Non-linearity and dynamics of low-voltage electrowetting and dewetting.

As an electrically controllable wetting effect, electrowetting on dielectrics (EWOD) is applied in diverse fields including optics, display technology and lab-on-a-chip systems. For the further development of EWOD applications, the reduction of the operation voltage is an essential issue. Recently, a low-voltage EWOD system with a threshold of 2 V was developed. In its sessile drop configuration, an aqueous electrolyte droplet with microliter scaled volume is actuated on an EWOD electrode in oil. The integration of this low-voltage EWOD system into a multiparameter measurement system enables the non-linearity and dynamics of the EWOD system to be online investigated during electrowetting and dewetting. The non-linearity was characterized by the hysteresis in the droplet deformation and that in the thickness variation of an oil layer, which is entrapped between the droplet and the electrode, in the nm range. The dynamics was evaluated with the characteristic time for the droplet deformation upon voltage jumps. This study of electrowetting and dewetting focuses on the conversion efficiency of the electrical energy in the deformation processes.

Frequency Dependence of Low-Voltage Electrowetting Investigated by Impedance Spectroscopy.

An electrowetting-on-dielectric (EWOD) electrode was developed that facilitates the use of low alternating voltages (≤5 VAC). This allows online investigation of the frequency dependence of electrowetting by means of impedance spectroscopy. The EWOD electrode is based on a dielectric bilayer consisting of an anodic tantalum pentoxide (Ta2O5) thin film (d = 59.35 nm) with a high relative permittivity (εd = 26.3) and a self-assembled hydrophobic silane monolayer. The frequency dependence of electrowetting was studied using an aqueous µL-sized sessile droplet on the planar EWOD electrode in oil. Experiments using electrochemical impedance spectroscopy and optical imaging indicate the frequency dependence of all three variables in the Young-Lippmann equation: the voltage drop across the dielectric layers, capacitance per unit area, and contact angle under voltage. The electrowetting behavior induced by AC voltages is shown to be well described by the Young-Lippmann equation for AC applications below a frequency threshold. Moreover, the dielectric layers act as a capacitor and the stored electrostatic potential energy is revealed to only partially contribute to the electrowetting.

Improving the Emergency Whole Blood Program.

The military has used the Emergency Whole Blood Program (EWBP) to treat combat casualties since World War I and it remains important in modern military campaigns. Despite frequent use, military medical providers receive little to no training on EWBP operations. The authors sought to improve the efficiency of the EWBP at a Forward Operating Base in southern Afghanistan during Operation Enduring Freedom 10-11 through the development of a prescreening program. A prescreened donor pool and registry were established by confirming blood type, rapid communicable disease testing, and completion and review of Defense Department (DD) form 572 before an emergency blood drive, during which prescreened donors were moved to the front of the line. Using prescreened donors, the mean time from EWBP activation to blood delivery to the operating room decreased from 43.3 (SD 4.4) to 26.7 minutes (SD 3.1) (p < 0.0004). The EWBP can be conducted more efficiently without compromising patient safety by implementing a robust prescreening program. Other improvements should include education of health care providers and commanders on prescreening and the EWBP. The use of fresh whole blood continues to be an important lifesaving resource in military medicine, and the Department of Defense should allocate resources to provide EWBP training and in-theater prescreening programs.

Improvements in design of spectra of multisine and binary excitation signals for multi-frequency bioimpedance measurement.

The paper discusses the usability of multi-frequency binary waveforms for broadband excitation in fast measurements of impedance spectrum of biological objects. It is shown that up to 70% of the energy of the amplitude spectrum of such two-level binary signals can be concentrated into the selected separate frequencies. The levels of selected frequency components are controllable in tens and hundreds of times. In this way we can underline the most important frequencies enhancing the corresponding amplitudes in the spectrum of excitation signal. As an implementation example, we consider the impedance spectroscopy in micro-fluidic devices for inline measurement of the conductivity of droplets in segmented flow. We use a thin-walled glass capillary with electrodes contacting the outer surface so that the contactless measurement of conductivity of liquid with biologic cells becomes possible.

Adsorption of poly(amido amine) (PAMAM) dendrimers on silica: importance of electrostatic three-body attraction.

Adsorption of poly(amido amine) (PAMAM) dendrimers to silicon oxide surfaces was studied as a function of pH, ionic strength, and dendrimer generation. By combining optical reflectometry and atomic force microscopy (AFM), the adsorbed layers can be fully characterized and an unequivocal determination of the adsorbed mass becomes possible. For early stages, the adsorption process is transport limited and of first order with respect to the dendrimer solution concentration. For later stages, the surface saturates and the adsorbed dendrimers form loose but correlated liquidlike surface structures. This correlation is evidenced by a peak in the pair correlation function determined by AFM. The maximum adsorbed amount increases with increasing ionic strength and pH. The increase with the ionic strength is explained by the random sequential adsorption (RSA) model and electrostatic repulsion between the dendrimers. The adsorbing dendrimers interact by the repulsive screened Coulomb potential, whose range decreases with increasing ionic strength and thus leads to increasing adsorbed densities. The pH increase is interpreted as an effect of the substrate and is quantitatively explained by the extended three-body RSA model. This model stipulates the importance of a three-body interaction acting between two adsorbing dendrimers and the charged substrate. The presence of the charged substrate weakens the repulsion between the adsorbing dendrimers and thus leads to higher surface densities. This effect can be interpreted as an additional attractive three-body interaction, which acts in addition to the usual two-body repulsion and originates from the additional screening of the Coulomb repulsion by the counterions accumulating in the diffuse layer.

Nano-patterning of solid substrates by adsorbed dendrimers.

Weakly charged solid substrates can be nano-patterned in liquid-like order with large and well-defined spacing by adsorbing poly(amido amine) (PAMAM) dendrimers; highly charged substrates lead to lower spacing due to electrostatic three-body attractions between the dendrimers and the substrate.

Thin adsorbed films of a strong cationic polyelectrolyte on silica substrates.

The adsorption of poly(diallyldimethyl ammonium chloride) (DADMAC) on planar silica substrates was examined as a function of ionic strength and pH. The study was carried out with reflectometry in an impinging-jet cell and complemented by atomic force microscopy (AFM) and ellipsometry investigations. The adsorption process is initially transport limited, whereby the adsorption rate increases somewhat with increasing ionic strength. This effect is caused by a simultaneous decrease of the hydrodynamic radius of the polymer. After a transient period, the adsorption process saturates and leads to an adsorption plateau. The plateau value increases strongly with increasing ionic strength. This increase can be explained by progressive screening of the electrostatic repulsion between the adsorbing polyelectrolyte chains, as can be rationalized by a random sequential adsorption (RSA) model. The adsorbed amount further increases with increasing pH, and this effect is probably caused by the corresponding increase of the surface charge of the silica substrate.

Electro-osmotic streaming on application of traveling-wave electric fields.

We describe ac electro-osmotic flow of an aqueous electrolyte on application of a traveling-wave electric field. Depending on the frequency of the applied traveling wave, the interaction of the electric double layer charge and the tangential electric field leads to fluid flow in the direction of the traveling wave. We have derived two theoretical models that describe this flow as a function of the amplitude of the applied electric potential, the signal frequency, and the material properties of the system. The first is based on a capacitative model and is limited to frequencies much lower than the double layer relaxation frequency. The second is an analytical solution of the electrokinetic equations and is also valid at higher frequencies. We provide experimental evidence that streaming takes place on application of a traveling wave of potential by tracing the movements of fluorescent latex beads over a spiral electrode structure. Streaming takes place at applied potentials low enough for the method to be easily integrated into lab-on-a-chip devices.