Electrical actuation of dielectric droplets by negative liquid dielectrophoresis

This paper presents an electrical actuation scheme of dielectric droplet by negative liquid dielectrophoresis. A general model of lumped parameter electromechanics for evaluating the electromechanical force acting on the droplets is established. The model reveals the influence of actuation voltage, device geometry, and dielectric parameter on the actuation force for both conductive and dielectric medium. Using this model, we compare the actuation forces for four liquid combinations in the parallel-plate geometry and predict the low voltage actuation of dielectric droplets by negative dielectrophoresis. Parallel experimental results demonstrate such electric actuation of dielectric droplets, including droplet transport, splitting, merging, and dispending. All these dielectric droplet manipulations are achieved at voltages < 100 V_rms. The frequency dependence of droplet actuation velocity in aqueous solution is discussed and the existence of surfactant molecules is believed to play an important role by realigning with the AC electric field. Finally, we present coplanar manipulation of oil and water droplets and formation of oil-in-water emulsion droplet by applying the same low voltage.     

Synthesis of titanium nitride thin films deposited by a new shielded arc ion plating

Thin films of titanium nitride (TiN) were deposited on stainless steel substrates by a modified deposition technique, double-layered shielded arc ion plating with vicarious circular holes (DL-SAIP). The results show that the TiN film with the distance of 10 mm between the double-layered shield plates had the least droplets. The deposition rate of the films prepared with the new technique was more homogeneous than that of all the other shielded arc ion plating. The film/substrate adhesion and microhardness values of the TiN films were higher than 40 N and 18 GPa, respectively. Thus such TiN thin films can be expected in applications.     

Measurement of microchannel fluidic resistance with a standard voltage meter

A simplified method for measuring the fluidic resistance (R_fluidic) of microfluidic channels is presented, in which the electrical resistance (R_elec) of a channel filled with a conductivity standard solution can be measured and directly correlated to R_fluidic using a simple equation. Although a slight correction factor could be applied in this system to improve accuracy, results showed that a standard voltage meter could be used without calibration to determine R_fluidic to within 12% error. Results accurate to within 2% were obtained when a geometric correction factor was applied using these particular channels. When compared to standard flow rate measurements, such as meniscus tracking in outlet tubing, this approach provided a more straightforward alternative and resulted in lower measurement error. The method was validated using 9 different fluidic resistance values (from ∼40 to 600 kPa s mm⁻³) and over 30 separately fabricated microfluidic devices. Furthermore, since the method is analogous to resistance measurements with a voltage meter in electrical circuits, dynamic R_fluidic measurements were possible in more complex microfluidic designs. Microchannel R_elec was shown to dynamically mimic pressure waveforms applied to a membrane in a variable microfluidic resistor. The variable resistor was then used to dynamically control aqueous-in-oil droplet sizes and spacing, providing a unique and convenient control system for droplet-generating devices. This conductivity-based method for fluidic resistance measurement is thus a useful tool for static or real-time characterization of microfluidic systems.     

Time evolution of the size distribution of nano-sphere droplets in the hexadecane-in-water miniemulsion stabilized by nonionic surfactants

Time evolutions of the droplet size distribution in miniemulsions, which is constituted of water/n-hexadecane/nonionic surfactants, were investigated by using light scattering techniques. A hard-sphere model is applied to characterize the polydispersity of miniemulsion droplets. Measuring the relative scattering intensity as a function of the volume fraction of dispersed phase, the variance of the droplets size distribution, σ², was evaluated. Miniemulsions developed gradually from monodisperse systems (σ²≅0.02) to polydisperse ones (σ²≥0.13) over 12 days after preparation. σ² increased rapidly in the early stage, and ceased to develop at about 6 days after preparation. The z-average hydrodynamic radius of miniemulsion droplets grew with time over the whole time range. The change with time of the total droplet number of miniemulsion is in agreement with that predicted by Smoluchowski's theory for diffusion-controlled coagulation. Although the characteristic coagulation time obtained here was much larger than that estimated by Smoluchowski's theory, the qualitative agreement between the theory and the experimental results obtained here is good. At the earlier stage of the destabilization process of miniemulsions, the growth mechanism of droplets may be explained in terms of a diffusion-controlled coagulation. Received: 1 April 2000 Accepted: 10 August 2000     

Combustion and microexplosion of collision-merged methanol/alkane droplets

The combustion characteristics of freely falling droplets, individually generated by the merging of colliding methanol and alkane droplets, were investigated and compared with those for pure methanol and alkanes. The merging of the nominally immiscible methanol and alkanes was manifested in an apparently adhesive, but unmixed, manner in all test conditions. An air bubble was found to be trapped at the colliding interfaces where they were “adhered,” with the trapping favored for head-on or near head-on collision orientations. The trapped air bubble ostensibly induced heterogeneous nucleation of the methanol, being facilitated by the relatively low limit of superheat of methanol. Consequently, the droplet exploded almost immediately upon ignition, leading to an extremely short overall lifetime. For collision orientations that were more off-centered, bubble trapping and thereby heterogeneous nucleation were not favored. However, delayed, albeit strong, microexplosion occurred through homogeneous nucleation of methanol at the contacting interface. The global burning rate was therefore again augmented. In general, microexplosion was facilitated for high-boiling-point alkanes such as hexadecane and tetradecane. The co-vaporization of methanol and alkane from their respective hemispherical segments constituting the adhered droplet also led to flame colors that were more bluish than yellowish, indicating the reduction of soot from alkane burning in the presence of methanol vapor. In light of the difficulty of forming stable methanol/oil emulsions, the potential of separate injection of oil and methanol in opposed jet arrangement, in direct-injection engines to facilitate collision, is suggested.     

Rheology and processing of salad dressing emulsions

In order to study the influence that the processing parameters exert on the rheology and stability of salad dressing emulsions, both steady-state shear and oscillatory measurements, as well as droplet size distribution tests were performed. Emulsions containing a mixture of egg yolk and sucrose stearate as emulsifier were prepared using two different emulsification machines, a rotor-stator turbine and a colloidal mill. An increase both in energy input and in the temperature of processing yields higher values of the steady-state viscosity, an increase in emulsion stability and, generally, lower droplet size and lower polydispersity. Furthermore, a plateau region in the loss modulus versus frequency plots appears as the energy input and processing temperature increase. This effect has been analyzed by calculating the relaxation spectra of these emulsions. The results have been discussed taking into account the relationship between several structural parameters, such as interparticle interactions and droplet size distribution, and the rheological response of these emulsions.     

A parametric study of droplet deformation through a microfluidic contraction: Shear thinning liquids

Numerical simulations of a droplet passing through an axisymmetric microfluidic contraction are presented, focusing on systems where one of the two liquids present is shear thinning. The simulations are performed using a transient Volume of Fluid (VOF) algorithm. When the droplet is shear thinning and the surrounding phase Newtonian, droplets deform in a similar way to Newtonian droplets that have a viscosity equal to the average viscosity of the shear thinning fluid while it is within the contraction. When the surrounding phase is shear thinning and the droplet Newtonian, droplets deform in a similar way to droplets contained within a Newtonian liquid that has a viscosity that is lower than that of the droplet. In both cases the behaviour of the shear thinning fluid can be broadly described in terms of a ‘characteristic’ Newtonian viscosity: However, determining the exact value of this viscosity without performing a full shear thinning simulation is not possible.     

Transport of salts and micron-sized particles entrained from a boiling water pool

The entrainment of soluble (KI, CsI) and non-soluble (Al₂O₃) substances through droplets, which are produced by disintegrating steam bubbles at the surface of a boiling water pool, is determined in a pilot-scale facility. Integral measurements are conducted at steady-state conditions in an atmosphere of either pure steam or an air–steam mixture. The ratio of the entrained liquid mass flow and the gas mass flow through the pool, the entrainment factor, is determined for air–steam ratios between 0 and 0.47 kg/kg in the gas atmosphere and at constant total pressures between 2 and 6 bar. The influence of the vertical temperature profile in the gas atmosphere on the convective velocity field is demonstrated by phase Doppler anemometry and particle image velocimetry measurements at a location 2.1 m above the pool surface. The influences of nucleation and natural convection are demonstrated during slow de-pressurization of the facility at rates below 420 Pa/s.     

Rheology and microstructure of gluten and soya-based o/w emulsions

Highly concentrated oil-in-water (o/w) emulsion stabilised by means of gluten and soya protein isolate (SPI) at low pH have been characterized by means of linear dynamic viscoelasticity and droplet size distribution analysis (DSD). The microstructure of these emulsions has been characterized at a colloidal level by using confocal laser scanning microscopy (CLSM) and light microscopy (LM). These emulsions always exhibited a behaviour characteristic of highly flocculated emulsions with a mechanical spectrum showing a well-developed plateau region. DSD results generally showed log normal bimodal profiles. Microstructure images revealed occurrence of a close packing of droplets with a broad distribution of sizes participating in the formation of a three dimensional flocculated network. The Mason model of elasticity of compressed emulsions has been used to correlate viscoelastic and microstructural parameters giving adequate fitting but underestimating the elastic properties obtained for the highest concentration of gluten. These deviations may be explained in terms of an enhancement of the elastic network formed in the aqueous phase in which the glutenin fraction must play an important role. Paper presented at the Annual European Rheology Conference (AERC) 2005, April 21-23, Grenoble, France.     

Effect of droplet size on the rheological properties of highly-concentrated w/o emulsions

Rheological properties of highly concentrated emulsions of the water-in-oil type were studied. Water phase (concentration approximately 91%) consists of a supersaturated aqueous solution of nitrate salts; water comprises less than 20% by mass. The average size of droplets, D, in the emulsions was varied. It was found that the emulsions are non-Newtonian liquids and flow curves measured in a sweep regime of shearing have clear low-shear-rate Newtonian domain. The complete flow curves are fitted by the Cross equation. The elastic modulus is practically constant in a very wide frequency range. Hence the viscoelastic relaxation processes might be expected at times >>100 s and in the short-term side of the curve at approximately 0.01 s. The elastic modulus (measured in oscillating testing and in elastic recovery as well) is proportional to D^-2 while the Newtonian viscosity is proportional to D^–1.The time effects were observed: it was found that the emulsions behave as rheopectic materials because prolonged shearing results in an increase of viscosity in the low shear rate domain of several orders of magnitude.Presented in part at the First Annual European Rheological Conference, Guimarães, Portugal, 11–13 September 2003