Piezoelectric Droplet Generator Research Articles

Towards Automation in 3D Cell Culture: Selective and Gentle High-Throughput Handling of Spheroids and Organoids via Novel Pick-Flow-Drop Principle.

3D cell culture is becoming increasingly important for mimicking physiological tissue structures in areas such as drug discovery and personalized medicine. To enable reproducibility on a large scale, automation technologies for standardized handling are still a challenge. Here, a novel method for fully automated size classification and handling of cell aggregates like spheroids and organoids is presented. Using microfluidic flow generated by a piezoelectric droplet generator, aggregates are aspirated from a reservoir on one side of a thin capillary and deposited on the other side, encapsulated in free-flying nanoliter droplets to a target. The platform has aggregate aspiration and plating efficiencies of 98.1% and 98.4%, respectively, at a processing throughput of up to 21 aggregates per minute. Cytocompatibility of the method is thoroughly assessed with MCF7, LNCaP, A549 spheroids and colon organoids, revealing no adverse effects on cell aggregates as shear stress is reduced compared to manual pipetting. Further, generic size-selective handling of heterogeneous organoid samples, single-aggregate-dispensing efficiencies of up to 100% and the successful embedding of spheroids or organoids in a hydrogel with subsequent proliferation is demonstrated. This platform is a powerful tool for standardized 3D in vitro research.

Stability Bounds for Micron Scale Ag Conductor Lines Produced by Electrohydrodynamic Inkjet Printing.

Continuous conducting lines of width 5–20 μm have been printed with a Ag nanoparticle ink using drop-on-demand (DOD) electrohydrodynamic (EHD) inkjet printing on Si and PDMS substrates, with advancing contact angles of 11° and 35°, respectively, and a zero receding contact angle. It is only possible to achieve stable parallel sided lines within a limited range of drop spacings, and this limiting range for stable line printing decreases as the contact angle of the ink on the substrate increases. The upper bound drop spacing for stable line formation is determined by a minimum drop overlap required to prevent contact line retraction, and the lower bound is governed by competing flows for drop spreading onto an unwetted substrate and a return flow driven by a Laplace pressure difference between the newly deposited drops and the fluid some distance from the growing tip. The upper and lower bounds are shown to be consistent with those predicted using existing models for the stability of inkjet printed lines produced using piezoelectric droplet generators. A comparison with literature data for EHD printed lines finds that these limiting bounds apply with printed line widths as small as 200 nm using subfemtoliter drop volumes. When a fine grid pattern is printed, local differences in Laplace pressure lead to the line width retracting to the minimum stable width and excess ink being transported to the nodes of the grid. After printing and sintering, the printed tracks have a conductivity of about 15%–20% of bulk Ag on the Si substrate, which correlates with a porosity of about 60%.

Experimental study on spreading and splashing behavior of continuous droplets impacting on heated wall

Continuous droplets impingement on the wall is common in industry and widely adopted in the fields of power machinery, materials science, agriculture, chemical industry and so on. In this work, the spreading and splashing behavior of continuous droplets impinging on the heated wall at Jacob numbers (Ja) and with different droplet Weber numbers (Wed) (Ja = -0.054 to 0.270, Wed = 30–1000) are studied experimentally. By means of the high-speed photography, the continuous droplets impacting on the heated wall produced by the ceramic piezoelectric droplet generator was observed. Next, the behaviors of the droplet impact on the wall under different conditions are divided into spreading, bouncing, coronal spatter and complete spatter, and the phase diagrams of droplet spreading are presented. In addition, the spreading and splashing factors are determined according to the experimental image of the droplet spreading. The analysis and mathematical fittings are carried out for the variation of spreading parameters of droplet with different Wed number and Ja number. Additionally, secondary droplets produced by the droplet splashing under different experimental conditions were observed, and the relationship between droplet splashing parameters, Wed number and Ja number were analyzed and modeled.

Hydrodynamic and heat transfer characteristics of droplet train spreading-splashing transition on heated surface

Heat transfer and hydrodynamic characteristics of droplet-induced spreading-splashing transition have been investigated experimentally and numerically. Specifically, the study involved the characterization of a train of monodispersed micron-sized droplets impinging on a heated surface covered by a thin liquid film at different droplet Weber number values. A single stream of mono-dispersed droplets was generated using a piezo-electric droplet generator with the ability to adjust parameters such as droplet impingement frequency, droplet diameter and droplet impingement velocity. A thin layer of Indium Tin Oxide was coated on a translucent sapphire substrate, which was used as a heating element. High-speed optical imaging and infrared thermal imaging techniques were used to characterize the hydrodynamics and heat transfer of droplet train impingement, respectively. It was observed that at low Weber number (We = 280), droplet impingements resulted in smooth spreading of the droplet-induced crown while splashing was observed at high Weber number (We > 489) cases. The effect of spreading-splashing transition on surface heat transfer has also been investigated at fixed flow rate conditions. Time-averaged temperature measurements indicate that the heat flux-surface temperature curves are linear at low surface temperature and before the onset of dry-out. However, a sharp increase in surface temperature was observed when dry-out appeared on the heater surface due to phase change. It was also found that strong splashing is unfavorable for heat transfer at high surface temperatures due to the instabilities seen in the liquid film, which leads to the onset of dry-out. Numerically, the Coupled Level Set-Volume of Fluid approach was used successfully to capture the hydrodynamic and thermal aspects of droplet train impingement on a heated surface. Furthermore, the dynamic mesh adaption technique was used in all simulations, which is capable of capturing the propagation of droplet-induced crown with high spatial and temporal resolutions. A good agreement was reached between experimental and numerical data in terms of droplet-induced crown morphology and surface temperatures. Thermal boundary layer displacement thickness profiles from numerical analysis were used to understand the coupled nature of hydrodynamics and heat transfer within the thin liquid film. In summary, the results indicate that droplet Weber number is a significant factor in spreading-splashing transition and droplet-induced liquid film heat transfer.

Dual-stream of monodisperse droplet generator

The generation of droplets with controllable sizes is important in various engineering applications. However, existing studies commonly focused on generating a single monodisperse droplet stream. The generation of dual-stream droplets with different controllable sizes and spacings has not been clearly investigated. Here we design a piezoelectric droplet generator characterized by two different-sized orifices in a metal plate. On the basis of Rayleigh breakup theory, the practical diameter ratio of the larger and smaller orifices is qualitied, with a value less than 2.4. Experiments are carried out on four different designed orifice plates. The ranges of excitation frequency that facilitate monodisperse dropletformation are identified. The formation process and transformation characteristics of the droplet streams are visually analysed with high speed shadowgraphy. Meanwhile, a non-dimensional number Rs is proposed to describe the volume flow rate fraction in the operation of orifice plates. Experimental results show that the design works with high stability.

Dynamics of a successive train of monodispersed millimetric-sized droplets impact on solid surfaces at low Weber number

An experimental study of successive impingement of a single train of monodispersed millimetric-sized droplets at low Weber number (We < 200) is presented. A piezoelectric droplet generator which is capable of adjusting droplet impact frequency and droplet diameter is fabricated and used to produce monodispersed millimetric-sized droplets. The interaction phenomena between the droplet-surface and droplet-film were observed using a high-speed camera at 4000 fps. The variation of the dimensionless spread factor and flatness factor of the liquid film formed by the successive impacts of droplets over time were recorded. The impact frequency of droplets was varied between 40 Hz and 85 Hz. The Weber number ranged from 29.6 to 196.6. And three types of solid surface with different static contact angles (71.5°, 82.7°, 90.1°) were tested in this work. The results showed that the impact frequency of droplets in the selected range had a minor effect on the maximum spread factor and post-impact oscillation. The maximum dimensionless spread factor of the liquid film formed by the successive impacts at the same dimensionless time tends to be greater when Weber number was higher and static contact angle was lower. The post-impact oscillation was more violent when Weber number was lower and static contact angle was larger. Empirical correlations for the maximum spread factor and average flatness factor under different conditions were obtained based on the experiment results.

Experimental and Numerical Visualization of Heat Transfer and Hydrodynamics Induced by Double Droplet Train Impingement

The objective of this study was to visualize and simulate the thermal physical process during double droplet train impingement for three different horizontal impact spacings (S = 0.65 mm, 1.2 mm and 2 mm). Two identical HFE-7100 droplet trains were produced using a piezoelectric droplet generator at a frequency of 6000 Hz with a corresponding droplet Weber number of 312. A translucent sapphire substrate with a thin film ITO coating was used as heater in the experiments. The heat transfer and hydrodynamics of double droplet train impingement have been visualized using IR thermal imaging and high speed optical imaging techniques, respectively. The double droplet train impingement process was also simulated numerically using the Coupled Level Set-Volume of Fluid (CLS-VOF) approach with dynamic mesh adaption (DMA). Humps were observed both numerically and experimentally between two adjacent impact craters due to the interactions caused by the impinging droplet trains. It was found that the hump height decreased when impact spacing increased. IR images show that higher impact spacing leads to better heat transfer performance, which could be due to the lower hump height at greater impact spacing conditions. It was also observed that higher impact spacing leads to better thermo-hydrodynamics within and outside the impingement zone. In summary, results show that horizontal impact spacing plays a significant role in double droplet train impingement cooling. This work was supported by the National Priority Research Program of the Qatar National Research Fund, Grant No.: NPRP 6-1304-2-525.

Post-impact drop vibration on a hydrophilic surface

Post-impact drop vibration on a hydrophilic surface has been studied experimentally in terms of the oscillation of spread and thickness factors. Water droplets were dropped from a piezoelectric droplet generator onto a smooth aluminum surface. Three different regimes featuring different oscillatory behaviors were classified based on Weber number. Quantitative characterization of the drop vibration was conducted for Regime I with We<30. Mobile contact line (MCL) and pinned contact line (PCL) vibrations were differentiated for the first time in analyzing the post-impact drop vibration. A time-frequency analysis shows that the transition from MCL vibration to PCL vibration features a tiny shift to a higher oscillation frequency. The normalized oscillation frequency can be well scaled by Ω∼We-1/2 as shown from the classical models. A new, empirical, unifying model was developed to account for the contact line movement in order to incorporate both MCL and PCL vibrations. The hysteresis-induced force at the three-phase contact line was found to alternate the equilibrium position of the vibrating drop with time, and thus making the oscillatory behavior nonlinear. It can be derived from the unifying model that the oscillation frequency should also scale as Ω∼We-1/2 and the damping factor should scale as ζ∼Re-1, which are validated by the presented experimental results. Finally, the good scaling of the damping factor indicates that the major damping mechanism, for both MCL and PCL vibrations, should originate from the viscous dissipation.

Heat transfer characteristics of double, triple and hexagonally-arranged droplet train impingement arrays

In this study, hydrodynamics and heat transfer of multiple droplet trains impinging a pre-wetted solid surface have been investigated experimentally. A piezo-electric droplet generator has been designed and constructed, which is capable of producing double, triple and hexagonally-arranged droplet trains. A translucent sapphire substrate coated with a thin layer of indium tin oxide (ITO) was used as flat heating element. The effects of droplet Weber number, impact spacing and impingement pattern on liquid film hydrodynamics and heat transfer have been evaluated using high speed optical imaging and IR thermal imaging techniques. High speed images show that a hump was formed between two impact craters for double droplet train impingement. Surface jet flows were observed among impact craters for triple and hexagonally-arranged droplet train impingement arrays. Heat transfer results reveal that horizontal impact spacing and impingement pattern play significant roles in cooling performance. For double droplet train impingement, it was found that higher impact spacing leads to better cooling performance both locally (i.e. within the impingement zone) and globally (i.e. outside the droplet impingement zone). For triple droplet train impingement, there is an optimum impact spacing for heat transfer. For hexagonally-arranged droplet train impingement arrays, lower impact spacing leads to better cooling performance locally. However, higher impact spacing leads to better cooling performance globally. Comparisons have been made between droplet train impingement and circular jet impingement for various impingement patterns. Heat transfer measurements show that droplet train impingement leads to better cooling performance for various impingement patterns. In summary, results reveal that the combined effects of the droplet Weber number and impingement pattern are significant factors in the study of droplet-induced surface heat transfer phenomena.

Experimental and Numerical Visualization of Droplet-Induced Crown Splashing Dynamics

The objective of this study was to visualize the droplet-induced crown splashing dynamics at high spatial and temporal resolutions. In this work, the effects of droplet train impingement on crown splashing dynamics were investigated experimentally and numerically. Experimentally, a HFE-7100 droplet train was produced using a piezo-electric droplet generator at a frequency ( f ) of 7500 Hz resulting in a droplet Weber number (We) of 489. Droplet-induced crown splashing dynamics was captured using a high-speed imaging system. It was observed that the free rim of the droplet-induced crown was smooth and axisymmetric during the early crown propagation phases (t*&lt; 5, where t* = 2πft). However, development of cusps was observed on the free rim during the intermediate phases (5&lt; t* &lt;8.5). It was found that the sites of the spikes distributed almost uniformly along the periphery of the free rim. At late phases (t* &gt; 8.5), fingering, detachment and secondary droplets (i.e. splashing) were observed on the free rim. Results show that the number of cusps (finally becoming fingers and spikes) based on experiments (ncups,exp) agrees well with the prediction (ncups,th) given by the Plateau-Rayleigh instability theory. Numerical simulations were carried out using a 3D transient coupled level set-volume of fluid (CLSVOF) solver with Courant number less than 1. A grid independence study was performed to ensure the results were independent of grid size. Reasonable agreement was reached between the numerical and experimental data in terms of crown morphology at different phases. This study should lead to a better understanding of the evolution of droplet-induced crown morphology at splashing conditions.

Effects of Screen Laminates on Droplet-Induced Film Hydrodynamics and Surface Heat Transfer

Formation of dry-out area could lead to a sharp increase in surface temperature during droplet impingement cooling. The objective of this study is to figure out an effective way of suppressing dry-out formation in droplet impingement cooling. In this work, HFE-7100 droplet train was produced using a piezo-electric droplet generator at a frequency of 6000 Hz with a droplet Weber number of 280. A translucent substrate was coated with a thin film ITO, which was used as a heater in the experiments. A copper screen laminates with a single punched hole (diameter = 3 mm) was placed over the heater surface at a distance of 0.3 mm to enhance surface heat transfer. Optical images showed that screen laminates effectively suppressed the formation of the dry-out area. It was also found that heat transfer was greatly improved when screen laminates were used. The heat transfer improvement could be attributed to the enhanced surface tension effects, which keep the whole surface wet at high surface temperatures. In summary, the results show that screen laminates effectively suppress the formation of dry-out area and greatly improve surface heat transfer.

Effects of High Frequency Droplet Train Impingement on Crown Propagation Dynamics and Heat Transfer

The objective of this study is to investigate the hydrodynamics and heat transfer phenomena due to high frequency droplet train impingement on a pre-wetted solid surface for electronic cooling applications. The effects of crown propagation dynamics and surface heat transfer were investigated experimentally and numerically. Experimentally, a single stream of mono-dispersed HFE-7100 droplets was generated using a piezo-electric droplet generator at a frequency ( f ) of 6000 Hz with a droplet Weber number (We) of 280. Droplet-induced crater and crown were imaged using a high speed camera system. Numerically, the ANSYS Fluent CFD tool was used to simulate the droplet train impingement process. A reasonable agreement was reached between experimental and numerical data in terms of crown propagation dynamics. Numerical simulations reveal that at the instant of initial spot formation, the magnitude of droplet velocity is almost identical to the crown's radial velocity. The instantaneous temperature field obtained by numerical simulations shows that heat transfer was most effective within the crown propagation region due to the radial momentum generated by the droplets, which leads to a large velocity gradient within the liquid film. A significant increase in surface temperature was observed beyond a radial position of 500 μm. In summary, high frequency droplet impingement leads to a very small temperature gradient in the radial direction within the droplet-induced impact crater. This study will benefit in understanding the relationship between the droplet parameters and surface heat transfer for different cooling applications involving impinging droplets.

Effects of High Frequency Droplet Train Impingement on Spreading-Splashing Transition, Film Hydrodynamics and Heat Transfer

The objective of this study is to investigate the effects of droplet-induced crown propagation regimes (spreading and splashing) on liquid film hydrodynamics and heat transfer. In this work, the effects of high frequency droplet train impingement on spreading-splashing transition, liquid film hydrodynamics and surface heat transfer were investigated experimentally. HFE-7100 droplet train was generated using a piezo-electric droplet generator at a fixed flow rate of 165 mL/h. Optical and IR images were captured at stable droplet impingement conditions to visualize the thermal physical process. The droplet-induced crown propagation transition phenomena from spreading to splashing were observed by increasing the droplet Weber number. The liquid film hydrodynamics induced by droplet train impingement becomes more complex when the surface was heated. Bubbles and micro-scale fingering phenomena were observed outside the impact crater under low heat flux conditions. Dry-out was observed outside the impact craters under high heat flux conditions. IR images of the heater surface show that heat transfer was most effective within the droplet impact crater zone due to high fluid inertia including high radial momentum caused by high-frequency droplet impingement. Time-averaged heat transfer measurements indicate that the heat flux-surface temperature curves are linear at low surface temperature and before the onset of dry-out. However, a sharp increase in surface temperature can be observed when dry-out appears on the heater surface. Results also show that strong splashing (We = 850) is unfavorable for heat transfer at high heat flux conditions due to instabilities of the liquid film, which lead to the onset of dry-out. In summary, the results show that droplet Weber number is a significant factor in the spreading-splashing transition, liquid film hydrodynamics and heat transfer.

Numerical and experimental investigations of crown propagation dynamics induced by droplet train impingement

In this study, hydrodynamics of HFE-7100 droplet train impinging on a pre-wetted solid surface was investigated experimentally and numerically. Experimentally, single stream of mono-dispersed droplets were produced using a piezoelectric droplet generator with the ability to adjust parameters such as droplet impingement frequency, droplet diameter and droplet velocity. Crown propagation events were imaged using a high-speed camera system given the high-frequency of droplet impingement. Relationships between droplet-induced crown propagation and crater formation were investigated experimentally. The high-frequency droplet impingement process was simulated numerically using CFD tool. Crown propagation dynamics were evaluated and analyzed experimentally and numerically, with reasonable agreement between the two methods. A revised theoretical crown propagation model based on numerical results is proposed in this paper, which takes into account the impinged liquid velocity distribution and film thickness at the moment of initial spot formation. The revised theoretical crown propagation model gives predictions with improved accuracy, which are in better agreement with the numerical results.

Simultaneous measurements of droplet size, flying velocity and transient temperature of in-flight droplets by using a molecular tagging technique

In the present study, a molecular tagging technique is introduced to achieve simultaneous measurements of droplet size, flying velocity and transient temperature of in-flight liquid droplets in a spray flow. For the molecular tagging measurements, a pulsed laser is used to “tag” phosphorescent 1-BrNp·Mβ-CD·ROH triplex molecules premixed within liquid droplets. After the same laser excitation pulse, long-lived laser-induced phosphorescence is imaged at two successive times within the phosphorescence lifetime of the tagged phosphorescent triplex molecules. While the sizes of the droplets are determined quantitatively based on the acquired droplet images with a precalibrated scale ratio between the image plane and the object plane, the displacement vectors of the in-flight droplets between the two image acquisitions are used to estimate the flying velocities of the droplets. The simultaneous measurements of the transient temperatures of the in-flight droplets are achieved by taking advantage of the temperature dependence of phosphorescence lifetime, which is estimated from the intensity ratio of the acquired phosphorescence image pair of the inflight droplets. The feasibility and implementation of the molecular tagging technique are demonstrated by conducting simultaneous measurements of droplet size, flying velocity and transient temperature of micro-sized water droplets exhausted from a piezoelectric droplet generator into ambient air at different test conditions in order to characterize the dynamic and thermodynamic behaviors of the micro-sized in-flight droplets. The unsteady heat transfer process between the in-flight droplets and the ambient air is also analyzed theoretically by using a lumped capacitance method to predict the temperature changes of the in-flight water droplets along their flight trajectories. The measured temperature data are compared with the theoretical analysis results quantitatively, and the discrepancies between measurement results and the theoretical predictions are found to be within 0.80 °C.

Droplet generation and characterization using a piezoelectric droplet generator and high speed imaging techniques

Accurate spray (droplet) characterization helps in the better understanding of the pesticide spray application process. The goal of this research was to evaluate the characteristics of a single droplet generated using a piezoelectric single droplet generator in 2 modes: Droplet-On-Demand (DOD) and continuous with 4 different orifice sizes. The droplet generator can produce a single droplet, or a continuous stream of uniformly sized droplets. Using a high speed imaging system and image processing algorithms, the effects of pulse width and pulse amplitude in DOD mode and of frequency and pulse amplitude in continuous mode on droplet characteristics (droplet size, droplet velocity and inter droplet spacing) were investigated for different orifice sizes. The system was able to produce droplet diameters within the range from 134.1 to 461.5 μm in DOD, and from 167.2 to 458.6 μm in continuous mode. In addition, droplet velocities ranged from 0.08 to 1.8 m/s in DOD mode and from 1.8 to 4.7 m/s in continuous mode. Validation using a droplet weighing method showed good agreement with the droplet image analysis results.

Study of the effects of single and multiple periodic droplet impingements on liquid film heat transfer

A study of heat transfer and film dynamics caused by single and multiple streams of impinging droplets using HFE-7100 as the cooling liquid under constant heat flux conditions is presented. Single and multiple streams of mono-dispersed droplets were produced using a piezoelectric droplet generator with the ability to adjust parameters such as droplet impingement frequency, droplet diameter, droplet velocity, and spacing between adjacent impinging droplet streams. In this study, a heater consisting of a thin layer of Indium Tin Oxide (ITO) as heating element, combined with a Zinc Selenide (ZnSe) substrate was used for characterizing the heat transfer behavior and hydrodynamic phenomena of impinged liquid films near or at the onset of critical heat flux (CHF). Film thickness inside the impact crater was measured using the Total Internal Reflection (TIR) technique. Hydrodynamic phenomena of the droplet impact craters were analyzed using a high speed imaging technique. Impact regimes of the impinging droplets were identified and classified, and their effects on heat transfer performance are discussed.The study supports the notion that forced convection is the main heat transfer mechanism inside the impact crater due mainly to the high frequency and periodic nature of droplet impingement. Furthermore, droplet impingement regimes such as spreading and splashing have been observed to play an important role in the overall heat transfer behavior. Spacing between adjacent impinging droplet streams is also an important factor in surface cooling when multiple droplet streams are used.

Investigation of droplet collisions for solutions with different solids content

The collision behaviour of droplets and the collision outcome are investigated for high viscous polymer solutions. For that purpose, two droplet chains produced by piezoelectric droplet generators are directed towards each other at a certain angle so that individual droplet pairs collide. For recording the collision event, one double-image and one high-speed CCD camera were used. One camera is positioned perpendicular to the collision plane recording the outcome of the collision, and the second camera is aligned parallel to the collision plane to assure that the droplet chains are exactly in one plane. A new approach for tracking droplets in combination with an extended particle tracking velocimetry algorithm has been developed. Time-resolved series of pictures were used to analyse the dynamics of droplet collisions. The three different water soluble substances were saccharose and 1-Ethenyl-2-pyrrolidone (PVP) with different molecular weights (K17, K30). The solvent was demineralised water. The solids contents ranged from 20 to 60 %, 5 to 25 % and 5 to 35 %, yielding dynamic viscosities in the range of 2–60 mPa s. Results were collected for different pairs of impact angles and Weber numbers in order to establish common collision maps for characterising the outcomes. Here, relative velocities between 0.5 and 4 m/s and impact parameters in the interval from 0 to 1 for equal-sized droplets (Δ = 1) have been investigated. Additionally, satellite formation will be discussed exemplarily for K30. A comparison with common models of different authors (Ashgriz and Poo in J Fluid Mech 221:183–204, 1990; Estrade et al. in Int J Heat Fluid Flow 20:486–491, 1999) mainly derived for low viscous droplets revealed that the upper limit of their validity is given by an Ohnesorge number of Oh = 0.115 and a capillary number of Ca = 0.577. For higher values of these non-dimensional parameters and hence higher dynamic viscosities, these models are unable to predict correctly the boundaries between collision scenarios. The model proposed by Jiang et al. (J Fluid Mech 234:171–190, 1992), which includes viscous dissipation, is able to predict the boundary between coalescence and stretching separation for higher viscosities (i.e. Oh > 0.115 and Ca > 0.577). However, the model constants are not identical for different solution properties. As a conclusion, an alteration of the collision appearance takes place because of the relative importance between surface tension and viscosity.

A piezoelectric drop-on-demand generator for accurate samples in capillary electrophoresis

In this work, we propose a piezoelectric droplet generator for injection of well-defined amounts of sample in capillary electrophoresis. We demonstrate stable, precise and drop-on-demand droplet formation for various solutions, with precise control of waveform driving piezoelectric crystal inside the ink-jet head. By tuning the waveform, we can also manipulate the droplet size and delivery frequency. This injector was used in sampling for capillary electrophoresis. As a state-of-the-art application, the analysis of theobromine, caffeine and theophiline using micellar electrokinetic chromatography was developed. The volume of sample (single droplet) analyzed in this experiment was 179pL (RSD=1.2%, n=10). The detection limits for caffeine, theobromine, and theophiline are 0.02, 0.08 and 0.06mML−1, respectively. Compared with conventional methods, the combination of picoliter droplet dispenser with capillary electrophoresis allows precise and accurate sampling, as well as for reduced sample consumption, which will prove to be an efficient tool in quantitative separation and analysis.

Effect of film thickness controlled by ink‐jet printing method on the optical properties of an electroluminescent polymer

AbstractInk‐jet printing (IJP) represents a highly promising liquid processed polymer deposition method for the film preparation of functional polymers in photo‐electronic devices. In this report, the results on the IJP of a fluorene‐based electroluminescent polymer, poly(9,9‐dihexylfluorene‐alt‐2,5‐dioctyloxybenzene) (PF6OC8), from a piezoelectric droplet generator are presented. The polymer film thickness has been found to show an approximate linear relation with the number of droplets per unit area; it is thus convenient to control the film thickness by the space of printed dots in IJP process. In comparison, spin coating approach is also used to prepare polymer films with different thicknesses by varying solution concentration and spinning speed. However, it is found that spin coating is difficult to control the film thickness quantitatively. The influence of film thickness on the photoluminescence (PL) properties of PF6OC8 films prepared by IJP and spin coating is comparatively investigated. For both ink‐jet printed and spin coated films, the intensity of PL spectra first increases and then decreases with increase in the film thickness, probably due to the exciton quenching in thicker films. When the polymer film thickness is at nanoscale, the major peak in the PL spectrum is the 0–0 vibronic emission at about 420 nm, and with increase in the film thickness, the 0–1 vibronic peak at about 440 nm becomes dominant. The red‐shifted PL spectra with increase in film thickness show the change from the 2D exciton state to the 3D one. Copyright © 2009 John Wiley &amp; Sons, Ltd.