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1 Thi document i downloaded from DR-NTU, Nanyang Technological Univerity Library, Singapore. Title A microfluidic enor for dynamic urface tenion meaurement Author() Citation Nguyen, Nam-Trung; Laemono, Sumantri; Chollet, Franck Alexi; Yang, Chun Nguyen, N. T., Laemono, S., Chollet, F. A. & Yang, C. (2006). A microfluidic enor for dynamic urface tenion meaurement. IEE Proceeding - Nanobiotechnology, 153(4), Date 2006 URL Right 2006 The Intitution of Engineering and Technology. Thi i the author created verion of a work that ha been peer reviewed and accepted for publication by IEE proceeding - nanobiotechnology, The Intitution of Engineering and Technology. It incorporate referee comment but change reulting from the publihing proce, uch a copyediting, tructural formatting, may not be reflected in thi document. The publihed verion i available at:

2 Microfluidic enor for dynamic urface tenion meaurement N.-T. Nguyen *, S. Laemono, F. A. Chollet, C. Yang *Correponding author. Addre: School of Mechanical and Aeropace Engineering, Nanyang Technological Univerity, Nanyang Avenue 50, Singapore Fax: ; Tel: ; Abtract: A novel microfluidic enor for meauring dynamic ga-liquid interfacial tenion i reported. The device conit of a microfluidic 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. Owing to the fixed flow rate ratio ued for the enor, urface tenion i the only parameter determining bubble formation frequency, which can be meaured 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. Meaurement were carried out for aqueou olution with different concentration of the ionic urfactant cetyl trimethyl ammonium bromide. Surface tenion of thee olution were calibrated with a commercial teniometer. The meaurement reult how a clear relation between urface tenion and formation frequency. The enor can be ued to identify the critical micelle concentration 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. 1 Introduction The dynamic urface tenion of an aqueou olution i an important parameter in many indutrial and dometic application. Dynamic 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

3 concentration, urfactant molecule tart to cluter and form micelle, urface tenion remain almot contant. Thi critical concentration i called the critical micelle concentration (CMC). Conventional meaurement method of urface tenion can be categorized in five group [1]: direct meaurement uing microbalance meaurement of capillary preure, analyi of capillary gravity force, gravity ditorted drop, reinforced ditortion of drop. All current commercially available ytem are baed on the above concept. In the firt group, urface tenion i meaured directly by a force enor. Commercial ytem ue a plate or a ring that i made of platinum-iridium alloy or platinum. The plate or ring have tandard dimenion, thu no calibration i required. In the econd group, urface tenion i derived from capillary preure, which can be meaured directly with a preure enor. The third group meaure gravity rie or ize of a droplet after detachment. In the fourth group, the hape of the droplet i ditorted by urface tenion and gravity, and meaurement of the geometry of a pendant drop allow the determination of urface tenion. Thi lat method require a CCD camera and a bulky computer for evaluation. Beide the above-mentioned conventional technique, there i a growing interet in interfacial tenion meaurement of mall ample. The tudy of interface on very mall particle and in finely dipered ytem i called microteniometry. The main application field of microteniometry are criminology, biology and pharmaceutical microreactor. The two method currently known for

4 microteniometry are: micropipette technique and atomic force microcopy. In micropipette technique, a droplet i firt captured at the tip of a micropipette. If the radian of curvature on both ide of the droplet i ued, the urface tenion can be calculated. Thi technique alo require bulky microcope and an image recording ytem. The econd approach i baed on direct force meaurement, where a force enor i needed. Both method of microteniometry have to deal with the problem of evaporation becaue the mall ample droplet can quickly evaporate. Monodipere 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 [2]. The main focu for thi phenomenon i it application in microreaction technology [3]. Mot of the previou work i baed on droplet formed between immicible liquid. The flow pattern inide a moving droplet can improve mixing ignificantly. Microdroplet have been ued for DNA analyi, protein crytalliation [4], analyi of human phyiological fluid, encapulation [5], and production of polymeric microbead [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 microbubble 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 to aqueou ample and the meaurement of ga-liquid interfacial tenion. The meaured ignal were

5 not table owing 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 microcale, urface tenion i dominant while volume-baed buoyancy force become negligible. Thu, further miniaturization i not in favourable 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 polymethyl methacrylate (PMMA) ubtrate by laer machining and direct thermal bonding. The bubble are formed at a T-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 relationhip 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. Detail on fabrication and characterization of the enor are decribed in the following. Surface tenion value were calibrated uing a commercial meaurement ytem. Finally, the reult are preented and dicued. 2 Senor concept and fabrication The enor concept i baed on the proce of bubble formation in a microchannel; ee Fig. 1. A bubble formation i a complex phyical phenomenon, the following analytical model only erve the purpoe of promoting the

6 relationhip between key parameter: the formation frequency and the urface tenion. The model aume a fixed flow rate ratio between air and ample liquid Q / Q Further aumption are mall bubble ize ( a 1 ) and incompreibility of air. A bubble in the model are formed at microcale 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 develop. The preence of the urface tenion gradient lead to the Marangoni force acting on the bubble. If the urfactant olution i dilute, the Marangoni 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 2 D F u urface tenion 2 A D C D i (1) where u, A D, D 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, CD 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. The coefficient C depend on the contact angle and the hape of the injection port. In thi model C i aumed contant. The effective drag urface A D grow with the bubble. If the bubble i a phere, the effective drag urface at the detachment moment i

7 A D Db 2 2 (2) where Db 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, owing to the continuou bubble growth the drag force i large enough to releae the bubble. Subtituting (2) into (1) reult in the bubble diameter: D b 2 C C D Di u 2 (3) The formation frequency can be etimated from the air flow rate Q a and the bubble volume V a: b f Q /V a b (4) Uing the bubble diameter Db and the relation Q a Q, the formation frequency in (4) can be expreed a: f 16 3 D C D / C 2 i 2 D u (5) 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 tenion uch a thoe of diluted urfactant. In thee olution, the denity change uually i of three to four order le than the urface tenion change. Our microfluidic enor conit of a microchannel network for bubble formation and an optical ytem for bubble detection. The configuration of the microchannel

8 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. 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 and detected by an optical enor. A croing air bubble change the intenity of the laer. The change in laer intenity i recorded a a pule in the output ignal of the optical enor. We ued Raytrace, an optical imulation tool ( to confirm thi detection concept. The laer i modelled by a number of light ray, whoe number repreent the laer intenity. Figure 2 depict the typical number of ray arriving at the receiving ide for different bubble poition. The poition are qualitatively depicted baed on the grid number in the model. The two peak caued by bubble entrance and bubble exit can be clearly oberved in Fig. 2. Some of the ray are diffracted while paing through the curved interface between air and liquid. To enter the fibre core, the incident light mut enter within a cone, which depend on the numerical aperture (NA) of the optical fibre, a value fixed by the refractive indice of the fibre core and it cladding. In our experiment, the optical fibre had a numerical aperture of NA=0.22, thu the maximum angle of the incident light hould theoretically be arcin( NA) Our microfluidic device i made of PMMA, the microchannel are machined into the ubtrate uing CO 2 laer. PMMA i one of the thermoplatic polymer that are

9 uually linear-linked and will often when heated above gla tranition temperature. PMMA ha a non-crytalline tructure and therefore poee good optical propertie with a 92 % tranmittance in the viible pectrum. The CO 2 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 M-300 Laer Platform (Univeral Laer Sytem Inc.). The ytem ha a 25 W CO 2 laer and a maximum beam peed of about 640 mm -1. 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 205 µm in depth. The larger microchannel for ample flow ha a width of 340 µm and a depth of 340 µm. The guide for the two optical fibre are engraved at a downtream poition. The optical fibre (AFS105/125Y, THORLABS Inc.) have a core diameter of 105 µm, a clad diameter of 125 µm, a buffer diameter of 250 µm, and a numerical aperture of After poitioning the fibre into the guide, the PMMA part with microchannel, optical fibre, and acce hole i covered by a econd PMMA part. The PMMA tack i then placed between a hotplate and an aluminium plate. The bonding preure can be adjuted by putting weight on top of the upper plate. For a better urface flatne of the PMMA part, two polihed ilicon wafer are placed on both ide of the PMMA-tack. Prior to the bonding proce, the PMMA part were carefully cleaned and rined in ethanol and DI-water. After bonding at 165 ºC, the bonded tack i annealed at 80 ºC for relieving tre. The bonding preure

10 wa kept at about 20 kpa. The total fabrication proce took about 4 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) are placed on a yringe pump (Cole-Parmer , 0.2 µlh -1 to 500 mlh -1, accuracy of 0.5 %). A both yringe (Hamilton GaTight) are driven by the ame tepper motor, the flow rate ratio can be adjuted by chooing yringe with a correponding ratio of cro-ection. In our experiment, the volumetric flow rate ratio between air and ample liquid i kept at 1:4. Figure 4 how the typical proce of bubble formation at the injection port. In thee image, diluted fluorecence (acid yellow 73) wa ued for a better contrat. For detecting the micro bubble, one optical fibre i poitioned and aligned to a laer ource (Melle Griot 58SDM101, 635nm), the other fibre i connected to an avalanche photodiode module (APD, C , Hamamatu, Japan), Fig. 5. The output ignal from the APD i recorded by a digital ocillocope (TDS220, 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 42 BrN) wa ued a urfactant to vary urface tenion value. Sample with different concentration ranging from 10-4 M to 10-2 M were teted. For calibration, the urface tenion of the ample wa meaured uing the teniometer FTA200 (Firt Ten Angtrom).

11 4 Reult and dicuion Figure 6 how the typical bubble ignal detected by the APD at a ample flow rate of 4mLh -1 for the different ample concentration. The reult agree qualitatively well with the imulation reult in Fig. 2. A optical imulation doe not include the different aborbance behaviour of air and the ample liquid, the imulated pule caued by the bubble i not a clear a in the meaurement. 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 CMC of about M, the formation proce become untable and the formation frequency fluctuate. Thi characteritic intability can be ued to detect CMC. In the table region up to CMC, the ratio of 1:5 between the pule width and the period i conitent with the fixed flow rate ratio of 1:4 between air and ample liquid. Thu, the formation frequency alo contain information about the pule width or the bubble ize. In the following, only the formation frequency i evaluated and dicued. Figure 7 depict the meaured frequency of bubble formation a a function of ample flow rate. The line are fitting curve of the meaured data. The ignal i table at low ample flow rate and high urface tenion. The characteritic of bubble formation frequency againt urface tenion are hown in Fig. 8. Table 1 ummarie the meaured urface tenion value of thee ample olution and the correponding meaured formation frequencie. The data how that urface tenion decreae with increaing concentration. The CMC i about 10-3 M, Fig. 8a. The reult how that bubble formation frequency continue to increae beyond CMC. A the urface tenion doe not change ignificantly at concentration higher than CMC, the frequency increae could be caued by

12 Marangoni force. A the bubble grow 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 develop. The preence of the urface tenion gradient lead to the Marangoni force acting on the bubble. If the urfactant olution i dilute, the Marangoni force i aumed to be negligible. The Mangaroni force i induced by non-uniform ditribution of urfactant cluter that make the formation and break-up of the bubble untable. Surface tenion value were calibrated according to the data lited in Table 1. The calibration curve i hown in Fig. 8a. The error data were calculated a the tandard derivation of the meaured frequency value. The abrupt change of the formation frequency and the large error due to intability at CMC and above can be clearly oberved in Fig. 8b. Up to CMC, bubble formation and detachment i dominated by the drag force, the formation frequency decreae almot linearly with increaing urface tenion. In thi range, the formation frequency can be ued directly to determining the urface tenion. A mentioned above, there i a harp change in formation frequency beyond CMC. The two region can be clearly oberved in Fig. 8b. In both Fig. 7 and Fig. 8b, the line repreent the fitting curve of the meaured data. 5 Concluion In thi paper, we reported 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. Owing to the mall change in denity, formation frequency depend only on the urfactant concentration or on the urface tenion. A the frequency increae monotonouly

13 with the concentration, bubble formation frequency can be related directly to the urfactant concentration. Up to CMC, the relation between urface tenion and formation frequency i almot linear. Thu, formation frequency can be ued to meaure urface tenion. Beyond CMC, the frequency change harply. Detecting 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 Minitry of Education Singapore, contract number RG11/02.

14 Reference 1 Drelich, J., Fang, Ch., White, C. L.: Meaurement of interfacial tenion in fluid-fluid ytem. in Encylopedia of Surface and Colloid Science (Marcel Dekker, 2002), pp Niiako, T., Torii, T., and Higuchi, T.: Droplet formation in a microchannel network, Lab on a Chip, 2002, 2, pp Bringer, M. R. et al.: Microfluidic ytem for chemical kinetic that rely on chaotic mixing in droplet Phil. Tran. R. Soc. Lond. A, 2004, 362, pp Zheng, B., Tice, J. D., and Imagilov, R. F.: Formation of arrayed droplet by oft lithography and two-phae fluid flow and application in protein crytallization, Adv. Mater., 2004, 125, pp Okuhima, S.; Niiako, T., Torii, T. and Higuchi, T.: Controlled production of monodipere double emulion by two-tep droplet breakup in microfluidic device, Langmuir, 2004, 20, pp Niiako, T., Torii, T., and Higuchi, T.: Novel microreactor for functional polymer bead, Chem. Eng. J., 2004, 101, pp Thoren, Y., Robert, R. W., Arnold, F. H., and Quake, S. R.: Dynamic pattern formation in a veicle-generating microfluidic device, Phy. Rev. Lett., 2001, 86, pp Dreyfu, R., Tabeling, P. and Willaime, H.: Ordered and diordered pattern in two-phae flow in microchannel, Phy. Rev. Lett., 2001, 90, pp Olthui, W., Volanchi, A., Bergveld, P.: Dynamic urface tenion meaured with an integrated enor-actuator uing electrolytically generated bubble, Senor and Actuator B, 1998, 49, pp

15 Lit of Table Table 1 Meaured urface tenion of aqueou olution of CTAB and the correponding formation frequencie

16 Table 1 Concentration, M Surface tenion mnm -1 Formation frequency at 4 mlhr -1,Hz Formation frequency at 8 mlhr -1,Hz

17 Lit of Figure Fig. 1 Concept of the microfluidic enor for dynamic urface tenion meaurement Fig. 2 Simulation reult of the optical detection concept for the microbubble Fig. 3 The completed microfluidic enor for dynamic urface tenion meaurement Fig. 4 Formation proce of an air bubble at the injection port: (a) Entry; (b) Bubble growth, (c) Bubble injection Fig. 5 Meaurement etup for enor characteriation Fig. 6 Typical bubble ignal detected by the APD at an air flow rate of 1 mlh -1 (contant flow rate ratio 1:4) Fig. 7 Meaured bubble formation frequency a function of ample flow rate at different urfactant concentration Fig. 8 Characteritic of bubble formation frequency againt urface tenion (a) Calibration curve, (b) Meaured bubble formation frequency a a function urface tenion at ample flow rate of 4 mlhr -1 and 8 mlhr -1

18 Sample flow Q Air flow D i Q a Air bubble Laer in D D b Laer out Photo diode Fig. 1

19 Number of ray Bubble poition Fig. 2

20 Laer in Wate outlet Laer out Sample inlet Air inlet Fig. 3

21 (a) (b) (c) Fig. 4

22 Laer diode (635 nm) Fiber alignment ytem Senor chip APD module Syringe pump Fig. 5

23 Pure DI water, c=0 M c=1x10-4 M c=5x10-4 M c=1x10-3 M c=5x10-3 M c=1x10-2 M Fig. 6

24 Bubble formation frequency f (Hz) c=1x10-2 M c=5x10-3 M c=1x10-3 M c=5x10-4 M c=1x10-4 M Sample flow rate Q (ml/h) Pure DI water, c=0 M Fig. 7

25 Bubble formation frequency f (Hz) Surfactant concentration (M) (a) Surface tenion (mn/m) Marangoni force region (b) Q =8 ml/hr Q =4 ml/hr Drag force region Surface tenion (mn/m) Fig. 8