Final Report Summary - EWETDYNAM (Electrowetting dynamics)
The research in the project EWETDYNAM focuses on electrowetting dynamics with regard to optimising the material choice for insulating and hydrophobic layers in electrowetting. The research focuses on ways to improve the thin film materials used in electrowetting with regard to price, stability and functionality. In house research has shown how reversible electrowetting can be performed on octadecyltrichlorosilane (OTS) monolayers deposited on silicon nitride and that this provides a less expensive alternative to Teflon AF that performs well. Tantalum pentoxide is a thin film material combining high dielectric constant with high breakdown strength and low pin-hole density. Therefore tantalum pentoxide is an ideal insulation layer material for electrowetting on dielectric because it allows electrowetting to take place at very low applied voltages.
A postdoc, Dr Kayla Calvert, was recruited in 2010 as part of Rise Pro Fellowship from the DAAD. Her PhD thesis in material science dealt with the anodisation of titanium and she applied her knowledge to optimising the anodisation of tantalum electrodes to tantalum pentoxide. Squares of silicon wafer coated with tantalum were acquired from TU Ilmenau. The tantalum surface was anodised electrochemically. The tantalum surface was connected as anode and a platinum electrode as cathode to a voltage source so that initially a low current flows until the tantalum pentoxide layer increases its thickness so that the maximum voltage (anodisation voltage) is reached at which the anodisation current begins to drop. The colour of the samples varies due to interference of light being affected by the varying thicknesses of the tantalum pentoxide layers. Tantalum pentoxide was characterised by impedance spectroscopy, atomic force microscopy and ellipsometry.
In June 2011, we again recruited an intern Zenko Kawabata as part of the Rise Pro Fellowship of the DAAD. He successfully extended the materials knowledge gained in the previous year to the application in a microfluidic system. Electrowetting was performed on tantalum pentoxide electrodes with silane hydrophobisation at applied voltages between 5 and 10V. The contact angle and contact diameter measured for droplets of water immersed in an oil bath by means of a MATLAB routine. The signal was based on a carrier frequency of 1 kHz and modulation at lower frequencies so that the dynamics of electrowetting could be recorded with the high speed camera. The maximum contact angle change decreased with increasing frequency. The applied voltage is between 6 and 10 V and is significantly lower than that required for other dielectric insulators.
For alternating current (AC) modulated signals the contact angle change depends on tantalum pentoxide layer thickness, applied voltage and the modulation frequency. As expected the contact angle change increases universally with applied voltage until contact angle saturation is reached. With increasing modulation frequency the contact angle change reduces because of the incomplete electrowetting and dewetting due to the rough surface. We studied how the contact angle change is affected by anodisation voltage of the sample and applied voltage. At lower anodisation voltages the layer thickness is smaller and the capacitance is higher but at applied voltages of greater than one third of the anodisation voltage it is known that the dielectric layer is not a simple capacitor and current starts to flow. Tantalum electrolytic capacitors have voltage limits in this range for this reason.
A finite element model was designed to model droplet generation by a two-fluid probe. The two-fluid probe, consisting of two concentrically-arranged tubings, is immersed in a beaker of cell medium so that oil is pumped through the outer tubing at a pumping speed less than fluid is drawn into the inner tubing. In this way, droplets of cell medium are entrained into the outlet tubing forming a segmented flow of homogeneously mixed droplets. The simulation allows the effect of changing the electrode geometry and working fluids to be ascertained.
Contactless impedimetric droplet sensing
The host institute requested that I finish the work on impedance spectroscopic measurement of droplets in segmented flow of my original Marie Curie Fellowship as part of the Sixth Framework Programme (FP6) Marie Curie Transfer of Knowledge (ToK) project 29857 InFluEMP. Although this work was not part of my proposal for the EWETDYNAM project, the freedom I had as Marie Curie Fellow allowed me to complete this work and present it in the form of publications and conference presentations. In collaboration with the TU Tallinn, a novel sensor for measuring the conductivity of aqueous droplets in segmented flow was constructed and modelled by finite element modelling. Previous electrical measurements in digital microfluidics were limited to measuring droplet presence and in turn droplet volume and speed. This sensor can measure conductivity: a variable quantity related to the droplet content that can be varied by changing the salt concentration of the aqueous medium. Two measurements setups were tested:
1. a HP 4194A impedance/gain-phase analyser and
2. the IMPSPEC system impedance measurement system.
Measurement with the HP device is more accurate while the measurement with the ImpSpec system is much more suited to a high-throughput process measurement environment due to cost and measurement speed requirements. The sensor system was modelled analytically and by means of a numerical model in Comsol that allowed the impedance of the area between the electrodes in the sensor to be measured. The online measurement of droplet content is of interest for the automation of lab-on-a-chip applications, for instance, for cytotoxicity tests. The sensing of droplets in segmented flow had previously been limited to the sensing of droplet presence and in turn droplet speed and volume. This work was the first to measure a variable property of droplets, the conductivity, by electrical means.
Contactless microwave droplet sensing
Through the use of microwave sensing it is possible to measure the variation of the dielectric properties (both conductivity and permittivity). Microwave sensing is well suited to the investigation of biological and chemical processes in microsystems, for example, cell growth, cell metabolism and molecular and ionic concentrations. High frequency sensors can integrate radiofrequency (RF) microwave detection in a microfluidic network for quick and precise biological and chemical analysis. In biosystems, it is of great importance to monitor parameters, such as, cell density, cell growth and cell viability. Cell metabolism is determined by biophysical and biochemical variables depend directly on cell growth and other metabolic processes. Microwave measurement technology has opened up a broad range of applications for measuring dielectric materials properties which are suitable for application as contactless nondestructive sensors. Microwave measurements offer many of the advantages of RF impedimetric sensors, but also possess additional advantages, particularly with regard to miniaturisation and their relative insensitivity to insulation layer thickness. A contactless microwave sensor is in the early stages of development. The compact sensor consists of a flow-through chamber; tubing can be reproducibly placed between strip line electrodes that act as the sensor element. The tubing can be easily placed in the sensor and replacement is uncomplicated. Media was pumped through the tubing and the variation of the material under test was measured by the microwave sensor. Investigation of the frequency response of the sensor was carried out using air, water and ethanol. This sensor has been used for measurement of the dynamics of single-use bioreactors but is also of more general significance for lab-on-a-chip systems for application in biology and analytical chemistry. This work is the result of two separate collaborations with colleagues in IBA and the IHP in Frankfurt/Oder.
Droplet switching
Droplet switching is of great interest for actuation in lab on a chip systems. Switching operations in transport of microfluidic compartments are of high interest in miniaturised biotechnology, cell cultivation and screening programs as well as for future applications in miniaturised and automated diagnostics and in particular for automated experiments in ultraminiaturised combinatorial chemistry and combinatorial screenings in multidimensional parameter spaces. In collaboration with the TU Ilmenau and the IPHT Jena, switching of aqueous nanoliter-sized aqueous droplets in a branched micro channel filled with tetradecane was realised using contactless non-galvanic electrical actuation. Reproducible switching operations were realised by the help of an image based trigger. In this way the direction of the segment transport could be chosen with very high reliability. This gives the possibility for the generation of an arbitrary chosen segment pattern in an outlet channel. It was possible to generate error-free segment sequences with thousands of segments and to use this technique for microfluidic encoding. This new method of droplet switching has several advantages over the previously presented droplet switching techniques. Firstly, there is no need to inject charge by direct contact with an electrode: contact with an electrode that is not hydrophobic can lead to fouling of the surface that will lead to cross-contamination of droplets or imperfect droplet transport across the surface. Secondly, the switching operation can be compactly performed with one set of electrodes being responsible for both droplet charging (through induced charge) and switching. Thirdly, the charge is induced on the surface of the droplet which means the charge disperses after switching and subsequent switching operations can be performed unproblematically.
A postdoc, Dr Kayla Calvert, was recruited in 2010 as part of Rise Pro Fellowship from the DAAD. Her PhD thesis in material science dealt with the anodisation of titanium and she applied her knowledge to optimising the anodisation of tantalum electrodes to tantalum pentoxide. Squares of silicon wafer coated with tantalum were acquired from TU Ilmenau. The tantalum surface was anodised electrochemically. The tantalum surface was connected as anode and a platinum electrode as cathode to a voltage source so that initially a low current flows until the tantalum pentoxide layer increases its thickness so that the maximum voltage (anodisation voltage) is reached at which the anodisation current begins to drop. The colour of the samples varies due to interference of light being affected by the varying thicknesses of the tantalum pentoxide layers. Tantalum pentoxide was characterised by impedance spectroscopy, atomic force microscopy and ellipsometry.
In June 2011, we again recruited an intern Zenko Kawabata as part of the Rise Pro Fellowship of the DAAD. He successfully extended the materials knowledge gained in the previous year to the application in a microfluidic system. Electrowetting was performed on tantalum pentoxide electrodes with silane hydrophobisation at applied voltages between 5 and 10V. The contact angle and contact diameter measured for droplets of water immersed in an oil bath by means of a MATLAB routine. The signal was based on a carrier frequency of 1 kHz and modulation at lower frequencies so that the dynamics of electrowetting could be recorded with the high speed camera. The maximum contact angle change decreased with increasing frequency. The applied voltage is between 6 and 10 V and is significantly lower than that required for other dielectric insulators.
For alternating current (AC) modulated signals the contact angle change depends on tantalum pentoxide layer thickness, applied voltage and the modulation frequency. As expected the contact angle change increases universally with applied voltage until contact angle saturation is reached. With increasing modulation frequency the contact angle change reduces because of the incomplete electrowetting and dewetting due to the rough surface. We studied how the contact angle change is affected by anodisation voltage of the sample and applied voltage. At lower anodisation voltages the layer thickness is smaller and the capacitance is higher but at applied voltages of greater than one third of the anodisation voltage it is known that the dielectric layer is not a simple capacitor and current starts to flow. Tantalum electrolytic capacitors have voltage limits in this range for this reason.
A finite element model was designed to model droplet generation by a two-fluid probe. The two-fluid probe, consisting of two concentrically-arranged tubings, is immersed in a beaker of cell medium so that oil is pumped through the outer tubing at a pumping speed less than fluid is drawn into the inner tubing. In this way, droplets of cell medium are entrained into the outlet tubing forming a segmented flow of homogeneously mixed droplets. The simulation allows the effect of changing the electrode geometry and working fluids to be ascertained.
Contactless impedimetric droplet sensing
The host institute requested that I finish the work on impedance spectroscopic measurement of droplets in segmented flow of my original Marie Curie Fellowship as part of the Sixth Framework Programme (FP6) Marie Curie Transfer of Knowledge (ToK) project 29857 InFluEMP. Although this work was not part of my proposal for the EWETDYNAM project, the freedom I had as Marie Curie Fellow allowed me to complete this work and present it in the form of publications and conference presentations. In collaboration with the TU Tallinn, a novel sensor for measuring the conductivity of aqueous droplets in segmented flow was constructed and modelled by finite element modelling. Previous electrical measurements in digital microfluidics were limited to measuring droplet presence and in turn droplet volume and speed. This sensor can measure conductivity: a variable quantity related to the droplet content that can be varied by changing the salt concentration of the aqueous medium. Two measurements setups were tested:
1. a HP 4194A impedance/gain-phase analyser and
2. the IMPSPEC system impedance measurement system.
Measurement with the HP device is more accurate while the measurement with the ImpSpec system is much more suited to a high-throughput process measurement environment due to cost and measurement speed requirements. The sensor system was modelled analytically and by means of a numerical model in Comsol that allowed the impedance of the area between the electrodes in the sensor to be measured. The online measurement of droplet content is of interest for the automation of lab-on-a-chip applications, for instance, for cytotoxicity tests. The sensing of droplets in segmented flow had previously been limited to the sensing of droplet presence and in turn droplet speed and volume. This work was the first to measure a variable property of droplets, the conductivity, by electrical means.
Contactless microwave droplet sensing
Through the use of microwave sensing it is possible to measure the variation of the dielectric properties (both conductivity and permittivity). Microwave sensing is well suited to the investigation of biological and chemical processes in microsystems, for example, cell growth, cell metabolism and molecular and ionic concentrations. High frequency sensors can integrate radiofrequency (RF) microwave detection in a microfluidic network for quick and precise biological and chemical analysis. In biosystems, it is of great importance to monitor parameters, such as, cell density, cell growth and cell viability. Cell metabolism is determined by biophysical and biochemical variables depend directly on cell growth and other metabolic processes. Microwave measurement technology has opened up a broad range of applications for measuring dielectric materials properties which are suitable for application as contactless nondestructive sensors. Microwave measurements offer many of the advantages of RF impedimetric sensors, but also possess additional advantages, particularly with regard to miniaturisation and their relative insensitivity to insulation layer thickness. A contactless microwave sensor is in the early stages of development. The compact sensor consists of a flow-through chamber; tubing can be reproducibly placed between strip line electrodes that act as the sensor element. The tubing can be easily placed in the sensor and replacement is uncomplicated. Media was pumped through the tubing and the variation of the material under test was measured by the microwave sensor. Investigation of the frequency response of the sensor was carried out using air, water and ethanol. This sensor has been used for measurement of the dynamics of single-use bioreactors but is also of more general significance for lab-on-a-chip systems for application in biology and analytical chemistry. This work is the result of two separate collaborations with colleagues in IBA and the IHP in Frankfurt/Oder.
Droplet switching
Droplet switching is of great interest for actuation in lab on a chip systems. Switching operations in transport of microfluidic compartments are of high interest in miniaturised biotechnology, cell cultivation and screening programs as well as for future applications in miniaturised and automated diagnostics and in particular for automated experiments in ultraminiaturised combinatorial chemistry and combinatorial screenings in multidimensional parameter spaces. In collaboration with the TU Ilmenau and the IPHT Jena, switching of aqueous nanoliter-sized aqueous droplets in a branched micro channel filled with tetradecane was realised using contactless non-galvanic electrical actuation. Reproducible switching operations were realised by the help of an image based trigger. In this way the direction of the segment transport could be chosen with very high reliability. This gives the possibility for the generation of an arbitrary chosen segment pattern in an outlet channel. It was possible to generate error-free segment sequences with thousands of segments and to use this technique for microfluidic encoding. This new method of droplet switching has several advantages over the previously presented droplet switching techniques. Firstly, there is no need to inject charge by direct contact with an electrode: contact with an electrode that is not hydrophobic can lead to fouling of the surface that will lead to cross-contamination of droplets or imperfect droplet transport across the surface. Secondly, the switching operation can be compactly performed with one set of electrodes being responsible for both droplet charging (through induced charge) and switching. Thirdly, the charge is induced on the surface of the droplet which means the charge disperses after switching and subsequent switching operations can be performed unproblematically.