Surface Tension Measurement with an Optofluidic Sensor

Transcription

1 1t International Conference on Sening Technology November 1-, 005 Palmerton North, New Zealand Surface Tenion eaurement with an Optofluidic Senor Abtract Nam-Trung Nguyen, Sumantri Laemono, Franck Alexi Chollet, Chun Yang School of echanical and Aeropace Engineering Nanyang Technological Univerity, Singapore Thi paper report a novel microfluidic enor for meauring dynamic ga/liquid interfacial tenion. The device conit of a mirofluidic chip with a microchannel network and an optical detection ytem. The ample i introduced into a main channel, while air i injected through a T-junction. A imple analytical model wa etablihed to identify the parameter, which can be ued for meauring the urface tenion. ue to the fixed flow rate ratio ued for the enor, bubble formation frequency i the only parameter, which can be meaured eaily by optical detection. While the bubble i repreented by a pule in the output ignal, the formation frequency i imply the frequency of the output ignal. eaurement were carried out for aqueou olution with different concentration of the ionic urfactant CTAB (Cetyl Trimethyl Ammonium Bromide). Surface tenion of thee olution were calibrated with a commercial teniometer (FTA00, Firt Ten Angtrom). The meaurement reult how a clear relation between urface tenion and formation frequency. The enor can be ued to identify the critical micelle concentration (CC) of urfactant. The enor potentially allow the ue of a minute amount of ample compared to the relatively large amount required for exiting commercial ytem. Keyword: microfluidic, urface tenion, lab on a chip 1 Introduction ynamic urface tenion of an aqueou olution i an important parameter in many indutrial and dometic application. ynamic urface tenion depend on the urfactant concentration in the olution. Up to a certain concentration, urface tenion i inverely proportional to the urfactant concentration. Above thi concentration, urfactant molecule tart to cluter and form micelle, urface tenion remain almot contant. Thi critical concentration i called the critical micelle concentration (CC). Conventional meaurement method of urface tenion can be categorized in five group: direct meaurement uing microbalance, meaurement of capillary preure, analyi of capillary gravity force, gravity ditorted drop, reinforced ditortion of drop [1]. All thee technique require a relatively large amount of ample. onodipere droplet have traditional application in the field of food cience, cometic, and pharmaceutic. Recently, The formation of droplet and bubble in microchannel attract the interet of microfluidic community []. The main focu for thi phenomenon i it application in microreaction technology []. ot of the previou work are baed on droplet formed between immicible fluid. The flow pattern inide a moving droplet caue chaotic advection and improve mixing ignificantly. icrodroplet have been ued for NA analyi, protein crytalliation [], analyi of human phyiological fluid, encapulation [5], and production of polymeric micro bead [6]. Uniform droplet can be prepared uing a imple T-junction [7] or a cro junction [8]. All the above-mentioned work ued two immicible liquid to form droplet. To the bet knowledge of the author, no previou work ued thi concept for meauring the interfacial tenion between the two liquid. Furthermore, the generation of micro bubble in a ga/liquid ytem wa not mentioned in the recent reported work. Olthui et al. [9] reported microbubble generation baed on electrolyi. Bubble formation wa detected electrically. Thi method wa limited by aqueou ample, and ga/liquid interfacial tenion. The reported reult how a bad reproducibility of bubble formation due to the large number of bubble evolving at the ame time from the electrode urface. Furthermore, bubble detachment and formation rely entirely on the balance between urface tenion and buoyancy force. In micro cale, urface tenion i dominant while volume-baed buoyancy force become negligible. Thu, further miniaturization i not in favour for thi concept. Thi paper preent a microfluidic enor for dynamic urface tenion meaurement. The enor chip ha two function: generation and detection of air bubble in a ample flow. The microfluidic enor wa fabricated on a PA ubtrate by laer machining and direct thermal bonding. The bubble are formed at a T-

2 1t International Conference on Sening Technology November 1-, 005 Palmerton North, New Zealand junction between a large channel for the ample liquid and a maller channel for injection of the air bubble. The T-configuration ha been widely ued for droplet formation in the work mentioned above. Our experiment alo howed that thi configuration reult in reliable and reproducible generation of air bubble. The generated air bubble are detected optically uing a pair of optical fibre poitioned on both ide of the ample channel. The characterization reult how a clear relation between the urface tenion and the formation frequency of the bubble, which can be eaily determined from the output ignal of the optical detection ytem. Following, we firt dicu a imple analytical model for the relation between urface tenion and the formation frequency of the bubble. The model alo help to identify other parameter of the ample liquid, which may affect the bubble formation frequency beide urface tenion. Next, detail on fabrication and characterization of the enor are decribed. Surface tenion value were calibrated uing a commercial meaurement ytem. Finally, the reult are preented and dicued. Even though accurate and imple, the ytem i far from perfect. It working principle reult in two fundamental limitation. Firt, the detection baed on difference of refractive indexe of air and liquid make it not poible to meaure ga. Second, it can only meaure average flow o not uitable for online monitoring. Theoretical model Figure 1 depict a imple model of bubble formation. Since bubble formation i a complex phyical phenomenon, thi model only erve the purpoe of undertanding the relation between key parameter uch a bubble ize, formation frequency, ample flow rate, and mot importantly urface tenion. The model aume a fixed flow rate ratio between air and ample liquid ( α = Q & a /Q& ). Further aumption are mall bubble ize ( α <<1) and incompreibility of air. Since bubble in the model are formed in micro cale and the flow are in teady tate, ma related force uch a inertial force, momentum force and buoyancy force are neglected. A the growing bubble i preent in a flowing urfactant liquid, the urfactant concentration at the bubble urface i not uniformly ditributed and thu a gradient of urface tenion on the bubble urface i developed. The preence of the urface tenion gradient lead to the arangoni force Q u S Q a i b Figure 1: A imple model of bubble formation. acting on the bubble. If the urfactant olution i dilute, the arangoni force i aumed to be negligible, and thu the force balance equation including only the drag force of the ample flow and the urface tenion at the injection port i expreed a: F drag 1 C = F ρu urface tenion A = C π i where u, A, i, and are the average velocity of the ample flow, the effective drag urface, the diameter of the injection opening, and the urface tenion, repectively. In addition, C and C are the drag coefficient and the coefficient for the urface tenion. The drag coefficient of a phere at a low Reynold number Re i calculated a C = /Re. The coefficient C (1) depend on the contact angle and the hape of the injection port. In thi model C i aumed contant. The effective drag urface A grow with the bubble. If the bubble i a phere, the effective drag urface at the detachment moment i: A πb = () where b i the diameter of the bubble. Initially the bubble i mall, the urface tenion i large enough to keep the bubble at the injection port. At the detachment moment, due to the continuou bubble growth the drag force i large enough to releae the bubble. Subtituting () into (1) reult in the bubble diameter: C = i () C ρ b u The formation frequency can be etimated from the air flow rate Q & and the bubble volume V a: a f = Q & a /V b () Uing the bubble diameter b and the relation Q & a = αq&, the formation frequency in () can be expreed a: α ρ u f = (5) 16( Ci / C ) From (5), the urface tenion can be meaured baed on the formation frequency f, if all other parameter are contant. The denity of the liquid ha the ame impact on the formation frequency a the urface tenion. Thu, thi method i only uitable for olution with contant denity and varying urface b

3 1t International Conference on Sening Technology November 1-, 005 Palmerton North, New Zealand =9. mn/m =9.1 mn/m =6. mn/m =7.5 mn/m Sample flow rate Q (ml/h) Q =8 ml/h Q =6 ml/h Q = ml/h Q Surface tenion (mn/m) (a) (b) Figure : Characteritic of bubble formation tenion uch a thoe of diluted urfactant. In thee olution, the denity change uually i of to order le than the urface tenion change. Equation (5) alo how a nonlinear relation between the formation frequency and the average ample velocity ( f u ) or ample flow rate ( f Q& ). Figure (a) how thi relation graphically. The characteritic of bubble formation frequency veru urface tenion at different ample flow rate are depicted in Fig. (b). Furthermore, if the arangoni force i conidered in equation (1) when the urfactant concentration i high, there will be an additional term for uch force in the numerator of (5), reulting in a higher frequency. Senor concept and fabrication Our microfluidic enor conit of a microchannel network for droplet formation and an optical ytem for bubble detection. The configuration of the microchannel network i depicted in Figure 1. Air bubble are injected through a mall microchannel, while the ample liquid come through a larger microchannel. The two channel form a T-junction, at which bubble formation occur. The influence of channel diameter i and on the formation frequency i apparent in (5). After being generated and tabilied in the long ample channel, the bubble are detected at a downtream poition. Bubble detection i baed on an optical concept. Laer light i guided into the microchannel by an optical fibre. After paing acro the main channel, the light i received on the other ide by a econd optical fibre, which lead it to an optical enor. The paing-by bubble change the intenity of the laer due to diffraction and aborption. Thu, the bubble can be well recorded a a pule in the output ignal of the optical enor. Our microfluidic device i made of polymethyl methacrylate (PA), the microchannel are machined into the ubtrate uing CO laer. PA i one of the thermoplatic polymer, which are uually linear-linked and will often when heated above gla tranition temperature. PA ha a noncrytalline tructure and therefore poee good optical propertie with a 9 % tranmittance in the Sample inlet Laer in Air inlet Wate outlet Laer out Fig. : The completed microfluidic enor for dynamic urface tenion meaurement. viible pectrum. CO laer ha a relatively long characteritic wavelength of 10.6 µm. Therefore, the ablation proce depend more on thermal energy, which in turn ha the ame ditribution a the laer intenity. We ued the commercial Univeral -00 Laer Platform (Univeral Laer Sytem Inc.). The ytem ha a 5 Watt CO laer and a maximum beam peed of about 60 mm/. The cro ection of the engraved microchannel depend on the intenity ditribution of the laer beam, it moving peed, the laer power and the thermal diffuivity of ubtrate material. The intenity of the laer beam ha a Gauian ditribution, thu the cro ection of the channel alo ha a Gauian hape. The injection channel and the guide for inerting the optical fibre are both 175 µm in width and 05 µm in depth. The larger microchannel for ample flow ha a width of 0 µm and a depth of 0 µm. The guide for the two optical fibre are engraved at a down tream poition. The optical fibre (AFS105/15Y, THORLABS Inc.) have a core diameter of 105 µm, a clad diameter of 15 µm, a buffer diameter of 50 µm, and a numerical aperture of 0.. After poitioning the fibre into the guide, the PA part with microchannel, optical fibre, and acce hole i covered by a econd PA part. The PA tack i then placed between a hotplate and a aluminium plate. The bonding preure can be adjuted by putting weight on top of the upper plate. For a better urface flatne of the PA part, two polihed ilicon wafer are placed on both ide of the PA-tack. Prior to the bonding proce, the PA part were carefully cleaned and rined in ethanol and I-water. After bonding at 165 ºC, the bonded tack i annealed at 80 ºC for relieving tre. The bonding preure wa kept at about 0 kpa. The total fabrication proce took about hour. Finally, tainle teel needle were glued to the acce hole. The completed device i hown in Fig.. Experiment In our experiment, the ample liquid i introduced into the main channel, while air join through the maller injection channel. A yringe filled with the ample liquid and an empty yringe (filled with air)

4 1t International Conference on Sening Technology November 1-, 005 Palmerton North, New Zealand Pure I water, c=0 c=1x10 - c=1x10 - c=5x10 - c=1x10 - c=5x10 - c=5x10 - c=1x10 - c=5x10 - c=1x10 - Figure : Typical bubble ignal detected by the AP at an air flow rate of 1 ml/h (contant flow rate ratio α=1:) are placed on a yringe pump (Cole-Parmer ) Since both yringe are driven by the ame tepper motor, the flow rate ratio can be adjuted by chooing yringe with a correponding ratio of cro ection. Small flow rate ratio a aumed in the analytical model lead to relatively untable droplet formation proce. In our experiment, the volumetric flow rate ratio between air and ample liquid i kept at 1:. For detecting the micro bubble, one optical fibre i poitioned and aligned to a laer ource (laer diode, 65 nm), the other fibre i connected to an avalanche photodiode module (AP, C560-01, Hamamatu, Japan). The output ignal from the AP i recorded by a digital ocillocope (TS0, Tektronix), which in turn i connected to a peronal computer (PC) over a erial cable. Thu, the bubble ignal can be recorded and analyzed later on the PC. Cetyl trimethyl ammonium bromide (CTAB, C 19 H BrN) wa ued a urfactant to vary urface tenion value. Sample with different concentration ranging from 10 - to 10 - were teted. For calibration, the urface tenion of the ample wa meaured uing the teniometer FTA00 (Firt Ten Angtrom). Table 1 ummarie the meaured urface tenion value of thee ample olution. The data how that urface tenion decreae with increaing concentration. The CC i about Table 1 eaured urface tenion of aqueou olution of CTAB. Concentration () Surface tenion (mn/m) Reult and icuion Figure how the typical bubble ignal detected by the AP at a ample flow rate of ml/h for the different ample concentration. The bubble are c=1x10 - Pure I water, c= Sample flow rate Q (ml/h) Figure 5: eaured bubble formation frequency a function of urface tenion at air flow rate of 1 ml/h and ml/h. repreented by the pule. The reult here clearly how that the bubble become maller and the formation frequency i higher with increaing urfactant concentration or decreaing urface tenion. Beyond the CC of about , the formation proce become untable, the formation frequency fluctuate. Thi intability characteritic can be ued to detect CC. In the table region up to CC, the pule width ratio of 1:5 i conitent with the fixed flow rate ratio of 1: between air and ample liquid. Thu, the formation frequency alo contain information about the pule width or the bubble ize. Following, only the formation frequency are evaluated and dicued. Figure 5 depict the meaured frequency of bubble formation a a function of ample flow rate. From the theory, the expected non-linear relation can be oberved clearly. The ignal i table and conitent at low ample flow rate and high urface tenion. The characteritic of bubble formation frequency veru urfactant concentration are hown in Fig. 6. The reult how that droplet formation frequency continue to increae beyond CC. Since the urface tenion doe not change ignificantly at concentration higher than CC, the frequency increae could be caued by arangoni force a indicated in the theoretical model. Figure 7 how the formation frequency a a function of urface tenion. Surface tenion value were calibrated according to the data lited in Table 1. For diluted olution with low urfactant concentration, the curve agree well with the general characteritic predicted by the theory and hown in Fig. (b). The abrupt change of the formation frequency at CC can be clearly oberved in Fig. 7. Up to CC, bubble formation and detachment i dominated by the drag force, the formation frequency decreae almot linearly with increaing urface tenion. In thi range, formation frequency can be ued directly for determining the urface tenion. Beyond CC, there 5

5 1t International Conference on Sening Technology November 1-, 005 Palmerton North, New Zealand i a harp change in formation frequency. The arangoni force caued by urface tenion gradient on the bubble urface become dominant in thi range and contribute to the increae of formation frequency. 6 Concluion Q=8 Q= Figure 6: eaured bubble formation frequency a function urface tenion. arangoni force region Q=8 Q= rag force region Surface tenion (mn/m) Figure 7: eaured bubble formation frequency a function urface tenion. In thi paper, we preented a microfluidic device for bubble formation and detection. The device ha a microchannel network to form the bubble. The bubble are detected by two optical fibre. A bubble paing by the detection point diffract a part of the incoming laer light. The change in intenity can be detected by the optical fibre placed on the other ide of the channel. ue to the mall change in denity, formation frequency depend only on the urfactant concentration or on the urface tenion. Since the frequency increae monotonouly with the concentration, bubble formation frequency can be related directly to the urfactant concentration. Up to CC, the relation between urface tenion and formation frequency i almot linear. Thu, formation frequency can be ued to meaure urface tenion. Beyond CC, the frequency change harply. etecting the harp change may allow the enor to determine thi critical value of a urfactant olution. Acknowledgment Thi work wa upported by the Academic Reearch Fund of the initry of Education Singapore, contract number RG11/0. 7 Reference [1] J. relich, Ch. Fang, C. L. White. eaurement of interfacial tenion in fluid-fluid ytem. in Encylopedia of Surface and Colloid Science, arcel ekker, 00, pp [] T. Niiako, T. Torii, and T. Higuchi. roplet formation in a microchannel network. Lab on a Chip, 00,, []. R. Bringer et al. icrofluidic ytem for chemical kinetic that rely on chaotic mixing in droplet, Phil. Tran. R. Soc. Lond. A, 00, 6, 1087 [] B. Zheng, J.. Tice, and R. F. Imagilov. Formation of arrayed droplet by oft lithography and two-phae fluid flow and application in protein crytallization. Advanced aterial, 00, 15, 165 [5] S. Okuhima, T. Niiako, T. Torii, and T. Higuchi. Controlled production of monodipere double emulion by two-tep droplet breakup in microfluidic device. Langmuir, 00, 0, 9905 [6] T. Niiako, T. Torii, and T. Higuchi. Novel microreactor for functional polymer bead. Chemical Engineering Journal, 00, 101, [7] Y. Thoren, R. W. Robert, F. H. Arnold, and S. R. Quake. ynamic pattern formation in a veicle-generating microfluidic device. Phyic Review Letter, 001, 86, 16 [8] R.reyfu, P. Tabeling, and H. Willaime. Ordered and diordered pattern in two-phae flow in microchannel. Phyic Review Letter, 001, 90, 1505 [9] W. Olthui, A. Volanchi, P. Bergveld. ynamic urface tenion meaured with integrated enor-actuator uing electrolytically generated bubble. Senor and Actuator B, 1998, 9, 16 6