Chueh-Yu Wu

Coalescence of magnetic islands

A detailed numerical analysis has been conducted of the instability described by Finn and Kaw in which parallel currents in neighboring islands tend to coalesce into larger units. The existence of this coalescence instability in the ideal magnetohydrodynamic limit is confirmed, but no evidence is found for a threshold in island width. The linear growth rates are found to be rapid compared with those for purely resistive processes, and the linear mode structure has only a weak dependence on resistivity. In the nonlinear regime, saturation of the mode in the ideal case is observed due to flux piling up at the X point, while in the nonideal case the merging process is observed to proceed to completetion.

A Model of Sonoluminescence

A bubble of air, trapped at the centre of a spherical container of water on the surface of which spherical sound waves are maintained by transducers, may emit light, a phenomenon known as sonoluminescence. The surface of the bubble expands and contracts in obedience to the Rayleigh–Lamb equation, which requires knowledge of the gas pressure on the surface of the bubble. In many investigations of bubble pulsations, it is assumed that the air in the bubble moves adiabatically. To understand sonoluminescence, however, it is necessary to allow for the possibility that shocks are generated within the bubble. We couple the Rayleigh–Lamb equation governing the bubble radius to Euler’s equations governing the motion of air in the bubble, and solve the two equations simultaneously. The air is modelled by a van der Waals gas. Results are presented for a number of slightly different conditions of excitation, but in which the response of the system is widely different. If the frequency of the sound is high, the gas in the bubble moves adiabatically and no light is emitted. As the frequency is reduced (for the same ambient bubble radius and driving pressure), the incoming bubble surface acts as a piston that generates an ingoing shock wave that passes through the centre of the bubble, and then, when it strikes the bubble surface, halts and reverses its inward motion; a sequence of such inwardly and outwardly moving shocks occur. The shock waves generate such high temperatures that the air near the centre of the bubble is almost completely ionized, and emits light, which we attribute to bremmstrahlung. The light created by the second inwardly moving shock exceeds that created by the first but, as the frequency of the sound is further reduced, the energy from the first shock rises, and the overall luminosity of the bubble increases. When the sound frequency is further reduced, two shocks are launched successively by the inward moving bubble surface, the second colliding with the first after it has passed through the centre of symmetry but before it can collide with the bubble surface. Even higher temperatures are reached and the luminosity of the bubble continues to increase with decreasing frequency. Details of these solutions are presented, and estimates are made of the luminosity of the bubble in different conditions of excitation. Two other families of solutions are presented. In one, the frequency and ambient bubble radius are fixed and the driving amplitude is varied. In the other, the frequency and driving amplitude are fixed and the radius is varied. The effects of changing only the molecular weight of the trapped gas is also examined.

Fabricating Shaped Microfibers with Inertial Microfluidics

DOI: 10.1002/adma.201400268 Using microfl uidics, a few different techniques have emerged for producing fi bers with different cross-sectional shapes; however, for most of these techniques the range of shapes is limited, such as hollow, semi-circular, and ribbon cross-sectional shapes. [ 10 ] One promising microfl uidic technique that is able to realize more complex fi ber cross-sectional shapes is the hydrodynamic focusing method developed by Ligler and coworkers. [ 11 ] This method uses recessed chevron and striped structures in the channel walls to focus the precursor fi ber solution and the sheathing liquids into a desired shape, which can be predicted with computational fl uid dynamics (CFD). Ligler and coworkers demonstrated that they could use custom software, Tiny-Toolbox, [ 12 ] to design their microfl uidic components and simulate the focusing process. In this Communication, we present the synthesis of shaped polymeric fi bers using a software-enabled inertial microfl uidic technique. Using a method described by Amini et al. [ 13 ] where fl uid streams can be sculpted into desired shapes in a microchannel containing a sequence of pillars, we designate one of the sculpted streams as a template for fi ber fabrication to produce fi bers with different noncircular cross-sectional shapes. We use a computer-aided design (CAD) tool, uFlow, that has a stored library of pre-computed fl uid deformations that are produced by individual pillars in the fl ow channel. [ 14 ] In uFlow, these individual pillar-induced deformations are design elements that can be combined to create a unique sequence of pillar positions along and transverse to the fl ow direction that will result in complex sculpted fl uid fl ows of miscible fl uids. As the CFD simulation step is built into the uFlow software, the tool is quick and simple to use, and accessible to users with all computational skill levels as it circumvents the need for any additional time-consuming simulation steps. The software allows the user to immediately observe the effects of adding pillars, changing lateral pillar position, pillar diameter and fl ow rate ratio on the shape of the fl ow deformation. Consequently, uFlow is a useful predictive tool for the rapid screening and design of microchannels for shaped multi-stream fl ows. While other software, optimized for creeping fl ow, can also be used to perform design for shaped fi bers, uFlow models fl ow at a higher Reynolds number, preferred for fi ber shaping because it is associated with large fl uid deformations following fl ow past a sequence of cylinders. We use the program to design channels containing different sequences of pillars specifi cally for fi ber generation using a common poly(ethylene glycol) diacrylate (PEG-DA) photopolymerization. For the experiments presented herein, three monomer solution streams are fl owed into the channel, and the central stream, which is the only stream containing photoinitiator, becomes the template for the solid fi ber. The two outer streams, which are non-reactive because of the absence Among synthetic fi bers, the circular cross-section is most prevalent; however, it is not uncommon to manufacture fi bers with noncircular cross-sections. We will use the term ‘shaped fi bers’ to describe fi bers with any cross-sectional shape that is not circular. Depending on the application, whether the fi bers are used in fabrics and textiles, [ 1 ] as insulating materials for sound and heat, [ 2 ] for light propagation, [ 3 ] as high surface area membranes and fi ltration materials, [ 4 ] or as engineered substrates for biological applications, [ 5,6 ] it has been observed that the cross-sectional shape affects the properties of the fi ber. For example, the crosssectional shape of fi bers manufactured for textile applications is reported to have an effect on bulkiness since packing density is infl uenced by shape, coeffi cient of friction, which imparts fabric roughness and infl uences overall tactility, fl exural rigidity, which affects the softness or stiffness of fabrics, visual properties such as luster and color, and wicking properties. [ 1 ] Shaped fi bers are also being considered for applications in tissue engineering. For example, it has been shown that the higher surface area afforded by shaped fi bers is useful in fi ber scaffolds for improved cell proliferation and more rapid scaffold degradation when compared to fi bers with circular cross-sections. [ 6 ] In addition, multifaceted or ridged fi ber substrates show improved cell orientation and alignment for applications such as guided cell growth when compared to smooth fi bers. [ 5 ]

Rapid Software-Based Design and Optical Transient Liquid Molding of Microparticles.

Microparticles with complex 3D shape and composition are produced using a novel fabrication method, optical transient liquid molding, in which a 2D light pattern exposes a photopolymer precursor stream shaped along the flow axis by software-aided inertial flow engineering.

Interchange instabilities in a compressible plasma

The interchange instability induced by gravity in a compressible plasma is investigated by numerical methods based on the equations of ideal magnetohydrodynamics. Eulerian computer codes have been developed to integrate in time both the linear and nonlinear forms of these equations. In the linear case the results are compared with a generalization of a normal mode analysis performed by Newcomb. Predictions regarding the growth rate and spatial distribution of these modes and their stabilization by sheared magnetic fields are confirmed in the computer runs. In the nonlinear case inclusion of diffusion and damping terms in the numerical code permits the unstable mode to be followed to a final stable state. The numerical results indicate that for a sheared magnetic field there is a displaced equilibrium state involving a strongly distorted plasma distribution.

Optimization of micropillar sequences for fluid flow sculpting

Inertial fluid flow deformation around pillars in a microchannel is a new method for controlling fluid flow. Sequences of pillars have been shown to produce a rich phase space with a wide variety of flow transformations. Previous work has successfully demonstrated manual design of pillar sequences to achieve desired transformations of the flow cross section, with experimental validation. However, such a method is not ideal for seeking out complex sculpted shapes as the search space quickly becomes too large for efficient manual discovery. We explore fast, automated optimization methods to solve this problem. We formulate the inertial flow physics in microchannels with different micropillar configurations as a set of state transition matrix operations. These state transition matrices are constructed from experimentally validated streamtraces for a fixed channel length per pillar. This facilitates modeling the effect of a sequence of micropillars as nested matrix-matrix products, which have very efficient numerical implementations. With this new forward model, arbitrary micropillar sequences can be rapidly simulated with various inlet configurations, allowing optimization routines quick access to a large search space. We integrate this framework with the genetic algorithm and showcase its applicability by designing micropillar sequences for various useful transformations. We computationally discover micropillar sequences for complex transformations that are substantially shorter than manually designed sequences. We also determine sequences for novel transformations that were difficult to manually design. Finally, we experimentally validate these computational designs by fabricating devices and comparing predictions with the results from confocal microscopy.

THE DECAY OF BUBBLE OSCILLATIONS

We study the initial value problem posed by the small amplitude free oscillations of a bubble in a viscous fluid. The solution consists of a linear combination of discrete normal modes and an integral over a continuous spectrum. The continuous spectrum dominates the solution for large times. As a result, the surface deformation ultimately decays algebraically and not as a modulated damped wave, as has sometimes been suggested.

Shaped 3D microcarriers for adherent cell culture and analysis

Standard tissue culture of adherent cells is known to poorly replicate physiology and often entails suspending cells in solution for analysis and sorting, which modulates protein expression and eliminates intercellular connections. To allow adherent culture and processing in flow, we present 3D-shaped hydrogel cell microcarriers, which are designed with a recessed nook in a first dimension to provide a tunable shear-stress shelter for cell growth, and a dumbbell shape in an orthogonal direction to allow for self-alignment in a confined flow, important for processing in flow and imaging flow cytometry. We designed a method to rapidly design, using the genetic algorithm, and manufacture the microcarriers at scale using a transient liquid molding optofluidic approach. The ability to precisely engineer the microcarriers solves fundamental challenges with shear-stress-induced cell damage during liquid-handling, and is poised to enable adherent cell culture, in-flow analysis, and sorting in a single format.Adherent cells: microcarriers for flow cytometryA new microcarrier for adherent cells is demonstrated which allows for accurate flow cytometry and high-speed imaging without risk of flow-induced damage. Microcarriers are attractive for accelerated cell culture, passaging and analysis, but they must be designed to promote cell growth and analysis without flow-induced cell damage. A team led by Dino Di Carlo at University of California, Los Angeles now report a 3D-shaped microparticle that features a region of extracellular matrix for cell adhesion and culture physically protected from shear flow. Key to the design is the intersection of two 2D patterns, leading to a shape which can align with flow inside the channel during cytometry, and also provides a cut-away region to protect cells during culture. These microcarriers may facilitate high-speed adherent cell screening for applications such as drug discovery.

Publisher’s Note: “Optimization of micropillar sequences for fluid flow sculpting” [Phys. Fluids 28, 012003 (2016)]

Publisher’s Note: “Optimization of micropillar sequences for fluid flow sculpting” [Phys. Fluids 28, 012003 (2016)] Daniel Stoecklein,1 Chueh-Yu Wu,2 Donghyuk Kim,2 Dino Di Carlo,2 and Baskar Ganapathysubramanian1 1Department of Mechanical Engineering, Iowa State University, Ames, Iowa 50011, USA 2Department of Bioengineering, University of California at Los Angeles, Los Angeles, California 90095, USA