Abstract:
A microfluidic sensor is disclosed that has a first inlet channel for a first fluid, a second inlet channel for a second fluid, and a measurement channel intersecting with both first inlet channel and the second inlet channel. A signal source system is provided for receiving a signal from a signal emitter, as is a signal detection system for receiving the signal from the signal source system. The signal source system and the signal detection system are for recording physical characteristics of at least one of the droplets in the measurement channel. A corresponding method is also disclosed.

Description:
FIELD OF THE INVENTION 
     This invention relates to microfluidic sensor for interfacial tension measurement and method for measuring interfacial tension and relates particularly, through not exclusively, a microfluidic device and methods for quick measurements of interfacial surface tension with a small quantity of a sample liquid. 
     BACKGROUND TO THE INVENTION 
     As shown in  FIG. 1 , known measurement methods of interfacial tension can be placed in five groups:
         direct measurement using microbalance;   measurement of capillary pressure;   analysis of capillary gravity forces;   gravity distorted drop; and   reinforced distortion of drop.       

     In the first method, surface tension is measured directly by a force sensor. Such systems use a plate or a ring of platinum-iridium alloy or platinum. The plates and rings have of standard dimensions, thus no calibration is required. In the second method, surface tension is proportional to capillary pressure, which can be measured directly with a pressure sensor. The third method measures gravity rise or size of a droplet after detachment. In the fourth method, the shape of the droplet is distorted by surface tension and gravity. Measuring the geometry of a pendant drop allows the determination of surface tension. For this measurement method, a CCD camera and computer evaluation is needed. The spinning drop technique evaluates the distortion of a drop and needs a CCD camera. 
     Besides these techniques, there is interest in interfacial tension measurement of small samples. The study of interfaces of very small particles and in finely dispersed systems is micro tensiometry. The main application fields of micro tensiometry are criminology, biology and pharmaceutical micro reactors. The two methods currently known for micro tensiometry are:
         (a) micropipette technique; and   (b) atomic force microscopy.       

     These are shown in  FIG. 2 . 
     In the micropipette technique of  FIG. 2(   a ), a droplet is first captured at the tip of a micropipette. Utilizing the radian of curvature on both sides of the droplet as shown in  FIG. 2(   a )A, the surface tension can be calculated. This technique requires a microscope and an image recording system. The second approach of direct force measurement as shown in  FIG. 2(   a )B. A force sensor is again required. 
     A miniaturized version of the direct measurement method depicted in  FIG. 1  is the use of atomic force microscopy to determine extremely small forces ( FIG. 2(   b )). The deflection of the micro cantilever is measured with a laser beam. Forces of the order of 1 pN can be measured. It has been proposed to use bubble generation and surface tension evaluation. The bubble is generated by electrolysis and detected electronically. The frequency of bubble formation is a measure surfactant concentration. A multi-well plate reader may be modified to evaluate surface tension. This technique utilizes the radius of curvature of the liquid surface acting as a fluidic lens and requires a camera system and an expensive commercial plate reader system. 
     All micro tensiometry techniques above require individual handling of a single droplet. As such, evaporation is a problem. Furthermore, the measurement is expensive and requires dedicated equipment. The bubble generation system is limited by the gas/liquid system of an aqueous sample. 
     It would be of advantage to be able to measure interfacial tension of micro droplets and bubbles in a simple configuration utilizing microfluidic technology. This should enable:
         a small sample size, higher accuracy, and faster results;   interfacial tensions of all immiscible systems (both liquid/liquid and gas/liquid);   lower cost and easier handling   be suitable for hand-held systems and portable field measurements; and   an integrated “lab-on-chip” device with a microchannel and optical wave guides is possible.       

     SUMMARY OF THE INVENTION 
     In accordance with a first preferred embodiment there is provided a microfluidic sensor comprising:
         (a) a first inlet channel for a first fluid;   (b) a second inlet channel for a second fluid;   (c) a measurement channel intersecting with both first inlet channel and the second inlet channel;   (d) a signal source system for receiving a signal from a signal emitter;   (e) a signal detection system for receiving the signal from the signal source system;   (f) the signal source system and the signal detection system being for recording physical characteristics of at least one of the droplets in the measurement channel.       

     According to a second preferred aspect there is provided a method for measuring physical characteristics of at least one droplet of a first fluid in a measurement channel of a microfluidic sensor, the method comprising:
         (a) forcing a first fluid along a first inlet and into the measurement channel;   (b) forcing a second fluid along a second inlet and into the measurement channel to form the at least one droplet;   (c) using a signal source system to provide a source signal and a signal detection system to detect the source signal;   (d) recording physical characteristics of the at least one droplet in the measurement channel by using the signal source system and the signal detection system.       

     The first inlet channel, the second inlet channel and the measurement channel may be in a substrate. The first fluid may be air and the droplets may be air bubbles. 
     The signal source system may be a source wave guide, the signal may be light, and signal detection system may be a detection wave guide. The source wave guide may be a source optical fibre, and the detection wave guide may be a detection optical fibre. The source wave guide and the detection waive guide may be in the substrate. 
     The signal source system and the signal detection system may be axially aligned on opposite sides of and intersect with the measurement channel. The signal source system and the signal detection system may be substantially identical. 
     The substrate may be transparent. The signal source system may be a light emitter and the signal detection system may be an optical sensor; one of the light emitter and the optical sensor may be above the measurement channel, and the other of the light emitter and the optical sensor may be below the measurement channel. 
     The physical characteristics may be at least one of: droplet length, droplet size, advancing edge shape, receeding edge shape, contact angle of the at least one droplet with the measurement channel, velocity of movement of the at least one droplet in the measurement channel, speed of movement of the at least one droplet in the measurement channel, and frequency of droplet formation. 
     The signal emitter may be a laser emitter, and the signal detector may be an optical sensor. The microfluidic sensor may further comprise a first fluid reservoir operatively connected to the first inlet channel, a second fluid reservoir operatively connected to the second inlet channel, and a waster reservoir operatively connected to an outlet end of the measurement channel. 
     The microfluidic sensor may further comprise a first pump operatively connected to the first fluid reservoir for forcing the first fluid into the first inlet channel and the measurement channel; and a second pump operatively connected to the second fluid reservoir for forcing the second fluid into the second fluid outlet and the measurement channel. 
     According to a third preferred aspect there is provided a tensiometer module comprising a receptor for a microfluidic sensor as described above, the receptor comprising electrical and optical connections for the microfluidic sensor and one of: a microcontroller and a digital signal processor. 
     According to a fourth preferred aspect there is provided computing apparatus comprising a tensiometer module as described above, and a screen. The tensiometer module may be removable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the present invention may be fully understood and readily put into practical effect, there shall now be described by way of non-limitative example only preferred embodiments of the present invention, the description being with reference to the accompanying illustrative drawings. 
       In the drawings: 
         FIG. 1  is an illustration of five prior art methods for measuring of interfacial tension; 
         FIG. 2  is an illustration of two prior art methods of micro tensiometry; 
         FIG. 3  is an illustration of three flow regimes of droplet formation in a microchannel; 
         FIG. 4  is a schematic illustration of a preferred embodiment of a microfluidic device; 
         FIG. 5  is an illustration of four components of  FIG. 4 ; 
         FIG. 6  is the evaluation of the optically detected signal for (a) pure water, and (b) 1 part surfactant to 80 parts water; 
         FIG. 7  is a graph of the frequency of droplet formation as a function of flow rate; 
         FIG. 8  is an illustration of counting the frequency of droplet formation; 
         FIG. 9  is a graph of the frequency of a droplet formation as a function of surfactant concentration; 
         FIG. 10  is a normalized graph corresponding to that of  FIG. 9 ; 
         FIG. 11  illustrates the change in droplet shape at the same flow rate due to the changes in surfactant concentration; 
         FIG. 12  illustrates the signals from the optical detection system corresponding to the droplet shapes of  FIG. 11 ; 
         FIG. 13  illustrates the method of counting the period of the droplet/bubble formation; 
         FIG. 14  are graphs of recorded signals from the optical sensor, (a) being the original time signal, and (b) being the time differential signal; 
         FIG. 15  is a graph of maximum values of time-differentiated signals on both sides of the droplet as a function of surfactant concentration; 
         FIG. 16  is an illustration of air bubble formations inside the measurement channel; 
         FIG. 17  shows detected signals of air bubbles at a constant flow rate with different surfactant concentrations; 
         FIG. 18  illustrates the measuring of droplet/bubble size; 
         FIG. 19  is two graphs of bubble generation frequency as a function of (a) surfactant concentration, and (b) surface tension; 
         FIG. 20  illustrates two forms of handheld terminal (a) with a computing platform (handheld computer, PDA, smart phone), and (b) stand alone; 
         FIG. 21  illustrates four different embodiments as variants of the device of  FIG. 4 ; 
         FIG. 22  illustrates an embodiment of a tensiometer module for  FIG. 21 ; and 
         FIG. 23  illustrates an embodiment for use in a detergent dispenser. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The formation of droplets and bubbles in microchannels may be used in microreaction technology, which can be used in both the chemical industry and for biochemical analysis. Microdroplets have been used for DNA analysis, protein crystallization, analysis of human physiological fluids, and cell encapsulation. The droplets are generated and manipulated using immiscible flows. The basic configuration is shown in  FIG. 3 . A carrier fluid such as oil disperses a sample fluid and splits it into single droplets, the size and frequency of the droplets depending on the flow rates (represented by the Reynolds number Re) and the interfacial tension (represented by the capillary number Ca). 
     To refer to  FIG. 3  the formation of droplet is shear-induced detachment. The balance of forces determines the final drop size at the end of the droplet growth, which is at the moment of detachment. The droplet size V droplet  and the volumetric flow rate of the sample {dot over (Q)} sample  determine the frequency of formation:
 
 f={dot over (Q)}   sample   /V   droplet   (1)
 
     The following forces may contribute to the detachment balance: 
     Drag force on droplet: 
                     F   drag     =       1   2     ⁢     C   D     ⁢     ρ   2     ⁢     u   2     ⁢     A   droplet   2               (   2   )               
Interfacial tension force:
 
F interfacial =C S σπD injection   (3)
 
Inertial force of the droplet:
 
                     F   inertial     =     ρ   ⁢           ⁢     V   droplet     ⁢       ⅆ   u       ⅆ   t                 (   4   )               
Momentum force:
 
                     F   momentum     =     ρ   ⁢         Q   .     sample       π   ⁢           ⁢       D   injection   2     /   4                   (   5   )               
Buoyancy force:
 
 F   buoyancy   =V   droplet (ρ carrier −ρ) g   (6)
 
     NOMENCLATURES 
     C D : drag coefficient 
     u: carrier flow velocity and droplet velocity 
     A droplet : projected area of the droplet 
     V droplet : volume of the droplet 
     C S : correction factor for surface tension force, depending on the injection angle (1 for our case of 90°) 
     D injection : hydraulic diameter of the injection channel 
     {dot over (Q)} sample : volumetric flow rate of the sample 
     {dot over (Q)} carrier : volumetric flow rate of the carrier 
     α={dot over (Q)} sample /{dot over (Q)} carrier : flow rate ratio 
     ρ: density of the sample liquid 
     ρ carrier : density of the carrier liquid 
     g: gravitational acceleration 
     σ: interfacial tension between sample liquid and carrier liquid 
     In the microscale, surface effects dominate over volume effects. Thus, all forces related to droplet volume and mass such as inertial force (4), momentum force (5) and buoyancy force (6) are negligible. The force balance is reduced to the two components of drag force and interfacial force, which are both surface forces: 
     
       
         
           
             
               
                 
                   
                     
                       F 
                       drag 
                     
                     = 
                     
                       F 
                       interfacial 
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       
                         1 
                         2 
                       
                       ⁢ 
                       
                         C 
                         D 
                       
                       ⁢ 
                       
                         ρ 
                         2 
                       
                       ⁢ 
                       
                         u 
                         2 
                       
                       ⁢ 
                       
                         A 
                         droplet 
                         2 
                       
                     
                     = 
                     
                       
                         C 
                         S 
                       
                       ⁢ 
                       σπ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         D 
                         injection 
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       A 
                       droplet 
                     
                     = 
                     
                       
                         1 
                         
                           ρ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           u 
                         
                       
                       ⁢ 
                       
                         
                           
                             
                               2 
                               ⁢ 
                               
                                 C 
                                 S 
                               
                             
                             
                               C 
                               D 
                             
                           
                           ⁢ 
                           π 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             D 
                             injection 
                           
                           ⁢ 
                           σ 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     Assuming that the droplet is a sphere with a diameter of the carrier channel D carrier  the projected area and the volume of the droplet are:
 
 A   droplet   =πD   carrier   2 /2  (8)
 
 V   droplet   =πD   carrier   3 /6
 
or
 
 V   droplet   =A   droplet   D   carrier /3  (9)
 
     Substituting (9) in (7) results in: 
     
       
         
           
             
               
                 
                   
                     V 
                     droplet 
                   
                   = 
                   
                     
                       π 
                       3 
                     
                     ⁢ 
                     
                       
                         ( 
                         
                           
                             
                               C 
                               S 
                             
                             
                               C 
                               D 
                             
                           
                           ⁢ 
                           
                             D 
                             injection 
                           
                           ⁢ 
                           
                             σ 
                             
                               
                                 ρ 
                                 carrier 
                               
                               ⁢ 
                               
                                 u 
                                 carrier 
                                 2 
                               
                             
                           
                         
                         ) 
                       
                       
                         3 
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     Substituting (10) in (1) results in the relation between droplet formation frequency and the interfacial tension: 
     
       
         
           
             
               
                 
                   f 
                   = 
                   
                     
                       
                         3 
                         ⁢ 
                         α 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           D 
                           carrier 
                           2 
                         
                       
                       
                         16 
                         ⁢ 
                         
                           
                             ( 
                             
                               
                                 C 
                                 S 
                               
                               ⁢ 
                               
                                 
                                   D 
                                   injection 
                                 
                                 / 
                                 
                                   C 
                                   D 
                                 
                               
                             
                             ) 
                           
                           
                             3 
                             2 
                           
                         
                       
                     
                     ⁢ 
                     
                       
                         
                           ρ 
                           carrier 
                           
                             3 
                             2 
                           
                         
                         ⁢ 
                         
                           u 
                           carrier 
                           4 
                         
                       
                       
                         σ 
                         
                           3 
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
     The results show the general relations between frequency and sample flow rate (f˜{dot over (Q)} sample   4 ) and between frequency and interfacial tension 
     
       
         
           
             ( 
             
               f 
               ∼ 
               
                 σ 
                 
                   - 
                   
                     3 
                     2 
                   
                 
               
             
             ) 
           
         
       
     
       FIG. 4  shows the schematics of a preferred embodiment of a microfluidic device  40 . The device  40  consists of two microchannels  42  joining at a T-junction  43 . The channels  41 ,  42  are a sample inlet  41  and a carrier fluid inlet  42 . After the junction  43  is a measurement channel  44 . The measurement channel  44  may be of any suitable length size and shape. It may be straight (as shown), curved, serpentine or the like. The carrier fluid is fed directly into the measurement channel  44  from inlet  42 , while the sample joins through the smaller inlet channel  41 . Downstream of the measurement channel  44 , at least one, but preferably two optical wave guides  411 ,  412  are positioned across the microchannel  44  for detecting the formed droplets. The optical wave guides  411 ,  412  are preferably optical fibers, and are axially aligned across the micro channel  44 . The wave guides  411 ,  412  can be integrated optical guides in the chip or hybrid-assembled optical fibers. Optical fiber with a core diameter of 105 μm may be used. The optical wave signals  411 ,  412  are preferably at least substantially identical. 
     The microfluidic device  40  may be fabricated in any material: silicon, SU-8, PDMS or PMMA. The microchannels  41 ,  42 ,  44  may be machined into the substrate  45  using a CO 2  laser. 
     The microchannel  44  may have a typical Gaussian shape ( FIG. 5(   a )). Dimensions of the channel cross-section such as width and depth depend on the laser power and the laser beam speed. Different laser parameters were applied for the different channel sizes depicted in  FIG. 5 , where:
         (a) is the measurement channel  44 ,   (b) is the sample inlet channel  41 ,   (c) is the junction  43 , and   (d) is the junction  47  of the optical fiber  411 ,  412  and the measurement channel  44 .       

     The two optical wave guides  411 ,  412  and preferably located near the outlet  48  of channel  44 . If the optical wave guides  411 ,  412  are optical fibers, the wave guides  411 ,  412  may be located in guides for accurately positioning the two optical fibers  411  and  412  for optical detection. After positioning the fibers  411  and  412 , the device  40  is bonded thermally at a temperature slightly above the glass temperature of PMMA. The channel guides  46  for the optical fibers  411  and  412  are sealed with adhesive to avoid leakage. 
     For detecting the droplets, one optical fiber  411  is positioned and aligned to a laser source  49  such, for example, a laser diode of a wavelength of, for example, 635 nm. The other optical fiber  412  is connected to a detector  410  such as, for example, an avalanche photodiode module (example: APD, C5460-01, Hamatsu, Japan). In this way it is possible to record physical characteristics of a droplet whilst still in the measurement channel  44  and as it passes between wave guides  411 ,  412 . The characteristics include length advancing and receding edge shape, contact angle speed of velocity of movement in the measurement channel  44 , and frequency of droplet formation. 
     The optical detection system is based on measuring the transmission of a laser beam across the measurement channel  44 . The system comprises an emitting sub-system to emit a beam that illuminates the channel  44  and a light detection sub-system measuring light on a limited surface. A laser diode  49  and the optical fiber  411  may be used as the emitting sub-system, and optical fiber  412  coupled to a photo detector  410  may be used as the detecting system. 
     An alternative arrangement could use waveguides integrated with the fluidic device instead of optical fibers to channel the light in and out of the channel  44 , allowing the distribution of light over different measurement sites. 
     Another version may integrate the laser diode and the photodetector directly on the microfluidic device  40  close to the measurement channel  44 . The system can also include other optical elements such as a lens to improve the sensitivity of the detection by providing a reshaped beam to illuminate the measurement channel  44 . Other parameter of the light beam may be measured to monitor the droplet, for example, the addition of a light polarizer would enable measurement of polarization changes. 
     An alternative is a capacitive detection system based on the capacitance change across the channel when a bubble/droplet is between electrodes. The detection system consists of two electrodes positioned across the channel  44 . An electronic circuit such as a capacitive bridge converts the capacitance change into a voltage. The frequency, time period, and bubble/droplet shapes follow the same methods. 
     Example 1 
     A carrier oil with a viscosity of, for example, 6.52×10 −2  Pa·s, may be passed to channel  44  through the carrier fluid inlet  42 . The sample fluid for inlet  41  may be pure DI-water (viscosity of approximately 10 −3  Pa·s) or water solution of diluted surfactants. The surfactants may mixed in different volume ratios to water (0.25:80, 0.5:80, 0.75:80, 1:80, 1.25:80, 1.5:80, 1.75:80, and 2:80). Each inlet  41 ,  42  is driven by a syringe pump (not shown). The diameters of the syringes have a ratio of 1 to 3. Thus, the total flow rate of oil is three times that of water. 
     When droplets form inside the microchannel  44 , the advancing and receding edge of the droplets have different contact angles and thus different radii of curvature. Using the optical detection concept described above in relation to  FIGS. 4 and 5 , it is possible to realize a closed loop control system with integrated micropumps for precisely generating liquid droplets or liquid plugs. This has potential in making compact droplet-based “labs on a chip”. 
     The frequency of droplet formation and the shape of the droplets depend on the flow rate of the sample, and the concentration of the surfactant.  FIG. 6  shows the typical signals of the optical detection with a sample flow rate of 50 μl/hour, the detection being by use of the optical fiber detection system  49 ,  411 ,  412 ,  410 . Decreasing the surface tension increases the formation frequency. However, the signal is distorted at high flow rates due to tiny satellite droplets. 
       FIG. 7  shows a linear relation between the frequency of droplet formation and the sample flow rate. The error bar is larger at higher flow rate because of the noise caused by satellite droplets. 
     As shown in  FIG. 8 , measuring the frequency of droplet/bubble formation is by counting the number of droplets or bubbles. If the recorded signal rises above a threshold voltage, an incremental counter increases its value. The number of droplet peaks over a fixed time represents the frequency of droplet formation. 
     In  FIG. 9 , by keeping the flow rates constant, the frequency will depend only on the concentration of the surfactants or the surface tension between the sample liquid and the carrier liquid. A simple evaluation circuit can count the frequency of the optically detected signal or the time period between two signal peaks. The measured frequency or time period can be correlated with the surface tension between the two phases. 
       FIG. 10  depicts the normalized frequency change (f−f 0 )/f 0  where f 0  is the frequency of droplet consisting of pure water. The curves show that the slower the flow rate, the larger is the frequency change. At lower flow rates the noise level is also lower due to a lack of satellite droplets. Small flow rates, such as those of the order of 100 nl/min, can be easily realized by different micro pump concepts, which can implemented in the same microfluidic system 
     As shown in  FIGS. 11 and 12 , each peak in the detected signal represents the size and shape of each droplet. The size of droplet can be measured by the width of each peak. Since the droplet is moving, the shape of the droplet is also determined by the interfacial tension. The shape change can be easily detected by the measured signal. With a high interfacial tension, the difference between the advancing and receding sides of the droplet is minimal. The difference increases with decreasing interfacial tension. The droplet transforms into a “bullet-like” shape (see  FIG. 11 ). The difference between the two sides can be evaluated and used as a measure of the interfacial tension. Measuring the time period of droplet/bubble formation is by determining the time between two rising edges of a signal. If the recorded signal rises above a threshold voltage, a timer (stand-alone or integrated in a microcontroller) starts counting. The timer stops counting of the signal rises above the same threshold again as is shown in,  FIG. 13 . 
     Time signals from the optical detection are fed to a digital signal processor (DSP). Next, the DSP calculates the time-differential signal of the original signal. The positive and negative peaks of the time-differential signal are detected as they represent the maximum slopes at the advancing and receding sides. The ratio or the difference between these two peaks also represents the interfacial tensions.  FIG. 14  shows the typical results of this.  FIG. 14(   a ) depicts the recorded time signals S(t) of droplets with different surfactant concentrations or different interfacial tensions. The signals show clearly that with decreasing surface tension the droplets are smaller and the difference between two droplets of different sizes is more easily distinguished.  FIG. 14(   b ) shows time-differential signals ds(t)/dt of the data shown in  FIG. 14(   a ). The positive peaks represent the receding side, while the negative peaks represent the advancing side. The difference between these two peaks is shown in  FIGS. 12 and 13 . The difference is a function of surfactant concentration of interfacial tension. 
       FIG. 15  shows the evaluation results of the time-differential signal. The curves show the peak values of the time-differential signal versus the surfactant concentration. Unlike the characteristics of the droplet frequency shown in  FIG. 9  and  FIG. 10 , the curve shown in  FIG. 15  has a maximum. That means it is possible to have a measurement range with high sensitivity. In the graph a circle represents a receeding edge and square represents an advancing edge. 
     As such, there are four way of evaluating the surface tension:
         time period between two droplets;   frequency of droplet formation;   size of droplet; and   difference between contact angles.       

     For an air/liquid system, air is introduced into the sample inlet channel  41 , while the carrier fluid channel  42  is for the sample to be measured. Both air and sample flows are driven by a syringe pump. The syringe for air may be a 0.25 mL syringe, while that for the sample may be a 1 mL syringe. The volumetric flow rate ratio between air and sample flows is kept at 1:4.  FIG. 16  shows the typical bubble formation inside the microchannel  44 . 
       FIG. 17  shows the time signal indicating the bubbles. The surfactant in use was CTAB (Cetyl Trimethyl Ammonium Bromide). Samples with different concentrations ranging from 0.0001 M/L to 0.01 M/L were tested. The surface tension of the sample decreases with the higher surfactant concentration. A higher frequency of bubble generation and a smaller bubble size can be observed. 
       FIG. 18  shows that the size of droplet/bubble can be used for measuring the droplet/bubble size. If the recorded signal rises above a threshold voltage, a timer (stand-alone or integrated in a micro controller) starts counting. The timer stops counting when the signal falls under the same threshold. The counted time represents the size of the droplet/bubble. 
       FIG. 19(   a ) depicts the clear dependence of bubble generation frequency on the surfactant concentration. For calibration, the surface tension of the samples was measured using a tensiometer such as, for example, FTA200 (First Ten Angstrom). The measured frequency versus the actual surface tension is depicted in  FIG. 19(   b ). The flow rate was 3 mL/hours. 
     The CMC (Critical Micelle Concentration) of a surfactant can be determined by obtaining the correlation of the surface/interfacial tension versus surfactant concentration. 
       FIG. 20  illustrates the two basic concepts of a handheld tensiometer with a microfluidic sensor:
         (a) a tensiometer module  201  is attached to a hand held PC  202 . The PC  202  is used as signal evaluation (look up table, polynom fitting, and so forth) and display on screen  203 . The tensiometer module  201  contains all the components required and is described above. The microfluidic device  40  is inserted into the module  201 , which provides fluidic, optical or electrical interconnects to the device  40 .   (b) a stand-alone device  205  with its own CPU or microcontroller, the data is displayed directly on the device LCD display  206 . The insertion mechanism for the microfluidic device  40  and components are the same as in  FIG. 20(   a ).       

       FIG. 21  depicts four different configurations of the sensor chip: 
       FIG. 21(   a ): The chip  2100  has two reservoirs  2101 —one for a sample and one for the carrier. In case of a liquid/liquid system, the reservoirs  2101  are filled with the corresponding liquids. The liquid samples are first drawn into a large microchannel section  2102  due to capillary force. The samples are stopped at a capillary stop valve  2103  which is where the microchannel becomes smaller. The chip  2100  is now ready for insertion into the tensiometer module  201  or  205 . The module  201 ,  205  provides pressure or vacuum to the reservoirs  2101  by means of an external pump and forces both liquids into the measurement channel  2104  with a constant flow rate. An optical wave guide  2105  leads light from the source  49  to the measurement channel  2104 . The other optical wave  2106  guide takes the light to an optical sensor  410  in the module  201 ,  205 . In case of impedance detection, optical guides are replaced by electrodes. In case of air/liquid system, one reservoir  2101  is left empty and, the external pump supplies air into the injection channel. The liquids are collected in a waste reservoir  2107 . The chip  2100  is ready for disposal after measurement. 
       FIG. 21(   b ): Similar to configuration in  FIG. 18(   a ), but there is no need for the optical wave guides  2105  and  2106 . If the chip  2100  is made of a transparent material such as polymer or glass, a light source and an optical sensor  2108  can be placed directly on the chip on opposite sides of channel  2104 . 
       FIG. 21(   c ): Similar to configuration in  FIG. 21(   a ), but two integrated micropumps  2109  are used for sample delivery. The micropumps  2109  may be checkvalve pumps, peristaltic pumps, valveless pumps, centrifugal pumps, electroosmotic pump, electrohydrodynamic pump and so forth. The pumps  2109  may be equipped with flow sensors for keeping the flow rate constant. Control signals for the pumps  2109  come from the tensiometer module  201 ,  205 . 
       FIG. 21(   d ) Similar to configuration in  FIG. 21(   b ), but the chip  2100  has two integrated micropumps  2109  in the same manner as  FIG. 21(   c ). 
       FIG. 22  shows a tensiometer module  201 ,  205 . The central component of this module  201 ,  205  is a microcontroller or a digital signal processor. 
     In case of optical detection, the module provides a light source  2202  and an optical sensor  2203 . 
     Control signals  2205  for the pump  2204  are from microcontroller  2201 . Signals  2207  from the optical sensor  2203  (or signals  2208  from the capacitive sensor) are evaluated in the microcontroller  2201 . When these integrated micropumps  2109 , signals  2209  for the micropump  2109  are from the microprocessor. 
     In case of external pumping, the modules provide a mini pump  2204  for pressure/vacuum supply to the sensor chip. The mini pump  2204  may be in the form of conventional check-valve pump, or a small syringe pump driven by a stepper motor. Before measurement, the syringes would be withdrawn to a charging position. 
     An insertion slot  2210  is provided for the chip  2100 , the slot  2100  having fluidic, optical and/or an electrical inter connects. Measurement results  2211  are sent from microcontroller  2201  to screens  203 ,  206 . 
       FIG. 23  shows the concept of a close-loop controlled detergent dispenser  2300  with a sensor  2100  and two pumps  2204 , one for air and the other for washing liquids. The sensor  2100  provides information about surface tension or CMC (Critical Micelle Concentration) of the washing liquid. The microcontroller  2201  uses this information to control the detergent dispenser  2201 . This concept can be integrated in a commercial washing machine to save detergent  2302  and protect the environment. 
     The preferred embodiments allow the fast determination of dynamic interfacial tension of a liquid/liquid system, or a gas/liquid system. The chip  40  can be designed for disposable use and easily be integrated in a more complex microfluidic system. Besides the advantage of a fast analysis, a handheld measurement device with this sensor has the potential to replace all current desktop system for determining surface tension in, for example, the petroleum industry. Surface tension, contact angle, and CMC (Critical Micelle Concentration) of a surfactant play an important role in the displacement of oil from the pore spaces of sedimentary rocks, in wetting and dewetting of oil from sand grains, in dewatering in refinery plants, and separation and flotation in oil recovery. Feedback-controlled detergent dosing for washing machines is another use. 
     Whilst there has been described in the foregoing description preferred embodiments of the present invention, it will be understood by those skilled in the technology concerned that many variations or modifications in details of design or construction may be made without departing from the present invention.