Fabrication of advanced particles and particle-based materials assisted by droplet-based microfluidics

Recent advances in the fabrication of complex particles and particle-based materials assisted by droplet-based microfluidics are reviewed. Monodisperse particles with expected internal structures, morphologies, and sizes in the range of nanometers to hundreds of micrometers have received a good deal of attention in recent years. Due to the capability of generating monodisperse emulsions and of executing precise control and operations on the suspended droplets inside the microchannels, droplet-based microfluidic devices have become powerful tools for fabricating complex particles with desired properties. Emulsions and multiple-emulsions generated in the microfluidic devices can be composed of a variety of materials including aqueous solutions, gels, polymers and solutions containing functional nanoparticles. They are ideal microreactors or fine templates for synthesizing advanced particles, such as polymer particles, microcapsules, nanocrystals, and photonic crystal clusters or beads by further chemical or physical operations. These particles are promising materials that may be applicable for many fields, such as photonic materials, drug delivery systems, and bio-analysis. From simple to complex, from spherical to nonspherical, from polymerization and reaction crystallization to self-assembly, this review aims to help readers be aware of the many aspects of this field.     

A turbulent dispersion model for particles or bubbles

A model for dispersed two-phase flow is derived based on a Boltzmann equation. This model is shown to be compatible with the two-fluid model, and includes the source of dispersion. In this model, dispersion is the result of the correlation of the liquid velocity fluctuations with the number density (perhaps more appropriately, with the trajectories of the individual dispersed units). Using this derived force, and a very simple assumption regarding the correlation of the presence of a dispersed unit and the carrier fluid velocity, a form for this force can be derived. This form gives a force which is proportional to the scalar (dot) product of the fluid Reynolds stress tensor with the gradient of bubble number density. For isotropic turbulence, the force is proportional to the gradient of number density. The constant of proportionality depends on the ratio of the dispersed unit relaxation time to the liquid turbulence time scale.     

Thermocapillary mobility of bubbles and electrophoretic motion of particles in a fluid

It is shown that for a collection of equal-sized bubbles in a viscous incompressible fluid, that is subjected to a spatially harmonic temperature field, a solution of the steady Stokes equations exists for which the flow velocity everywhere is irrotational and proportional to the local temperature gradient. Moreover, the bubbles remain at rest. As a consequence, for a suspension of equal-sized bubbles, which on average is spatially uniform, the effective thermocapillary mobility tensor is identical to the effective thermal conductivity tensor, apart from a simple proportionality factor. Analogous results hold for the electrophoretic motion of equal-sized particles in an electrolyte solution, provided the Debye length of the solution is much less than the particle radius.While this article was in print I was informed by Prof. A. Acrivos that the theorem derived above is contained in the paper Y. Wang, R. Mauri, and A. Acrivos, Thermocapillary migration of a bidisperse suspension of bubbles. J. Fluid Mech. 261 (1994) 47–64.     

Modeling radiation characteristics of semitransparent media containing bubbles or particles

Modeling of radiation characteristics of semitransparent media containing particles or bubbles in the independent scattering limit is examined. The existing radiative properties models of a single particle in an absorbing medium using the approaches based on (1) the classical Mie theory neglecting absorption by the matrix, (2) the far field approximation, and (3) the near field approximation are reviewed. Comparison between models and experimental measurements are carried out not only for the radiation characteristics but also for hemispherical transmittance and reflectance of porous fused quartz. Large differences are found among the three models predicting the bubble radiative properties when the matrix is strongly absorbing and/or the bubbles are optically large. However, these disagreements are masked by the matrix absorption during calculation of radiation characteristics of the participating medium. It is shown that all three approaches can be used for radiative transfer calculations in an absorbing matrix containing bubbles.     

Room-temperature metal stamping by microfluidics

We show the possibility to fabricate highly controlled metal micropatterns on a variety of substrates, such as semiconducting or metallic materials, exploiting a combination of spontaneous galvanic displacement reactions with microfluidics. The process is reliable and quite versatile and allows the fabrication of complex patterns of different metals on a number of substrates in few minutes on a conventional laboratory bench.     

Comparison of attachment process of particles to air and oily bubbles in flotation

The attachment between particles and bubbles is a very important basic unit for mineral flotation. In the present study, methylation processing for glass spheres with trimethylchlorosilane was used to prepare particles with different surface hydrophobicity, and a visualization setup was assembled based on high-speed camera technology to investigate the dynamic attachment process of particles onto air/oily bubbles. More importantly, the difference in the attachment process between particles and air/oily bubbles was successfully quantified with the software Image Pro Plus 6.0. It was found that the wetting film ruptured at about 36 ms during the attachment between the hydrophobic glass sphere M2 and air bubbles, while it occurred at about 28 ms during the attachment between M2 and oily bubbles. Moreover, the formed three-phase contact line between hydrophobic particles and oily bubbles was larger than that between hydrophobic particles and air bubbles, suggesting a more stable attachment of the former. In contrast, no successful attachment of the hydrophilic glass sphere M0 onto air or oily bubbles was observed. These findings strongly demonstrated that oily bubbles had both strong collecting power and excellent selectivity for hydrophobic particles in flotation. This study improved the understanding of the difference between oily-bubble and conventional flotations.     

Attenuation of laser-generated shock waves in Plexiglas

A simple analytic model is derived for describing the attenuation of a shock wave in a Plexiglas plate. At the same time, experimental measurements are presented with a well-designed optical-fiber sensor based on detection-beam deflection. The amplitude of the shock-wave pressure is measured experimentally and calculated numerically for analytic expressions at different distances from the region of the surface breakdown by the radiation of a single-pulse Nd:YAG laser. Good agreement between the experimental and the calculated values of the shock-wave pressure is established.     

Understanding of attachment efficiency and induction time between bubbles and pyrite particles in flotation

The fundamental flotation models are useful for understanding flotation mechanisms. However, these models cannot be used for design and optimization of flotation circuits because it is very difficult to determine induction times experimentally during flotation despite using advanced high-speed cameras. Thus, there is a need to develop the model that can be used for practical applications, which is the main objective in this work. The developed models were successful in predicted P_a (attachment efficiency) and t_i (induction time) at various chemical conditions. The values of P_a (attachment efficiency) were strongly affected by particle size, collector concentration and pulp pH while the values of t_i (induction time) were affected by particle size and collector concentration and the values of k (flotation rate constant) were affected by collector concentration only. It means that P_a is much more sensitive than t_i and k to detect the changes in the flotation experimental conditions. The slopes of t_i vs P_a functions were larger at different particle sizes than those at different collector concentrations, demonstrating that the effect of particle size was more dominant than the effect of collector concentration on P_a. This paper demonstrated that understanding of P_a is essential for better analysing flotation mechanisms.     

Optical diagnostics of laser-generated indium plasma

The optical emission from a plasma torch formed at the surface of an indium target under the action of laser radiation with a wavelength of λ=1.06 μm and a power of (3–5)×10⁸ W/cm² was studied. Wide-scan (200–600 nm) emission spectra, relative intensity distributions of the emission In(I) and In(II) lines, and the dynamics of the emission from the plasma of the nucleus of the erosion torch were measured. The total emission spectrum of the plasma represents In(I) and In(II) lines on a continuous emission background. The most intense lines at 451.1, 410.2, 325.8 (6), and 271.1 (4) nm correspond to the upper excited states with energies in the range from 3.02 to 4.84 eV. The principal mechanism of occupation of the excited states of indium atoms and ions in the laser-generated plasma is the two-electron recombination. The emission duration for the In(II) transitions in plasma of the laser torch nucleus does not exceed 80–100 ns, while that for the In(I) transitions amounts to 400–700 ns. The corresponding upper limits of the recombination times for In(III) and In(II) ions are 50 and 500 ns, respectively.     

Air-coupled piezoelectric detection of laser-generated ultrasound

A pulsed laser has been used to generate ultrasonic transients in samples of metal and fiber-reinforced polymer composite material. These have been detected using an air-coupled piezoelectric transducer. It is demonstrated that such a transduction system can be used for longitudinal waves in bulk material, Rayleigh waves at solid surfaces and Lamb waves in thin plates.     

The determination of the morphology of melanocytes by laser-generated periodic surface structures

We generated self-organized, grating-like structures on the surface of polyethylene-terephthalate by ArF excimer laser ablation. Tilting the polymer films with angles larger than 60°, we got parallel grooves. The surface changes caused by the laser illumination were qualified by AFM. The period of the structures was approximately 1 μm, which is commensurable with the characteristic size of melanocytes. We examined the cell shape of normal epidermal melanocytes (M-C) and of melanocytes from the skin of one neurofibromatosis 1 patient (M-NFS) in vitro. The number of dendrites per cell, the length of dendrites and the orientation of several hundred cells were evaluated by an image-processing program. It was proven that the cell morphology and orientation is determined by the topography of the structured polymer substrate. All cells are aligned parallel to the grooves and show the typical bipolar shape. In contrast, on the untreated part of the substrate, the cells are randomly oriented, and the NF cells have more dendrites.     

Introduction: mixing in microfluidics

In this paper we briefly review the main issues associated with mixing at the microscale and introduce the papers comprising the Theme Issue.     

On the calculation of laser-generated ultrasound pulses

The generation of ultrasound pulses with pulsed lasers has received considerable attention in recent years. Promising applications are to nondestructive evaluation and to materials characterization, where it is convenient to have a broad-band source requiring no contact with the sample. Quantitative applications in nondestructive evaluation require theoretical calculations incorporating realistic sample and source properties. This has, until recently, dictated approaches which were very computation-intensive. After the results obtained through these approaches are reviewed, we will describe and illustrate a very recent formulation which enables such calculations to be accomplished more easily. Numerical results will be presented to demonstrate the advantages of this approach, with emphasis on the effects of finite source dimensions, and on the initial waveform. We also show that this approach may be used to investigate surface-wave generation.     

激光激发薄管中超声兰姆波的数值模拟

用有限元方法,对薄管中热弹机制产生的激光超声进行了研究。在考虑材料热物理参数随温度变化的前提下,得到了薄铝管中的温度场和表面的超声导波,描绘了薄铝管中的逆时针向不同接收点处表面导波的时域波形图。由波形图可知,薄圆管中的激光超声导波是典型的L(0,m)模态的超声Lamb波,同时数值结果验证了管道中L(0,2)模式是传播速度最快且频散较小的导波,为激光超声导波在管道无损检测中的应用打下了一定的基础。     

Optical and acoustic detection of laser-generated microbubbles in single cells

Acoustically monitored laser-induced optical breakdown (LIOB) has potential as an important tool to diagnose and treat living cells. Laser-induced intracellular microbubbles are readily detectable using high-frequency ultrasound, and LIOB can be controlled to operate within two distinct regimes. In the nondestructive regime, a single, short-lived bubble can be generated within a cell, without affecting its immediate viability. In the destructive regime, the induced photodisruption quickly can kill a targeted cell. To generate and monitor this range of bioeffects in real time, we have developed a system integrating an ultrafast laser source with optical and acoustic microscopy. Experiments were performed on monolayers of Chinese hamster ovary (CHO) cells. A 793 nm, 100 fs laser pulsed at 3.8 kHz was tightly focused within each cell to produce the photodisruption, and a 50 MHz ultrasonic transducer monitored the resultant bubble via continuous pulse-echo recordings. Photodisruption was also observed using bright field microscopy, and cell viability was assessed following laser exposure with a trypan blue assay. By controlling laser pulse fluence and exposure duration, either nondestructive or destructive LIOB could be produced. The intracellular position of the laser focus was also varied to demonstrate that cell viability was affected by the specific location of material breakdown.     

Influence of the hydrophobic force model on the capture of particles by bubbles: A computational study using Discrete Element Method

Discrete Element Method computer simulations have been carried out to analyse the influence of the hydrophobic force model on the capture of particles by a central bubble. Two hundred particles, with diameters ranging between 24 and 66 μm, were randomly positioned within a maximum distance from the surface of a bubble of 2 mm in diameter. Initial particle velocities were random in direction and value and they followed Gaussian distributions with standard deviations between 0.0 and 1.0 m/s. Three possible models, named A, B and C have been used in the simulations. The models correspond to different published relationships of the hydrophobic force with the distance between particle and bubble surfaces, d. Model A corresponds to a hydrophobic force that decays in the form 1/d; the hydrophobic force given by Model B uses a relationship in the form 1/d²; Model C predicts a force that decays in an exponential way in the form exp(?d/λ). These models have also been compared with a base case in which the hydrophobic force only acted when the particles were in contact with the bubble. Therefore, we could better discern between the influence of the initial particle velocities and the long range component of the hydrophobic force. The differences in the capture efficiency of the particles predicted by the three models were drastic. All particles were captured by the bubble in the cases simulated using Model A for any particle–bubble surface distance smaller than 1 mm. However, only 40% and 60% of the particles were captured even for particles located at distances of less than 50 μm from the bubble surface in the cases simulated using Models B and C (λ = 1 μm), respectively. In fact, the capture of particles seems to be more strongly influenced by how the hydrophobic force decays with interparticle distance in the range of tens of micrometres rather than by the differences between the models in the range of micrometres. Therefore, this work should aid in the future determination of a general hydrophobic force model through an experimental comparison of the kinetics of collision of particles against bubbles in flotation cells with the simulation results.     

Simple synthesis of ZnO 3D-hierarchical nanostructures by microfluidics process

Journal of Materials Science: Materials in Electronics - In this study, hierarchical zinc oxide (ZnO) nanostructures assembled by nanosheets were synthesized via a simple and rapid process without...     

Jets in bubbles

Different types of jet formation in collapsing cavitation and gas bubbles near a rigid boundary are explored by using an advanced boundary-integral technique which incorporates the transition from simply connected to multiply connected bubbles (i.e. toroidal bubbles). Physical interpretation and understanding is facilitated by the calculation of the evolving bubble shape, fluid velocities and pressures, the partitioning of kinetic, potential and gravitational energies, the circulation around the bubble and the Kelvin impulse associated with both the complete bubble and the high-speed liquid jet. In the most vigorous jet formation examples considered it is found that upto 31% of the total energy and upto 53% of the Kelvin impulse is associated with the jet. Practical implications of this study beyond the usual damage mechanisms imply that the level of bubble compression will be signiffcantly lessened leading to lower bubble gas temperatures and thence the corresponding change in the chemical reactivity of its contents or the emission of light. Calculations also suggest interesting phenomena around a standoff distance of 1?2 maximum bubble radii where the circulation around the bubble and the kinetic energy of the jet appear to have maximum values. The practical implications and experimental confirmation of this are yet to be explored.     

Experimental study of laser-generated shear waves using interferometry

The shear displacements generated by short laser pulses have been measured in aluminum semicylindrical samples, both in the thermoelastic and ablation regimes. We measured the waveforms at different angles and obtained the angular distribution pattern of the amplitudes. For the thermoelastic regime good agreement has been found between the measured and the theoretically predicted shear waveforms. In the ablation regime, the absolute values of the amplitudes are comparable to the ones of compressional waves. The shear waveforms are difficult to interpret, particularly in the case where both thermoelastic and ablative effects play a role, because the phases of the shear pulses are opposite to one another in these two regimes. To measure the in-plane displacements, and hence the shear displacement field generated by the pulsed laser, a speckle heterodyne interferometer was used.This article is dedicated to Professor Dr. Paul Höller on the occasion of his 65th birthday.