Manjeet Dhindsa

Ion and Liquid Dependent Dielectric Failure in Electrowetting Systems

Electrowetting devices often utilize aqueous solutions with ionic surfactants and inorganic salts to modify the electrowetting response. It has been observed in low-voltage electrowetting devices (thin dielectric, <12 V) that a frequent onset of dielectric failure (electrolysis) occurs with use of ionic solutes such as potassium chloride (KCl) or sodium dodecyl sulfate. More detailed current-voltage investigations reveal less dielectric failure for the larger size ions. Specifically, improved resistance to failure is seen for surfactant ions carrying a long alkane chain. Therefore, a catanionic surfactant (in which both ions are amphiphilic) was custom synthesized, and elimination of dielectric failure was observed in both negative and positive voltage. Because water is a small molecule that easily penetrates dielectrics, further experiments were performed to show that dielectric failure can also be eliminated by use of larger size polar molecules such as propylene glycol. In addition to these results, important parameters such as conductivity and interfacial tensions are reported.

Electrowetting on Superhydrophobic Surfaces: Present Status and Prospects

Electrowetting devices with an initial superhydrophobic water contact angle (>150°) have now been demonstrated on a variety of structured substrates. These substrates are more complex than a conventional superhydrophobic surface since electrowetting requires an electrical conductor that is coated with a high-performance dielectric and a hydrophobic fluoropolymer. Substrate structures that have been studied include silicon nanoposts and nanowires, carbon nanofibers and nanotubes, and polymer microposts. Even though these structured surfaces are geometrically diverse, there are several consistencies in electrowetting behavior for all these platforms. As an electrowetting bias of 10s of volts is applied between a saline drop and the substrate, the macroscopically observed contact angle is typically decreased from >150° to ∼100°. As the voltage is increased an electromechanical force promotes capillary wetting between the substrate structures, and the saline drop transitions from the Cassie state to the Wenzel state. The Wenzel state presents a new energy minimum for the system, and in all current experiments the wetting is irreversible. Transition from the Wenzel state back to the Cassie state has been demonstrated by means of liquid boiling or addition of a second non-polar liquid. The importance of these recent investigations includes the dynamic tuning of the wetting on a superhydrophobic surface, and improved understanding of electrowetting on, and into, structured surfaces.

Experimental Validation of the Invariance of Electrowetting Contact Angle Saturation

Abstract Basic electrowetting theory predicts that a continued increase in applied voltage will allow contact angle modulation to zero degrees. In practice, the effect of contact angle saturation has always been observed to limit the contact angle modulation, often only down to a contact angle of 60 to 70°. The physical origins of contact angle saturation have not yet been explained successfully and unequivocally. At best, scientists have produced multiple disconnected hypotheses (droplet ejection, charge injection, a thermodynamic limit, etc.) that do not satisfactorily hold for the large body of electrowetting experimental results. Herein we experimentally demonstrate that when using DC voltage, electrowetting contact angle saturation is invariant with electric field, contact line profile, interfacial tension, choice of non-polar insulating fluid, and type of polar conductive fluid or ionic content. The selected experiments were performed and designed using conventional electrowetting materials, without bias toward supporting a particular theory. Because the experimental results show such a strong invariance of saturation angle to multiple parameters, electrowetting saturation parallels many of the trends for Taylor cone formation. However, the contact line geometry is distinct from a Taylor cone, suggesting that some other (though related) form of electrohydrodynamic instability might cause saturation. Although this work does not unequivocally prove what causes contact angle saturation, it reveals what factors play a very limited or no role, and how dominant factors causing saturation may change with time of voltage application. This study thereby provides additional direction to the continued pursuit of a universal theory for electrowetting saturation.

Recent Progress in Arrayed Electrowetting Optics

Electrowetting devices can now be formed in arrays covering thousands of square centimeters of glass. New research is pointing the way toward exciting applications for laser radar, 3D displays, adaptive camouflage, electronic paper, retroreflector communication and lab-on-a-chip.

Electrowetting without Electrolysis on Self-Healing Dielectrics

An electrowetting system with protection against dielectric breakdown is presented. It comprises an electrolyte and a Parylene-C film deposited on an aluminum electrode. The system demonstrates virtually instantaneous self-healing (within 100 ms) after dielectric breakdown under both DC and certain AC electrowetting conditions. DC current response during electrowetting on intentionally damaged Parylene-C is presented. Also presented is a characterization of DC offset voltages and duty cycle percentages required for electrolysis free AC electrowetting between 10 Hz and 4 kHz.

Laplace barriers for electrowetting thresholding and virtual fluid confinement.

Reported are Laplace barriers consisting of arrayed posts or ridges that impart ∼100 to 1000 s of N/m(2) Laplace pressure for fluid confinement, but the Laplace pressure is also small enough such that the barriers are porous to electrowetting control. As a result, the barriers are able to provide electrowetting flow thresholding and virtual fluid confinement in noncircular fluid geometries. A simple theoretical model for the barriers and experimental demonstrations validate functionality that may be useful for lab-on-chip, display devices, and passive matrix control, to name a few applications.

Active-Matrix Microelectrode Arrays Integrated With Vertically Aligned Carbon Nanofibers

In this letter, we have successfully integrated vertically aligned carbon nanofibers (VACNFs) onto active matrix thin-film transistor (TFT) and demonstrate a new microelectrode array (MEA) platform. The materials and processes of the bottom gate inverted staggered TFT structure were designed to be compatible with the requisite high-temperature (~700degC) and direct current plasma-enhanced chemical vapor deposition VACNF growth process. The critical device integration issues are elaborated, and initial device characteristics are reported. This device platform provides great potential as an advanced MEA for direct cell sensing, probing, and recording with a high electrode density and active addressability.

Composite Dielectrics and Surfactants for Low Voltage Electrowetting Devices

In this work electrowetting operation at <12 V has been achieved through implementation of composite dielectrics and surfactants. The composite dielectrics consist of A12O3 and Si3N4 covered by a thin film of hydrophobic Cytop fluoropolymer. These composite dielectrics exhibit higher capacitance and therefore lower voltage electrowetting operation. Atomic layer deposition of A12O3 and plasma-enhanced chemical vapor deposition of Si3N4 were chosen because they can be deposited at low temperature, are pin-hole free, and because they can be conformably coated on 3D surfaces. Surfactants were also explored to lower the water/oil interfacial surface tension and thereby further reduce the voltages required for electrowetting. These results are important for continued development of arrayed electrowetting optics for displays and beam steering applications.

Electrowetting on Arrayed Carbon Nanofibers

Electrowetting on arrayed carbon nanofibers has been demonstrated. The testing structure consists of Si/carbon nanofibers/dielectric/fluoropolymer/liquid. Nanofibers were patterned on 5×5 μm pitch which resulted in superhydrophobic behavior for water (θ>150°). Via electrowetting, liquid contact angle was shown to be irreversibly reducible to θ∼100°. Electrowetting using a competitive two-liquid (oil/water) system was shown to be reversible. The nanofibers terminate in a <25 nm radius tip. This small tip size results in effectively zero capacitance for water in the dewetted state. As electrowetting increased with applied voltage, the wetting state was confirmed by measuring the capacitance and stored energy density for the liquid/nanofiber system.

A new electrowetting lab-on-a-chip platform based on programmable and virtual wall-less channels

Microscale liquid handling based on electrowetting has been previously demonstrated by several groups. Such liquid manipulation however is limited to control of individual droplets, aptly termed digital microfluidics. The inability to form continuous channels thus prevents conventional microfluidic sample manipulation and analysis approaches, such as electroosmosis and electrophoresis. In this paper, we discuss our recent progress on the development of electrowettingbased virtual channels. These channels can be created and reconfigured on-demand and preserve their shape without external stimulus. We also discuss recent progress towards demonstrating electroosmotic flows in such microchannels for fluid transport. This would permit a variety of basic functionalities in this new platform including sample transport and mixing between various functional areas of the chip.