Production of monodisperse water-in-oil emulsions consisting of highly uniform droplets using asymmetric straight-through microchannel arrays

This paper reports the production of monodisperse water-in-oil (W/O) emulsions using new microchannel emulsification (MCE) devices, asymmetric straight-through MC arrays that were hydrophobically modified. The silicon asymmetric straight-through MC arrays consisted of numerous pairs of microslots and circular microholes whose cross-sectional sizes were 10 μm. This paper primarily focused on investigating the effect of the osmotic pressure of a dispersed phase (Π_d) on MCE. This paper also investigated the effects of the type of continuous-phase oils and the dispersed-phase flux (J _d) on MCE. The dispersed phases were Milli-Q water and Milli-Q water solutions containing sodium chloride. The continuous phases were decane (as control), hexane, medium chain triacylglyceride (MCT), and refined soybean oil (RSO) solutions containing tetraglycerin monolaurate condensed ricinoleic acid ester (TGCR) as a surfactant. At Π_d of exceeding threshold, highly uniform aqueous droplets with coefficients of variation of less than 3% were stably generated via hydrophobic asymmetric straight-through MCs. Monodisperse W/O emulsions with average droplet diameters between 32 and 45 μm were produced using the alkane–oil and triglyceride–oil solutions as the continuous phase. This work also demonstrated that the hydrophobic asymmetric straight-through MC array had remarkable ability to produce highly uniform aqueous droplets at very high J _d of up to 1,200 L m⁻² h⁻¹.     

Microchannel emulsification for mass production of uniform fine droplets: integration of microchannel arrays on a chip

We present a novel microchannel emulsification (MCE) system for mass-producing uniform fine droplets. A 60 × 60-mm MCE chip made of single-crystal silicon has 14 microchannel (MC) arrays and 1.2 × 10⁴ MCs, and each MC array consists of many parallel MCs and a terrace. A holder with two inlet through-holes and one outlet through-hole was also developed for simply infusing each liquid and collecting emulsion products. The MCE chip was sealed well by physically attaching it to a flat glass plate in the holder during emulsification. Uniform fine droplets of soybean oil with an average diameter of 10 μm were reliably generated from all the MC arrays. The size of the resultant fine droplets was almost independent of the dispersed-phase flow rate below a critical value. The continuous-phase flow rate was unimportant for both the droplet generation and the droplet size. The MCE chip enabled mass-producing uniform fine droplets at 1.5 ml h⁻¹ and 1.9 × 10⁹ h⁻¹, which could be further increased using a dispersed phase of low viscosity.     

Temperature effect on microchannel oil-in-water emulsification

Microchannel (MC) emulsification is a promising technique to produce monodisperse emulsions by spontaneous interfacial-tension-driven droplet generation. The purpose of this study was to systematically characterize the effect of temperature on droplet generation by MC emulsification, which is a major uncharted area. The temperature of an MC emulsification module was controlled between 10 and 70°C. Refined soybean oil was used as the dispersed phase and a Milli-Q water solution containing sodium dodecyl sulfate (1 wt%) as the continuous phase. Monodisperse oil-in-water (O/W) emulsions with a coefficient of variation below 4% were produced, and at all the operating temperatures, their average droplet diameter ranged from 32 to 38 μm. We also investigated the effect of flow velocity of the dispersed phase on droplet generation characteristics. The maximum droplet generation rate (frequency) from a channel at 70°C exceeded that at 10°C by 8.1 times, due to the remarkable decrease in viscosity of the two phases. Analysis using dimensionless numbers indicated that the flow of the dispersed phase during droplet generation could be explained using an adapted capillary number that includes the effect of the contact angle of the dispersed phase to the chip surface.     

Integrated micro Pirani gauge based hermetical package monitoring for uncooled VO x bolometer FPAs

This paper presents the design and fabrication of a micro Pirani gauge using VO _x as the sensitive material for monitoring the pressure inside a hermetical package for micro bolometer focal plane arrays (FPAs). The designed Pirani gauge working in heat dissipating mode was intentionally fabricated using standard MEMS processing which is highly compatible with the FPAs fabrication. The functional layer of the micro Pirani gauge is a VO _x thin film designed as a 100 × 200 μm pixel, suspended 2 μm above the substrate. By modeling of rarefied gas heat conduction using the Extended Fourier’s law, finite element analysis is used to investigate the sensitivity of the pressure gauge. Also the thermal interactions between the micro Pirani gauge and bolometer FPAs are verified. From the fabricated prototype, the measured device TCR is about −0.8% K⁻¹ and the sensitivity about 1.84 × 10⁻³ W K⁻¹ mbar⁻¹.     

Effects of actuating waveform,ink property,and nozzle size on piezoelectrically driven inkjet droplets

Computational fluid dynamics and micro-flow visualization (μ-FV) have been complementarily performed to study the evolution of a single droplet ejected from a bend-mode piezoelectric inkjet printhead. The numerical simulation is characterized by the coupled piezoelectric-structural-fluid solution procedure and verified by the μ-FV results. The in-house numerical code is subsequently applied to investigate the influences of electric voltage φ _pp, pulse shape, ink property, and nozzle diameter D _n on the droplet volume, velocity, and configurations. φ _pp studied ranges from 14 to 26 V and pulse shape is explored by varying the key time intervals with fixed voltage slopes. The influence of ink property is examined by investigating the dynamic viscosity μ and surface tension σ separately. Investigation on the effects of nozzle diameter is also conducted by decreasing D _n from 26 to 11 at 3 μm interval. The computed results are found in good agreement with the experimental ones. New findings are to discover the critical ranges of electric waveform parameters, μ, and σ outside which the phenomena of satellite droplets and puddle formation at the nozzle opening are absent. In addition, the imbedded physical rationales for these critical ranges are provided. The results are also new in terms of the identifications of the critical σ and D _n for the reference of improving the droplet quality.     

Rapid quantification of bio-particles based on image visualisation in a dielectrophoretic microfluidic chip

We studied an imaging-based technique for the rapid quantification of bio-particles in a dielectrophoretic (DEP) microfluidic chip. Label-free particles could be successively sorted and trapped in a continuous flow manner under the applied alternating current (AC) conditions. Both 2 and 3 μm polystyrene beads at a concentration of 1.0 × 10⁷ particles ml⁻¹ could be rapidly quantified within 5 min in our DEP system. Capturing efficiencies higher than 95% could be 2 μm polystyrene beads with a linear flow speed, applied voltage and frequency of 0.89 mm s⁻¹, 20 V_p-p and 5 MHz. Yeast cells (Candida glabrata and Candida albicans) could also be captured even at a lower concentration of 2.5 × 10⁵ cells ml⁻¹. Images of aggregative particles taken from the designed trapping area were further processed based on the intensity of relative greyscale followed by correction of the particle numbers. The imaging-based quantification method showed higher agreement than that of the conventional counting chamber method and proved the stability and feasibility of our AC DEP system.     

Droplet generation in micro-sieve dispersion device

Microfluidic devices with micro-sieve plate as the dispersion medium have been widely used for the mass production of emulsions. While unfortunately, few studies have so far been made for the droplet generation rules in those devices. In this work, the droplet generation processes in micro-sieve dispersion devices are investigated with specially designed micro-sieve pore arrays. The effects of channel structure, pore arrangement, and feeding method of dispersed phase on the average size and distribution of droplets are studied carefully. It is found the dimensionless average droplet diameters (d _av/d _e) in micro-sieve dispersion devices can be represented by a linear relation with Ca^−1/4 of continuous phase, the same as the scaling law in T-junction microchannels. The flow distribution among pores and the steric hindrance between droplets affect the diameter distribution of generated droplet very much. Monodispersed droplets with polydispersity index less than 5% can be made at Ca number larger than 0.01 and phase ratio (Q _D/Q _C) less than 1/6 in the present investigation.     

Generation of uniform drops via through-hole arrays micromachined in stainless-steel plates

This study investigated the generation of oil drops using new symmetric and asymmetric through-hole-array devices made of stainless steel. The through-hole-array devices were built by piling up six stainless-steel plates, each having circular micro-holes with a diameter of 300 or 500 μm or micro-slots with a shorter line of 300 or 500 μm. Drops were generated by injecting a dispersed phase (refined soybean oil) via the through-hole array into a compartment filled with a continuous phase (Milli-Q water solution containing one of two emulsifiers). The drop detachment from symmetric and asymmetric through holes was observed in real time and analyzed. Uniform oil drops with average diameters of 1.0–4.1 mm and coefficients of variation of typically less than 6% were generated using symmetric and asymmetric through-hole-array devices. The resultant drop diameters for asymmetric through-hole arrays were significantly smaller than those for symmetric through-hole arrays. This paper also discusses experimental results regarding the effects of the microstructure, the dimensions of the through holes, and the type of emulsifier on drop generation and the resultant drop diameter.     

Control of serial microfluidic droplet size gradient by step-wise ramping of flow rates

This paper describes a method to control and detect droplet size gradient by step-wise flow rate ramping of water-in-oil droplets in a microfluidic device. The droplets are generated in a cross channel device with two oil inlets and a water inlet. The droplet images are captured and analyzed in a time sequence in order to quantify the droplet generation frequency. It is demonstrated that by controlling the ramping of the oil flow rates it is possible to manipulate the ramping of droplet sizes. Increasing or decreasing of droplet sizes is achieved for a step-wise triangular ramping profile of the oil flow rate. The dynamic behavior of droplets due to the step-wise flow pulses is investigated. Uniform linear size ramping of water-in-oil droplets from 73 to 83 μm in diameter is generated with an oil flow ramping range from 1 to 11 μL/min in a minimum of five steps while water flow rate is held constant at 2 μL/min.     

Generating gas/liquid/liquid three-phase microdispersed systems in double T-junctions microfluidic device

This article describes the generation of microdispersed bubbles and droplets in a double T-junctions microfluidic device to form immiscible gas/liquid/liquid three-phase flowing systems. Segmented gas plugs are controllably prepared in water at the first T-junction to form gas/liquid two-phase fluid with the perpendicular flow cutting method. Then using this two-phase fluid as the cross-shearing fluid for the oil phase at the second T-junction, the gas/liquid/liquid three-phase flowing systems are prepared. Interestingly, it is found that the break-up of the oil droplets is mainly dominated by the cutting effect of the gas/liquid interface or the pressure drop across the emerging droplet, but independent with the viscous shearing effect of the continuous phase, even at the capillary number (Ca = u _wμ_w/γ_ow) higher than 0.01. The size laws and the distributions of the bubbles and droplets are investigated carefully, and a mathematical model has been developed to relating the operating conditions with the dispersed sizes.     

Incomplete crossed hypercubes

In this paper, a new interconnection network called the incomplete crossed hypercube is proposed for connecting processors of parallel computing systems. The incomplete crossed hypercube architecture denoted by CI _n−m ⁿ is made by combining two complete crossed hypercubes CQ _n and CQ _n−m for 1≤m≤n. Several topological properties of CI _n−m ⁿ are analyzed. In particular, accurate mean internode distance formulas of both CQ _n and CI _n−m ⁿ are given. Compared with the incomplete enhanced hypercube EI _n−m ⁿ , CI _n−m ⁿ has shorter mean internode distance for large n. An optimal routing algorithm for CI _n−m ⁿ is also described which guarantees the generation of a shortest path from a node to another in CI _n−m ⁿ .     

Pre-programmable polymer transformers as on-chip microfluidic vacuum generators

This paper develops novel polymer transformers using thermally actuated shape memory polymer (SMP) materials. This paper applies SMPs with thermally induced shape memory effect to the proposed novel polymer transformers as on-chip microfluidic vacuum generators. In this type of SMPs, the morphology of the materials changes when the temperature of materials reaches its glass transition temperature (T _g). The structure of the polymer transformer can be pre-programmed to define its functions, which the structure is reset to the temporary shape, using shape memory effects. When subjected to heat, the polymer transformer returns to its pre-memory morphology. The morphological change can produce a vacuum generation function in microfluidic channels. Vacuum pressure is generated to suck liquids into the microfluidic chip from fluidic inlets and drive liquids in the microchannel due to the morphological change of the polymer transformer. This study adopts a new smart polymer with high shape memory effects to achieve fluid movement using an on-chip vacuum generation source. Experimental measurements show that the polymer transformer, which uses SMP with a T _g of 40°C, can deform 310 μm (recover to the permanent shape from the temporary shape) within 40 s at 65°C. The polymer transformer with an effective cavity volume of 155 μl achieved negative pressures of −0.98 psi. The maximum negative up to −1.8 psi can be achieved with an effective cavity volume of 268 μl. A maximum flow rate of 24 μl/min was produced in the microfluidic chip with a 180 mm long channel using this technique. The response times of the polymer transformers presented here are within 36 s for driving liquids to the end of the detection chamber. The proposed design has the advantages of compact size, ease of fabrication and integration, ease of actuation, and on-demand negative pressure generation. Thus, this design is suitable for disposable biochips that need two liquid samples control. The polymer transformer presented in this study is applicable to numerous disposable microfluidic biochips.     

Pressure drop of gas–liquid Taylor flow in round micro-capillaries for low to intermediate Reynolds numbers

In this paper, a model is presented that describes the pressure drop of gas–liquid Taylor flow in round capillaries with a channel diameter typically less than 1 mm. The analysis of Bretherton (J Fluid Mech 10:166–188, 1961) for the pressure drop over a single gas bubble for vanishing liquid film thickness is extended to include a non-negligible liquid film thickness using the analysis of Aussillous and Quéré (Phys Fluids 12(10):2367–2371, 2000). This result is combined with the Hagen–Poiseuille equation for liquid flow using a mass balance-based Taylor flow model previously developed by the authors (Warnier et al. in Chem Eng J 135S:S153–S158, 2007). The model presented in this paper includes the effect of the liquid slug length on the pressure drop similar to the model of Kreutzer et al. (AIChE J 51(9):2428–2440, 2005). Additionally, the gas bubble velocity is taken into account, thereby increasing the accuracy of the pressure drop predictions compared to those of the model of Kreutzer et al. Experimental data were obtained for nitrogen–water Taylor flow in a round glass channel with an inner diameter of 250 μm. The capillary number Ca _gl varied between 2.3 × 10⁻³ and 8.8 × 10⁻³ and the Reynolds number Re _gl varied between 41 and 159. The presented model describes the experimental results with an accuracy of ±4% of the measured values.     

Characterization of an electro-thermal micro gripper and tip sharpening using FIB technique

A micromachined electro-thermal gripper, first introduced by Ivanova et al. (Microelectron Eng 83:1393–1395, 2006), represents a promising candidate for the manipulation and handling of micro or even nano-scaled objects. To further optimize the performance of the device, a detailed electrical and mechanical characterization is needed. Due to the so-called duo-action gripper approach (i.e., a separate actuator for closing and opening action) these investigations focused on the maximum (minimum) opening width being 11.5 μm (3.3 μm), while in rest position a value of 4 μm is feasible. The maximum, electrical input power is limited to 80 mW/actuator element, resulting in a current density of up to 1.27 MA cm⁻² in the corresponding metal layers. When applying, however, larger current densities the probability of device failure increases substantially as in combination with an enhanced temperature of about 200°C electromigration effects occur in the metallization. Furthermore, the cut-off frequency and parasitic effects during actuation such as the z-deflection and the increase in length of each arm both showing values of up to 3 μm have been investigated as a function of operation parameters. Finally, the tips of the gripper were sharpened using Focused Ion Beam technique to a radius of less than 1 μm for gripping operations in space-restricted environments or for the manipulation or handling of sub-μm scaled objects.     

Random 2 XORSAT Phase Transition

We consider the 2-XOR satisfiability problem, in which each instance is a formula that is a conjunction of Boolean equations of the form x ⊕ y=0 or x ⊕ y=1. Formula of size m on n Boolean variables are chosen uniformly at random from among all ((n(n-1)) || (m)){n(n-1)\choose m} possible choices. When c<1/2 and as n tends to infinity, the probability p(n,m=cn) that a random 2-XOR formula is satisfiable, tends to the threshold function exp (c/2)⋅(1−2c)^1/4. This gives the asymptotic behavior of random 2-XOR formula in the SAT/UNSAT subcritical phase transition. In this paper, we first prove that the error term in this subcritical region is O(n ⁻¹). Then, in the critical region c=1/2, we prove that p(n,n/2)=Θ(n ^−1/12). Our study relies on the symbolic method and analytical tools coming from generating function theory which also enable us to describe the evolution of n^1/12 p(n,\fracn2(1+mn^-1/3))n^{1/12}\ p(n,\frac{n}{2}(1+\mu n^{-1/3})) as a function of μ. Thus, we propose a complete picture of the finite size scaling associated to the subcritical and critical regions of 2-XORSAT transition.     

Micro-droplet formation in non-Newtonian fluid in a microchannel

Micro-droplet formation from an aperture with a diameter of micrometers is numerically investigated under the cross-flow conditions of an experimental microchannel emulsification process. The process involves dispersing an oil phase into continuous phase fluid through a microchannel wall made of apertured substrate. Cross-flow in the microchannel is of non-Newtonian nature, which is included in the simulations. Micro-droplets of diameter 0.76–30 μm are obtained from the simulations for the apertures of diameter 0.1–10.0 μm. The simulation results show that rheology of the bulk liquid flow greatly affects the formation and size of droplets and that dispersed micro-droplets are formed by two different breakup mechanisms: in dripping regime and in jetting regime characterized by capillary number Ca. Relations between droplet size, aperture opening size, interfacial tension, bulk flow rheology, and disperse phase flow rate are discussed based on the simulation and the experimental results. Data and models from literature on membrane emulsification and T-junction droplet formation processes are discussed and compared with the present results. Detailed force balance models are discussed. Scaling factor for predicting droplet size is suggested.     

Low voltage electroosmotic pump for high density integration into microfabricated fluidic systems

A low voltage electroosmotic (eo) pump suitable for high density integration into microfabricated fluidic systems has been developed. The high density integration of the eo pump required a small footprint as well as a specific on-chip design to ventilate the electrolyzed gases emerging at the platinum (Pt) electrodes. For this purpose, a novel liquid–gas (lg) separator was invented. This lg-separator separated the gas bubbles from the liquid and guided them away from the eo pump. Its operational principle was solely based on the geometry of tapered sidewalls. An eo pump sandwiched by two lg separators (microchannels in the range of 10 μm, footprint of 100 μm × 15 μm) was experimentally investigated. The lg-separator was able to reliably separate and ventilate an emerging gas flow of 2 pl s⁻¹. The eo pump achieved flow rates of 50 pl s⁻¹ at actuation voltages of 5 V.     

Continuous generation of hydrogel beads and encapsulation of biological materials using a microfluidic droplet-merging channel

In this paper, we describe a method for encapsulation of biomaterials in hydrogel beads using a microfluidic droplet-merging channel. We devised a double T-junction in a microfluidic channel for alternate injection of aqueous fluids inside a droplet unit carried within immiscible oil. With this device, hydrogel beads with diameter <100 μm are produced, and various solutions containing cells, proteins and reagents for gelation could merge with the gel droplets with high efficiency in the broad range of flow rates. Mixing of reagents and reactions inside the hydrogel beads are continuously observed in a microchannel through a microscope. By enabling serial injection of each liquid with the dispersed gel droplets after they are produced from the oil-focusing channel, the device simplifies the sample preparation process, and gel-bead fabrication can be coupled with further assay continuously in a single channel. Instantaneous reactions of enzyme inside hydrogel and in-situ formation of cell-containing beads with high viability are demonstrated in this report.     

Wavelength Assignment for Realizing Parallel FFT on Regular Optical Networks

Routing and wavelength assignment (RWA) is a central issue to increase efficiency and reduce cost in Wavelength Division Multiplexing (WDM) optical networks. In this paper, we address the problem of wavelength assignment for realizing parallel FFT on a class of regular optical WDM networks. We propose two methods for sequential mapping and shift-reversal mapping of FFT communication pattern to the optical WDM networks concerned. By sequential mapping, the numbers of wavelengths required to realize parallel FFT with 2ⁿ nodes on WDM linear arrays, rings, 2-D meshes and 2-D tori are 2^{n − 1}, 2^{n − 1}, 2^{max (k,n − k) − 1} and 2^{max (k,n − k) − 1} respectively. By shift-reversal mapping, the numbers of wavelengths required are max (3× 2^{n − 3},2), 2^{n − 2}, max (3× 2^{max (k,n − k) − 3},2) and 2^{max (k,n − k) − 2}. These results show that shift-reversal mapping outperforms sequential mapping. Our results have a clear significance for applications because FFT represents a common computation pattern shared by a large class of scientific and engineering problems and WDM optical networks as a promising technology in networking has an increasing popularity.