Thermocapillary actuation of droplets on chemically patterned surfaces by programmable microheater arrays

We have designed a microfluidic device for the actuation of liquid droplets or continuous streams on a solid surface by means of integrated microheater arrays. The microheaters provide control of the surface temperature distribution with high spatial resolution. These temperature gradients locally alter the surface tension along droplets and thin films thus propelling the liquid toward the colder regions. In combination with liquophilic and liquophobic chemical surface patterning, this device can be used as a logistic platform for the parallel and automated routing, mixing and reacting of a multitude of liquid samples, including alkanes, poly(ethylene glycol) and water.     

Numerical simulation of droplet impacting liquid surface by SPH

Droplet impacting liquid surface is not only the extremely prevalent phenomenon in the nature and industrial production but also the extremely complicated problem of strong non-linear transient impact and free-surface flow. On the basis of the two-dimensional viscous incompressible N-S equations, this paper conducts a study of numerical simulation on the problem of droplet impacting liquid surface (water beads) of water container in certain initial velocity by the method of SPH (smoothed particle hydrodynam...     

Propulsion of droplets on micro- and sub-micron ratchet surfaces in the Leidenfrost temperature regime

Spatially periodic systems with localized asymmetric surface structures (ratchets) can induce directed transport of matter (liquid/particles) in the absence of net force. Here, we show that propulsion for the directed motion of water droplets levitating on heated ratchet surfaces in the Leidenfrost (film boiling) regime is significantly enhanced as the ratchet period decreases down to micro- and sub-micrometers. At the temperature range slightly above the threshold temperature of droplet motion, sub-micron ratchets yield water droplet velocities reaching ~40 cm/s, a speed that has never been achieved with any chemical and topological gradient surfaces. This dramatic increase in the droplet velocity is attributed to an enhanced heat transfer through the local contacts between ratchet peaks and bottom of the droplet. A hydrophobic coating on the ratchet surfaces is found to further increase the droplet velocity and decrease the threshold temperature of the droplet motion. The results suggest that miniaturized ratchet surfaces can potentially be used in diverse applications requiring control over fluid transport and heat transfer such as two phase cooling systems for microprocessors and fuel injection for combustion technology and that for those applications the design of ratchet dimensions and surface chemistry are critically important.     

Numerical simulation of the coupling problems of a solid sphere impacting on a liquid free surface

This paper presents a model, using a phase-field method, that is able to simulate the motion of a solid sphere impacting on a liquid surface, including the effects of capillary and hydrodynamic forces. The basic phenomena that were the subject of our research effort are the small scale mechanism such as the wetting property of the solid surface which control the large scale phenomena of the interaction. The coupled problem during the impact will be formulated by the inclusion of the surface energies of the solid surface in the formulation, which gives a reliable prediction of the motion of solid objects in/on/out of a liquid surface and the hydrodynamic behaviours at small scales when the inertia of fluid is less important than its surface tension. Numerical results at different surface wettabilities and impact conditions will be presented and compared with the experiments of Duez el al. [C. Duez, C. Ybert, C. Clanet, L. Bocquet, Nat. Phys. 3 (2007) 180–183] and Lee and Kim [D. Lee, H. Kim, Langmuir 24 (1) (2008) 142].     

Fluid dynamic behavior of dispensing small droplets through a thin liquid film

This paper presents a technology for dispensing droplets through thin liquid layers. The system consists of a free liquid film, which is suspended in a frame and positioned in front of a piezoelectric printhead. A droplet, generated by the printhead, merges with the film, but due to its momentum, passes through and forms a droplet that separates on the other side and continues its flight. The technology allows the dispensing, mixing and ejecting of picolitre liquid samples in a single step. This paper overviews the concept, potential applications, experiments, results and a numerical model. The experimental work includes studying the flight of ink droplets, which ejected from an inkjet print head, fly through a free ink film, suspended in a frame and positioned in front of the printhead. We experimentally observed that the minimum velocity required for the 80 pl droplets to fly through the 75 ± 24 μm thick ink film was of 6.6 m s^?1. We also present a numerical simulation of the passage of liquid droplets through a liquid film. The numerical results for different initial speeds of droplets and their shapes are taken into account. We observed that during the droplet–film interaction, the surface energy is partially converted to kinetic energy, and this, together with the impact time, helps the droplets penetrate the film. The model includes the Navier–Stokes equations with continuum-surface-tension force derived from the phase-field/Cahn–Hilliard equation. This system allows us to simulate the motion of a free surface in the presence of surface tension during merging, mixing and ejection of droplets. The influence of dispensing conditions was studied and it was found that the residual velocity of droplets after their passage through the thin liquid film well matches the measured velocity from the experiment.     

Dry-spot nucleation in thin liquid films on chemically patterned surfaces

We systematically study the influence of chemical patterning on the instability of thin liquid films induced by chemical heterogeneities on a flat, horizontal, and partially wetting substrate. We consider common geometric shapes like wedges, circles, and stripes and determine the time required for nucleation of a dry-spot as a function of film thickness, contact angle, pattern dimensions, and geometry. Moreover, we characterized the resulting liquid distribution and identified conditions that avoid the formation of residual droplets on the less wettable regions, which is usually undesirable in technological applications.     

Molecular dynamics simulations of diffusion of submonolayer polar liquid lubricant films on solid surfaces

A fundamental understanding of the diffusion phenomena of submonolayer polar liquid films is important for achieving reliable lubrication between moving mechanical parts separated by a nanometer-sized gap. To acquire this understanding, we conducted molecular dynamics (MD) simulations of diffusion phenomena of submonolayer polar perfluoropolyether (PFPE) Zdol films on solid surfaces. To improve the accuracy of these simulations, we developed an all-atom model that includes hydrogen-bond potential and refined atomic charges for Zdol molecules and tested it through MD simulations of spreading of step-shaped submonolayer PFPE films. Our MD simulations reproduced the experimentally observed effects of polar end groups on the diffusion speed and molecular conformation of Zdol. We then conducted MD simulations of self-diffusion of submonolayer Zdol films; these simulations demonstrated that as the thickness of the submonolayer Zdol films decreases, molecular conformation becomes flatter and the self-diffusion coefficient decreases. These changes in molecular conformation partially explain our experimental finding that the spreading of step-shaped submonolayer polar PFPE films slows down with decreasing initial thickness.     

Flow resistance of a liquid droplet confined between two hydrophobic surfaces

Flow resistance of a liquid droplet confined between two hydrophobic surfaces has been investigated experimentally and factors contributing to the flow resistance have been studied. A water droplet has been sheared between two hydrophobic surfaces and shear resistance has been measured. The experimental results show that the shear resistance at low shear velocities is primarily caused by asymmetrical surface tension due to the contact angle hysteresis. A droplet on a rough hydrophobic surface remains almost symmetrical under shear and exhibits extremely low friction. The shear resistance at high shear velocities is affected by viscous force. Furthermore, sliding angles of water droplets on micropatterned hydrophobic surfaces have been measured to clarify the effects of surface topography on flow resistance. Surfaces with many prismatic structures raised out of the plane tend to exhibit low sliding angles.     

Creating, transporting, cutting, and merging liquid droplets by electrowetting-based actuation for digital microfluidic circuits

Reports the completion of four fundamental fluidic operations considered essential to build digital microfluidic circuits, which can be used for lab-on-a-chip or micro total analysis system (/spl mu/TAS): 1) creating, 2) transporting, 3) cutting, and 4) merging liquid droplets, all by electrowetting, i.e., controlling the wetting property of the surface through electric potential. The surface used in this report is, more specifically, an electrode covered with dielectrics, hence, called electrowetting-on-dielectric (EWOD). All the fluidic movement is confined between two plates, which we call parallel-plate channel, rather than through closed channels or on open surfaces. While transporting and merging droplets are easily verified, we discover that there exists a design criterion for a given set of materials beyond which the droplet simply cannot be cut by EWOD mechanism. The condition for successful cutting is theoretically analyzed by examining the channel gap, the droplet size and the degree of contact angle change by electrowetting on dielectric (EWOD). A series of experiments is run and verifies the criterion.     

Molecular dynamics simulation of ultra-thin liquid bridging between surfaces having two-dimensional roughness

By using the molecular dynamics, formation of molecularly thin liquid bridge between solid surfaces having molecular-scale roughness was simulated. Upon comparing the projected profiles of liquid bridges between solid surfaces with different roughness, it was found that the layered molecular arrangement inside the liquid bridge was disturbed by the molecular-scale roughness on the surfaces and the projected profiles were able to be approximated by arcs.     

A novel method for label-free detection of ricin using liquid crystals supported on chemically functionalized surfaces

In this paper, we report a simple and novel liquid-crystal based sensor for ricin detection. The method relies on the use of liquid crystals (LCs) 5CB to amplify and report the presence of ricin captured by an affinity ligand. A merit of this approach is that the ricin can be imaged on the chemically functionalized surfaces and transduced into optical signal by using LCs, the optical signal caused by the orientational transition of the LCs could be easily identified with polarized light microscopy. In addition, the sensor exhibited high sensitivity with strong selectivity, experimental results showed that the minimum detection concentration was as low as 10 μg/mL, demonstrating the feasibility of using these devices to detect and positively identify ricin. Furthermore, the success of LC-based sensor reported here does not use complex instrumentations and does not involve any labeling steps.     

哑铃型弹丸超高速撞击防护板数值仿真

对哑铃型弹丸超高速撞击Whipple防护板进行数值仿真分析,用前板弹孔直径和后板毁伤直径表征防护板的毁伤特性,得到撞击速度、前板厚度、前后板间距等因素变化对防护板毁伤特性的影响规律。在相同条件下的撞击研究结果表明：随着撞击速度的增加,前板弹孔直径和后板毁伤直径均增大;随着前板厚度的增加,前板弹孔直径增大,后板毁伤直径减小;随着板间距的增加,后板毁伤直径增大。     

Optical separation of droplets on a microfluidic platform

This paper describes the optical separation of microdroplets according to their refractive indices. The behavior of the droplets was characterized in terms of the optical force and the hydrodynamic effects present upon illumination of the droplets in a direction normal to the flow direction in a rectangular microfluidic channel. The optical forces acting on the droplets and the resultant droplet trajectories were analyzed and compared with the numerically predicted values. The relationship between the drag force and optical force was examined to understand the system performance properties in the context of screening applications involving the removal of unwanted droplets. Two species of droplets were compared for their photophoretic displacements by varying the illumination intensity. Because the optical forces exerted on the droplets were functions of the refractive indices and sizes of the droplets, a variety of chemical species could be separated simultaneously.     

Driving and sorting of the fluorescent droplets on digital microfluidic platform

In this paper, we present a digital microfluidic droplet sorting platform to achieve automated droplet sorting based on fluorescent detection. We design and fabricate a kind of digital microfluidic chip for manipulating nano-liter-sized liquid droplets, and the chip is integrated with a fluorescence-initiated feedback system for real-time sorting control. The driving and sorting characteristics of fluorescent droplets encapsulating fluorescent-labeled particles are studied on this platform. The droplets dispensed from on-chip reservoir electrode are transported to a fluorescence detection site and sorted according to their fluorescence signals. The fluorescent droplets and non-fluorescent droplets are successfully separated and the number of fluorescent particles inside each droplet is quantified by its fluorescent intensity. We realize droplet sorting at 20 Hz and obtain a linear relationship between the fluorescent particle concentrations and the fluorescence signals. This work is easily adapted for sorting out fluorescent-labeled microparticles, cells and bacteria and thus has the potential of quantifying catalytic or regulatory bio-activities.     

Pneumatic dispensing of nano- to picoliter droplets of liquid metal with the StarJet method for rapid prototyping of metal microstructures

This study presents a new, simple and robust, pneumatically actuated method for the generation of liquid metal micro droplets in the nano- to picoliter range. The so-called StarJet dispenser utilizes a star-shaped nozzle geometry that stabilizes liquid plugs in its center by means of capillary forces. Single droplets of the liquid metal can be pneumatically generated by the interaction of the sheathing gas flow in the outer grooves of the nozzle and the liquid metal. For experimental validation, a print head was build consisting of silicon chips with a star-shaped nozzle geometry and a heated actuator (up to 280°C). The silicon chips are fabricated by Deep Reactive Ion Etching (DRIE). Chip designs with different star-shaped geometries were able to generate droplets with diameters in the range of the corresponding nozzle diameters. The StarJet can be operated in two modes: Either continuous droplet dispensing mode or drop on demand (DoD) mode. The continuous droplet generation mode for a nozzle with 183?μm diameter shows tear-off frequencies between 25 and 120?Hz, while droplet diameters remain constant at 210?μm for each pressure level. Metal columns were printed with a thickness of 0.5–1.0?mm and 30?mm height (aspect ratio >30), to demonstrate the directional stability of droplet ejection and its potential as a suitable tool for direct prototyping of the metal microstructures.     

Identifying and analyzing critical factors impacting on passenger comfort employing a hybrid model

With the increase in air travel, comfort is becoming an important issue with which airlines differentiate themselves in a competitive market. Passengers are paying more attention to their comfort experience during flights. However, passenger comfort is affected by various factors that result from the complex cabin system. Besides, the factors have interrelations between one another. How to identify the key factors and comb through their interrelations to improve passenger comfort efficiently is a critical problem in cabin design. To solve this problem, a hybrid model is proposed to identify the key factors and discuss the interrelation between them based on the questionnaire data. First, factor analysis is conducted to segment complex influencing factors into groups and to extract the most important factors. Decision Making Trial and Evaluation Laboratory (DEMATEL) method and fuzzy set theory are combined in the next analysis, considering both the interrelation between factors and the fuzziness of subjective comfort perception concurrently. It forms a structural model and then visualizes the causal relationships among factors through a causal diagram. The results of an empirical study show that the proposed model is capable of identifying the critical factors that affect passenger comfort effectively and could offer a priority of factors to improve passenger comfort.     

Thermal actuation and confinement of water droplets on paper-based digital microfluidics devices

In this paper, the thermocapillary actuation is implemented to manipulate and confine the fluid droplets in a paper-based digital microfluidics (PB-DMF) device. The main advantage of using the thermocapillary actuation over the traditional electrowetting-on-dielectric actuation in the DMF devices is its ability to work with lower operating DC voltages. The proposed device is fabricated by the low-cost screen printing method using very low-cost materials. In order to overcome the weak controllability of the device over the droplets, a new thermal confinement technique is proposed which simply embedded in the device electrode pattern. A new thermally actuated valve is also designed to work based on thermocapillary actuation for switching on or off the droplets. The fabricated DMF device and the thermal valve are both combined with a microfluidics paper-based analytical device to form a hybrid paper chip in which the droplets are driven by both channel-based and droplet-based devices. The device operation is tested by using a biochemical glucose colorimetric detection assay.     

Infiltration and dimensional scaling of inkjet droplets on thick isotropic porous materials

We study the imbibition of picoliter (pL)-sized inkjet droplets on controlled pore glass membranes (CPG), as a suitable model for isotropic three-dimensional porous materials. We do so using a variety of liquids, i.e., water, formamide and diiodomethane, and measure the evolution of the imbibition process using high-speed digital imaging. Here, experiments were conducted on 2–280 nm CPG membranes with drops with initial volumes ranging from 100 to 600 pL. We derive scaling laws for imbibition through dimensional analysis and advance the argument that the rate of absorption is related to two-dimensionless groups \(V = v(t) \epsilon /v_{\text{tot}} ;\,T = tkp_{\text{c}} /\mu \,\left( { \epsilon /v_{\text{tot}} } \right)^{2/3} ,\) where v(t) is the imbibed volume, as determined from experiments, t is the time, v _tot the total liquid volume, \( \epsilon \) the porosity, μ the liquid viscosity, k the permeability, and p _c the Laplace capillary pressure. We show this scaling to well describe the system at intermediate T values and report that \(V \propto T^{0.8} .\)