unperturbed APM This m turn suggests that the APM devrce could be useful m determmmg whether hqmd-tosohd phase changes result m mtlmate bondmg of a solid to the APM surface In such an expenment, the quartz APM device surface could first be modtfied by coatmg it Hrlth a second solid matenal (which would, of course, have to adhere well to the quartz) to allow the study of bonding between this second matenal and the substance undergomg the phase change If the bmdmg of the solid phase to the APM device 1s sufiiaent to insure a non-slip boundary, and both media are modeled as lsotroplc, then the effect of the phase change on APM propagation can be estimated If the acoustic shear wave velocity m the adjacent solid phase is higher than that of the plate, there should be a number of APMs which propagate m the quartz device Hrlthout increased loss However, when the shear velocity m the adjacent phase IS lower than that of the plate, APMs become ‘leaky’, Hrlth the degree of attenuation depending on the relative acoustic impedances of the plate and adjacent solid Most matenals which undergo a hqmd-solid phase change m the vlcuuty of room temperature (e g , water, organic solvents, and Ga) have a lower shear wave velocrty m the solid phase than quartz, resulting m leaky APM propagation when sohdfication occurs on the APM sensor Solving the wave equation in the plate, subject to the boundary condltlons Imposed at the lower free surface and upper contacted surface, allows determmatlon of the APM propagatton loss At the lower plate surface a stress-free boundary IS obtamed At the upper surface, the non-&p boundary between the plate and contacting medium reqmres (1) contmmty of particle dlsplacement, and (2) a balancing of shear stresses
across the mterface These latter boundary condotlons result m radiation of acoustrc energy from the plate into an adjacent sohd medium havmg lower shear wave velocity The couphng of acoustic energy from the plate into the adJacent medium leads to APM attenuation a,, which, for Z,/Z, < 1, IS gven by a, = $j tanh-‘(Z,/Z,)
(2)
where n IS the APM transverse mode mdex, 1 1s the APM wavelength along the surface (32 pm), and b 1s the plate thickness Z1 and Z, are the shear wave Impedances of the solid phase contactmg the plate and the quartz plate, respectively The ratio of these impedances IS gven by z, p’p’ ‘I2 -= (3) z2
(
PP
>
where p’ and p’ are the mass density and shear stiffness of the sohd contacting the quartz, respectively, p and ~1are the mass density and shear stiffness of quartz Ice properties at - 16 “C are [9] p’ = 917 kg/m3, p’ = 3 06 x 10’ N/m2, ZI = 1 68 x lo6 Rayls, for quartz p = 2650 kg/ m3, p = 3 99 x 10” N/m’, Z, = 103 x 10’ Rayls Consequently, Z,/Z, = 0 16 Plotted m Fig 7 1s the APM attenuation calculated from eqn (2) versus the ratio Z,/Z, of shear-wave impedances As Z,/Z, approaches unity, the acoustic nnpedance match improves between the plate and the adjacent sohd, leading to more rapid leakage of acoustic energy from the plate and higher attenuation The propagation loss L (m dB) correspondmg to CX,1s gven by L = 8 69aJ
(4) where I 1s the path length (center-to-center) between transducers For the path length of our
Rg 7 A theorekal estimate of the attenuation caused by havmg a senu-miimtc elastic sohd 111contact wltb the AFW devxe 2, and Z, are the shear wave unpadances of the adJacent solId and the quartz APM substrate, respechvcly
698
device (I = 0 75 cm), a reasonable estimate of sohdficatlon loss 1s obtained when a transverse mode index n = 3 1s used m eqn (2) It should be pointed out that the geometry of the particular APM device employed does not allow resolution of mdlvldual transverse modes [ 11, consequently, excltatlon at 158 MHz could result in several (n = O-5) transverse APMs bemg excited In any case, the mode index of 3 used m eqns (2) -(4) yields L = 14 dB, close to the value observed The n value which Dves reasonable estimates of loss when the device 1s m contact with ice may be regarded as an ‘average index’ for superposltlon of modes excited The freeang process of Fig 2 is exammed in greater detail m Fig 8, revealing that the transltlon from hqmd to solid 1s not mstantaneous Because the latent heat of fusion for water 1s large compared with Its heat capacity, the water does not supercool sufficiently for all of it to freeze simultaneously when nucleation begms, we have never observed the sort of dlscontmmty m the data which would be expected d all the water sohdlfied at once Consequently, for a penod of time (about two mmutes m the case of Fig 8), a two-phase system at 0 “C exists Dunng this time, the measured loss mcreases smoothly, the magmtude of the loss 1s consistent with the vlscoslty of the two-phase system increasing from about 2 cP, which corresponds to hqmd water supercooled to - 8 “C, to a final value of 7 CP It is conceivable that many small crystals may bind to the quartz surface and grow with time, creating an average loss somewhere between that associated with pure hquld and pure (bound) sohd The two-phase system presents a problem of considerable conceptual complexity, the solution to which awalts a
number of future expenments, mcludmg nucroscope visual observation and carefully controlled temperature gradients (e g , to ensure that nucleation starts on the quartz surface, cool only the quartz or coat the quartz with a thm, highthermal-conductlwty metal) c0lle1usIolls
It has been demonstrated that the APM device 1s a sensltlve momtor for phase transitions occurrmg on a surface A very small quantity of matenal 1s suffiaent for study, smce sensmg occurs m the first few rmcrons Information about the WScoslty of the hqmd phase IS obtamed, and for reasonably stiff sohds hke ice, a large and easily monitored change m msertlon loss occurs when solid forms on the APM device surface The effect of addltlves, such as ethylene glycol, can be momtored and the thawmg caused by the antifreeze venfied In addition, useful mformation IS gathered concermng the sticking or binding of the solid phase to the surface The surface of the plezoelectnc substrate may be coated with a thmfilm matenal to allow exammatlon of adhesion between the substance changmg phase and the thm film Some obvious uses for tis new sensor include momtonng the icing of alrcraft wmgs and other surfaces where the freezmg of water (and the stlckmg of ice) cause problems and evaluatmg the efficacy of chemical or blolo@cal agents designed to prevent frost damage to crops Many such apphcatlons reqmre the remote detection of freenng and thawmg events, where temperature alone 1s not a reliable guide
The authors are grateful to Diana Bkr of Sandra National Laboratones for suggestmg this solution to the ice detection problem and Barbara Lammle of Sandra National Laboratones for techmcal assistance Thus work was performed at Sandra National Laboratones and supported by the U S Department of Energy under contract DE-ACO4-76DPOOO789 I I I
0
,
50
100
Time
I 150
200
250
(set)
Fla 8 InsertIon loss measured wlule freezmn water on the GM devxe The three re.lpons denoted ‘LK&KI’, ‘Llqmd + Sohd’, and ‘Sohd’ are marked by the vertaxl hnes Completion of freezmg 1s determmed to be the trme at whch &e ice temperature starts to decrease after mmng at 0 “C durmg the freezmg process (see Fig 2)
1 S J Martm. A J bcco, T M Nxmczyk and G C Frye, Character&Uon of SH acoustic plate n&e hqmd sens&, Sensors and Actuators, 20 (1989) 253-268 2 A J Bcco, S J Martm, % M. Nnx~ayk and G C Frye, m R W Murray, R E Dessey, W R Hememan, J Janata and W R Se& (eds ), Chemical Sensors ami h4uzromstrumentatton, Amencan Chemxal Surety, Washmgton, DC, 1989, m press
3 S J Martm, A J bcco, G C Frye, T M Nuxnczyk and I Adluhetty, Sensmg m hquds with SH plate mode devxes, Proc 1988 IEEE Ulrrasonrcs Symp, IEEE, New York, 1989, m press 4 A J bcco and S J Martm, Acoustx wave vlscoslty sensor, Appl Phys L.&t, 50 (1987) 1474-1476 5 T M N~emczyk, S J Martm, G C Frye and A J Rxco, Acoustoelectnc rnteract~on of plate modes wth solutions, J Appl Phys , 64 (1988) 5002-5008 6 A J bcco and S J Martm, Proc Symp Electroless
699 Dep of Met& rmd Alloys, Vol 88-12, The Electrochemical Society, Penmngton, NJ, 1988, pp 142-153 7 R C Weast (ed ). CRC Handbook of ChemMy cmd Physics, Chcoucal Rubber Co , Boca Raton. FL, 54th edn ,1973, pp F43-F45 8 M Wmdbolz (ed ), The Merck Index, Merck & Co, Rahway,NJ, 10th edn, 1983, p 4218 9 Amerzcan Insfltute of Phystcs Handook, McGraw-Hffl, New York, 3rd edn , 1972, pp 2-49
Sensors and Actuators, A21 -A23 ( 1990) 693 -699
Liquid-Solid Phase Transition Detection with Acoustic Plate Mode Sensors: Application to Icing of Surfaces R C HUGHES, S J MARTIN, G C FRYE and A J RJCCO Sandta NatIonal Laboratones, Albuquerque, NM 87185 (rr S A )
Abstract
An acoustic plate mode (APM) device operatmg at 158 MHz has been used to momtor hqmdto-solid phase transitions Shear-honzontal plate modes propagating m a quartz plate probe the phase of a medium m contact wth the plate When hqmd contacts the device, VISCOUS damping of the APM IS the dominant means of energy transfer, with the resultmg APM attenuation servmg as a measure of hqmd viscosity When the hqmd freezes, acoustic energy IS mechanically coupled from the plate into the adjacent solid, leading to a dramatic increase m APM propagation loss Because effective coupling of acoustic energy from the plate into the adJacent medmm depends on contmulty of shear displacement across the solid/ solid interface, the magmtude of the propagation loss IS an mdlcafion of the intimacy of contact between the solid and the APM surface Possible applications of ths sensor include a monitor for au-plane wmg icmg
Introduction
We have recently demonstrated that acoustic wave devices utdlzmg shear honzontal (SH) acoustic plate modes (APMs) can be extremely useful m prohng the sohd/hqmd interface [l-6] Momtormg perturbations of the propagation charactenstlcs of the SH-APM allows the measurement of hqmd vlscoslty [4], mass density of mterfaclal films [ 1,6], density of chemical species bound at the interface [l--3], and electrical properties of solution [5] As deplcted m Fig 1, SH-APMs are excited and detected by mterdlgtal transducers patterned on one face of a piezoelectnc quartz plate Apphcation of an altematmg electrical potential to one of the transducers imposes an alternating strain field m the plezoelectnc plate, which launches a mechanical wave mto the bulk of the quartz plate Multiple reflection of this shear wave by the upper and lower faces leads to wave confinement so that 0924-4247/90/%3 50
the plate forms an acoustic wavegmde The superposltlon of reflected waves results m an SH plate mode characterized by smusoldal vanahon of shear displacement across the plate, with maxima at the upper and lower surfaces [1] When the APM reaches the second transducer, it generates an electrical signal which conveys mformatlon about the attenuation and phase delay expenenced by the acoustic mode as it traversed the plate Because both surfaces of the APM device substrate undergo equal displacement as the wave propagates, either face can be used to probe a contacting medmm An SH plate mode has displacement parallel to the surface of the device and perpendicular to the &rection of mode propagation When hqmd contacts the quartz plate, this shear dsplacement entrains a thm layer of hqmd, causing VISCOUS damping of the APM When the meclmm contactmg the device surface undergoes a phase change from hqmd to solid, its behavior changes from VISCOUSto elastic Consequently, the APM no longer entrains a thm layer adjacent to the plate, instead radlatmg shear waves mto the bulk of the contacting sohd The radiated power “leaks away” from the propagatmg APM, causing substantial attenuation The loss to the solid phase IS
Teflon Cell
Llquld I” Contact wth Surface
I
I
I
Input Transducer
I
Pleroelscfrlc ouortz
Plats
output
\ ACOUSllC Plate Made
Transducer
Rg 1 Schematic of acoustic plate mode (APM) sensor for detectIon of hqmd-sohd phase transitions Interd@al transducers on ST-cut quartz exate and detect the shear-honxontal AF’M wluch probes the medium contactmg the sensor 0
Elsevler Sequola/Prmted
m The Netherlands
694
related to how well the acoustic impedance of the adJacent solid matches that of the quartz plate Because SH plate modes suffer mmor attenuatlon when a liquid contacts the device surface, but undergo substantial attenuation with a solid m contact, they are very well suited to detectmg liquid-solid phase changes In this paper we report the observation of such phase transitions using SH-APM devices, m particular the freezmg and melting of water and galhum on the APM device surface The capablhty to detect freezmg could be extremely useful m the momtormg of lcmg conditions, such as occur on airplane wings, and for mvestlgatmg the physical chemistry of hqmd-solid phase transitions m general
of tixed frequency and amphtude was input to the device whde momtonng changes m the output signal amplitude A Hewlett-Packard 8656A syntheslzed source provided an RF signal at 158 MHz while a Hewlett-Packard 8508A vector voltmeter monitored the signal level from the output transducer The difference m mput and output signal levels IS a measure of the insertion loss, which includes both transduction and propagation losses Data from the vector voltmeter and RTD were recorded as a function of time by computer (Hewlett-Packard 9816) This mstrumentatlon makes It possible to consistently momtor the large changes m propagation loss which occur upon freezmg
Experiment
Results
The APM device used m this study 1s Identical to those used to measure the vlscoslty of liquids [ 1,4] Interdlgtal transducers were defined photohthographlcally on an ST-cut quartz plate from 200-nm-thick Au-on-Cr metalhzatlon Transducers were composed of 50 finger-pairs each, Hrlth a perlodlclty of 32 pm, this penodlaty, together with an APM propagation velocity of approxlmately 5100 m/s, results m SH-APM excitation being most efficient at 158 MHz The unmetalhzed side of the device was lapped to obtain a thickness of 165 pm, then pohshed to an optical fimsh The device was mounted m a 25 5 mm x 12 7 mm gold-plated-steel flatpack wth a 20 5 mm x 3 7 mm openmg to allow hqmd to contact the unelectroded side of the device The unelectroded face of the device was bonded (m the regon surrounding the acoustic path) to the openmg m the flatpack using a bead of elastomer (room temperature vulcamzmg &cone rubber) Electrical contact was made between transducer bonding pads and flatpack feedthroughs by 25pm-diameter gold leads attached wth an ultrasonic bonder The flatpack was mounted m a brass test fixture contammg impedance-matching networks As illustrated m Fig 1, hqmd was held m contact wth the sensing surface by an open Teflon cell sealed by compression to the metal flatpack About one rmlhhter of hqmd was placed m the cell, water was doubly dlstllled and Ga was 99 999% pure A mlmature platmum RTD (resatance temperature device, & = 100 Q) was freely suspended m the liquid to measure the temperature Controlled, slow temperature ramps were obtained by placing the entire assembly m a vanable temperature environmental chamber To measure changes m APM propagation loss ansmg when the contacting liquid freezes, a signal
The results of a liquid-to-solid phase change usmg distilled water are &splayed m Fig 2 Insertion loss is plotted as a function of water temperature, which was ramped from 20 “C down to - 8 “C and back to 12 “C Though not shown m Fig 2, the mcrease m insertion loss when water was mltlally added to the dry APM device surface at 25 “C amounts to about 5 dB over the 0 75-cm path length between input and output transducers The sequence of events whch occurs durmg the phase changes of Fig 2 are numbered as follows (1) As the water cools, its vlscoslty increases, causing gradually higher viscous attenuation The water supercools to about -8 “C before freezmg begms (2) As nucleation of the first ice crystals occurs, the temperature of the water/ice nuxture humps immediately to 0 “C, there is little increase m insertion loss at this pomt
66 64 s p
62
g
60
0
56
z t:
56
x =
64 62 I -15
-10
-5
0 Temperature
5
10
16
("Cl
Rg 2 Loss vs temperature for dlstrlled water as temperature cycles from 20°C to -8°C and back to 12 “C The tme sequence IS m&cated by arrows and the nrcled numbers A fill descnption of the state of the system correspondmg to each number is gwcn m the text
695
-10
-5
0
5 10 15 Temperature (Z)
20
25
Fig 3 Loss vs temperature vvlth dtstdled water contactrng APM devxq as shown 1x1Rg 2, except that ethylene glycol was added to the sold ILXat -8 “C The amount of ethylene glycol added was sutkcnt to depress the freenng point to -5°C
(3) The two-phase water/Ice rmxture continues to freeze at constant temperature (0 “C), w&h a steady increase m attenuatton accompanymg the phase change (4) When all the water has turned to lee, the temperature of the KZ decreases toward the temperature of the su~o~dlng environmental chamber, -8°C (5) The chamber t~rn~rat~e IS ramped up to 20 “C, causmg the tee to melt The lee/water nuxture remains at 0 “C as meltmg proceeds, mth the APM device ln~~lon loss retra~ng the path followed durmg freezmg (6) When the ice has completely melted, the water tem~rat~e mcreases, resultmg m a decrease m the VISCOUS attenuatton To further test the versatility of the APM sensor for measurmg phenomena associated w&h phase transttions, a second freezmg expenment was carned out, agam startmg with d&died water In t&s case, ethylene glycol (the myor constituent of antifreeze) was added after the ICX was fully formed and had cooled to - 8 “C These data are dlsplayed m Fig 3 m a manner simdar to that of Fig 2 (I) Water IS added to the dry surface of the device, resulting m about 5 dB Increased loss due to the VISCOUS coupling between the shear wave and the hqmd (2) Decreasing temperature causes an increase m the vlscoslty (and about 2 dB further mcrease m loss) of the water as it enters the supercooled regme (3) The supercooled water begms to freeze, but the m&al crystalhzatlon produces only a small increase in loss (4) The loss increases as the two-phase system freezes at 0 “C
(5) After all the water has frozen, temperature decreases below 0 “C (6) When the temperature of the ice reaches - 8 “C, sufficient ethylene glycol 1s added to the cell to make a 15% solution by we&t (7) The solution melts at - 5 “C (over a SOmm penod) In this case the loss does not retrace the path It followed durmg freezmg the meltmg pomt has been depressed by the addltlon of the ethylene glycol (8) In comparison to the begmmng of this run, when the water was pure, the melted solutmn causes increased msetion loss, consistent wth the higher viscosity of the ethylene glycol/water nuxture [4] To correlate the magmtude of viscous damping of the APM with bqmd nscoslty, a serves of glycerol/water nurtures havmg a range of V~SCOSLties between 1 and 62 cP were used With hqmd contactmg the entrre unelectroded side of the APM wave path, mcludmg the transducer regtons, changes m loss AL (m dB) due to changes m hqmd shear vrscos&yAq (m Parse) are found to be gven by AL = 25(~)]~
(1)
The dependence of viscous loss on Mscoslty ~11 be discussed below m ~nn~~on w&h the Increase m loss oceumng m Figs 2 and 3 durmg the coohng of hqmd water The nature of the so~d/~lId mterface which forms when a 1lqmd-~hd phase change occurs on the APM device surface was mvestigated by momtonng the freezmg of gallium metal Shown m Ftg 4 1s the varration m loss measured as Ga sohdilfies on the APM sensor The dashed hne mdicates the loss obtamed wrth a free (unperturbed) deuce surface Placmg hqmd Ga (meltmg pomt 29 8 “C) on the device surface resulted m
+
Bonded sold
Go
34 %
5 lo g .Q $4 t
32 30 28 26 24 22 I
------
-
x,Free surface 5
10
16
20
26
i
Time, mln Rg 4 Insertion loss of APh4 sensor as Ga sohdsfies on the surface The Ga delanates spontaneously shortly after sohd&ation, causing loss to return to near the unperturbed (free surface) value
696
approximately 5 dB of viscous loss Sohdlficatlon of the Ga lmtlally contnbuted an additional 13 dB of loss After about two minutes, however, the loss unexpectedly dropped preclpltously, to near the free-surface value This was apparently due to delarnmatlon of the solid Ga from the quartz, at the conclusion of this expenment, the piece of solid Ga was found to be completely free on the quartz device surface
(4
Discussion
The results shown above indicate that APM devices are very sensltlve to hqmd-solid phase transitions Llqmd water contactmg the device contnbutes 5-7 dB of loss, depending on liquid temperature, ice contacting the device contnbutes an additional 11- 12 5 dB of loss The loss values thus provide unambiguous and reproducible mformation about whether a medmm contacting the device surface IS entirely liquid or entirely solid, temperature measurement alone can be amblguous, due to the posslblhty of supercooling The all-hqmd and all-solid phases are dlstmctly separable, with viscous properties determmmg the energy transfer to the hqmd, while elastic properties dictate loss to the solid phase, these two sltuatlons are readily modeled Because the specific structure of the two-phase nuxture dunng freezing 1s unknown, the response of the APM sensor to the mixed hqmd/sohd phase 1s not readily modeled Loss with an Adjacent Lzqurd Phase With a hqmd m contact mth the APM devtce, loss 1s determined by the hquld viscosity The surface components of the plate modes generate a damped shear wave m the hqmd which decays exponentially with distance from the surface, as shown m Fig 5 (a) The decay length at 158 MHz for hqmd water at 20 “C 1s about 50 nm [4] Thus, events occurnng only microns away, such as the freezing of the water at the top of the cell, are not detected by the APM device This 1s supported by the data of Figs 2 and 3, which show that when the RTD probe detects the onset of freezing, there 1s little accompanying change m the APM sensor output This indicates that crystallization probably did not begm on the APM device surface The increase m loss measured durmg coohng can be attnbuted to an increase m the shear vlscoslty of the water The coohng data of Fig 2 are reproduced as the solid points (0) m Fig 6 The known dependence of water shear vlscoslty on temperature [7] allows calculation, using eqn (l), of the viscous loss expected durmg cooling
@I Rg 5 Shear displacement at the APM surface contactmg a mednnn (a) Llqmd water m contact with the APM device results m entramment of a thm bqmd layer, leadmg to approxnnately 5 dB of loss over a 7 5 mm acoust~ path length (b) Bmdmg of ice to the surface of the APM devrce results m ra&ahon of acoustic energy out of the plate and 11 - 12 5 dB of loss
Tmpsrature
td*g
C)
Fig 6 Insertion loss measured durmg the coolmg of hqmd water, pnor to freezmg, taken from the data of Fig 2 (0) The sohd hne shows the loss calculated from eqn 1 due to changes m water vlscoslty with temperature
This loss, indicated by the sohd line ( -) 6, agrees reasonably with that measured cooling (0)
m Fig durmg
Loss with an Adjacent Sobd Phase While the viscosity of an adJaCXnt hqmd determines propagation loss, with an adJacent solid it is the elastic properties which dictate loss If the solid 1s sufliclently well-bonded to the APM device, a non-slip boundary condition unll exist at the solid/solid interface In this case, shear motion of the device substrate will couple shear motion into the adJacent solid (Fig 5(b)), leading to the
697
radiation of acoustic energy into the latter and greatly increasing propagation loss How mtimate the contact between the two sohds must be IS not fully understood, but we have found that slmply placing a solid, such as a piece of glass, on the APM device surface does not produce any measurable propagation loss The sohdlficahon of Ga on the APM device surface provides further insight into the effects of the nature of the sohd/sohd contact on propagatlon loss It should be pointed out that the observed spontaneous delammation of the Ga from the quartz surface IS qmte reasonable m light of the large contra&on ( -3 1%) whch accompames Its sohdlficatlon [8] The sudden decrease m loss observed (Fig 4) suggests that bondmg between the Ga and the quartz IS transitory, 1e , the solid lmtlally bonds sufficiently well to ensure a non-&p boundary, but then suddenly debonds, allowmg boundary shppage and an unperturbed APM This m turn suggests that the APM devrce could be useful m determmmg whether hqmd-tosohd phase changes result m mtlmate bondmg of a solid to the APM surface In such an expenment, the quartz APM device surface could first be modtfied by coatmg it Hrlth a second solid matenal (which would, of course, have to adhere well to the quartz) to allow the study of bonding between this second matenal and the substance undergomg the phase change If the bmdmg of the solid phase to the APM device 1s sufiiaent to insure a non-slip boundary, and both media are modeled as lsotroplc, then the effect of the phase change on APM propagation can be estimated If the acoustic shear wave velocity m the adjacent solid phase is higher than that of the plate, there should be a number of APMs which propagate m the quartz device Hrlthout increased loss However, when the shear velocity m the adjacent phase IS lower than that of the plate, APMs become ‘leaky’, Hrlth the degree of attenuation depending on the relative acoustic impedances of the plate and adjacent solid Most matenals which undergo a hqmd-solid phase change m the vlcuuty of room temperature (e g , water, organic solvents, and Ga) have a lower shear wave velocrty m the solid phase than quartz, resulting m leaky APM propagation when sohdfication occurs on the APM sensor Solving the wave equation in the plate, subject to the boundary condltlons Imposed at the lower free surface and upper contacted surface, allows determmatlon of the APM propagatton loss At the lower plate surface a stress-free boundary IS obtamed At the upper surface, the non-&p boundary between the plate and contacting medium reqmres (1) contmmty of particle dlsplacement, and (2) a balancing of shear stresses
across the mterface These latter boundary condotlons result m radiation of acoustrc energy from the plate into an adjacent sohd medium havmg lower shear wave velocity The couphng of acoustic energy from the plate into the adJacent medium leads to APM attenuation a,, which, for Z,/Z, < 1, IS gven by a, = $j tanh-‘(Z,/Z,)
(2)
where n IS the APM transverse mode mdex, 1 1s the APM wavelength along the surface (32 pm), and b 1s the plate thickness Z1 and Z, are the shear wave Impedances of the solid phase contactmg the plate and the quartz plate, respectively The ratio of these impedances IS gven by z, p’p’ ‘I2 -= (3) z2
(
PP
>
where p’ and p’ are the mass density and shear stiffness of the sohd contacting the quartz, respectively, p and ~1are the mass density and shear stiffness of quartz Ice properties at - 16 “C are [9] p’ = 917 kg/m3, p’ = 3 06 x 10’ N/m2, ZI = 1 68 x lo6 Rayls, for quartz p = 2650 kg/ m3, p = 3 99 x 10” N/m’, Z, = 103 x 10’ Rayls Consequently, Z,/Z, = 0 16 Plotted m Fig 7 1s the APM attenuation calculated from eqn (2) versus the ratio Z,/Z, of shear-wave impedances As Z,/Z, approaches unity, the acoustic nnpedance match improves between the plate and the adjacent sohd, leading to more rapid leakage of acoustic energy from the plate and higher attenuation The propagation loss L (m dB) correspondmg to CX,1s gven by L = 8 69aJ
(4) where I 1s the path length (center-to-center) between transducers For the path length of our
Rg 7 A theorekal estimate of the attenuation caused by havmg a senu-miimtc elastic sohd 111contact wltb the AFW devxe 2, and Z, are the shear wave unpadances of the adJacent solId and the quartz APM substrate, respechvcly
698
device (I = 0 75 cm), a reasonable estimate of sohdficatlon loss 1s obtained when a transverse mode index n = 3 1s used m eqn (2) It should be pointed out that the geometry of the particular APM device employed does not allow resolution of mdlvldual transverse modes [ 11, consequently, excltatlon at 158 MHz could result in several (n = O-5) transverse APMs bemg excited In any case, the mode index of 3 used m eqns (2) -(4) yields L = 14 dB, close to the value observed The n value which Dves reasonable estimates of loss when the device 1s m contact with ice may be regarded as an ‘average index’ for superposltlon of modes excited The freeang process of Fig 2 is exammed in greater detail m Fig 8, revealing that the transltlon from hqmd to solid 1s not mstantaneous Because the latent heat of fusion for water 1s large compared with Its heat capacity, the water does not supercool sufficiently for all of it to freeze simultaneously when nucleation begms, we have never observed the sort of dlscontmmty m the data which would be expected d all the water sohdlfied at once Consequently, for a penod of time (about two mmutes m the case of Fig 8), a two-phase system at 0 “C exists Dunng this time, the measured loss mcreases smoothly, the magmtude of the loss 1s consistent with the vlscoslty of the two-phase system increasing from about 2 cP, which corresponds to hqmd water supercooled to - 8 “C, to a final value of 7 CP It is conceivable that many small crystals may bind to the quartz surface and grow with time, creating an average loss somewhere between that associated with pure hquld and pure (bound) sohd The two-phase system presents a problem of considerable conceptual complexity, the solution to which awalts a
number of future expenments, mcludmg nucroscope visual observation and carefully controlled temperature gradients (e g , to ensure that nucleation starts on the quartz surface, cool only the quartz or coat the quartz with a thm, highthermal-conductlwty metal) c0lle1usIolls
It has been demonstrated that the APM device 1s a sensltlve momtor for phase transitions occurrmg on a surface A very small quantity of matenal 1s suffiaent for study, smce sensmg occurs m the first few rmcrons Information about the WScoslty of the hqmd phase IS obtamed, and for reasonably stiff sohds hke ice, a large and easily monitored change m msertlon loss occurs when solid forms on the APM device surface The effect of addltlves, such as ethylene glycol, can be momtored and the thawmg caused by the antifreeze venfied In addition, useful mformation IS gathered concermng the sticking or binding of the solid phase to the surface The surface of the plezoelectnc substrate may be coated with a thmfilm matenal to allow exammatlon of adhesion between the substance changmg phase and the thm film Some obvious uses for tis new sensor include momtonng the icing of alrcraft wmgs and other surfaces where the freezmg of water (and the stlckmg of ice) cause problems and evaluatmg the efficacy of chemical or blolo@cal agents designed to prevent frost damage to crops Many such apphcatlons reqmre the remote detection of freenng and thawmg events, where temperature alone 1s not a reliable guide
The authors are grateful to Diana Bkr of Sandra National Laboratones for suggestmg this solution to the ice detection problem and Barbara Lammle of Sandra National Laboratones for techmcal assistance Thus work was performed at Sandra National Laboratones and supported by the U S Department of Energy under contract DE-ACO4-76DPOOO789 I I I
0
,
50
100
Time
I 150
200
250
(set)
Fla 8 InsertIon loss measured wlule freezmn water on the GM devxe The three re.lpons denoted ‘LK&KI’, ‘Llqmd + Sohd’, and ‘Sohd’ are marked by the vertaxl hnes Completion of freezmg 1s determmed to be the trme at whch &e ice temperature starts to decrease after mmng at 0 “C durmg the freezmg process (see Fig 2)
1 S J Martm. A J bcco, T M Nxmczyk and G C Frye, Character&Uon of SH acoustic plate n&e hqmd sens&, Sensors and Actuators, 20 (1989) 253-268 2 A J Bcco, S J Martm, % M. Nnx~ayk and G C Frye, m R W Murray, R E Dessey, W R Hememan, J Janata and W R Se& (eds ), Chemical Sensors ami h4uzromstrumentatton, Amencan Chemxal Surety, Washmgton, DC, 1989, m press
3 S J Martm, A J bcco, G C Frye, T M Nuxnczyk and I Adluhetty, Sensmg m hquds with SH plate mode devxes, Proc 1988 IEEE Ulrrasonrcs Symp, IEEE, New York, 1989, m press 4 A J bcco and S J Martm, Acoustx wave vlscoslty sensor, Appl Phys L.&t, 50 (1987) 1474-1476 5 T M N~emczyk, S J Martm, G C Frye and A J Rxco, Acoustoelectnc rnteract~on of plate modes wth solutions, J Appl Phys , 64 (1988) 5002-5008 6 A J bcco and S J Martm, Proc Symp Electroless
699 Dep of Met& rmd Alloys, Vol 88-12, The Electrochemical Society, Penmngton, NJ, 1988, pp 142-153 7 R C Weast (ed ). CRC Handbook of ChemMy cmd Physics, Chcoucal Rubber Co , Boca Raton. FL, 54th edn ,1973, pp F43-F45 8 M Wmdbolz (ed ), The Merck Index, Merck & Co, Rahway,NJ, 10th edn, 1983, p 4218 9 Amerzcan Insfltute of Phystcs Handook, McGraw-Hffl, New York, 3rd edn , 1972, pp 2-49