Browsing by Subject "Fluid dynamics"

Acoustic waves, as a form of mechanical vibration, not only induces the force directly on the object, but also induces the motion of the medium that propagates throughout the system. The study of acoustofluidic mainly focuses on the exploration of the underlying mechanism of the acoustic waves and fluid motion and the methodology of applying this technique to practical applications. Featuring its contactless, versatile, and biocompatible capabilities, the acoustofluidic method makes itself an ideal tool for biosample handling. As the majority of the bio-related samples (e.g., cell, small organism, exosome) possess their native environment within liquids, there is an urgent need to study the acoustic induced fluid motion in order to cooperate with the development of the acoustic tweezing technique. While both the theoretical study and application exploration have been established for the combination of acoustics and microfluidics, the fluid motion on a larger scale is still under-developed. One reason is that, although the acoustofluidic methods hold great potential in various biomedical applications, there is a limited way to form an organized motion in a larger fluid domain, which may lead to the imprecise manipulation of the target. On the other hand, the theoretical study for the microfluidic domain is on the basis of a simplified model with certain assumptions, when applying to the larger fluid area, and significantly influences both the accuracy and computation cost. In this dissertation, we have first developed a series of theoretical and numerical methods in order to provide insights into the acoustofluidic phenomenon in different domain scales. Specifically, we explored the non-linear acoustic dynamics in fluids with the perturbation theory and Reynolds’ stress theory. Then we presented that the vortex streaming can be predicted and designed with our theoretical and numerical study, which can be utilized for various fluid systems and expanded to practical biomedical applications. The boundary-driven streaming and Reynolds’ stress-induced streaming are studied and applied to the digital acoustofluidic droplet handling platform and droplet spinning system, respectively. We demonstrated that within the digital acoustofluidic platform, the droplet can be manipulated on the oil layer in a dynamic and biocompatible manner. Meanwhile, in the droplet spinning system, we can predict and guide the periodic liquid-air interface deformation, as well as the particle motion inside the droplet. We demonstrated that with the theoretical and experimental study, this platform can be utilized for the nanoscale particle (e.g., DNA molecule and exosome) concentration, separation, and transport. Next, based on our study of the acoustically induced fluid motion, we developed an integrated acoustofluidic rotational tweezing platform that can be utilized for zebrafish larvae rapid rotation (~1s/rotation), multi-spectral imaging, and phenotyping. In this study, we have conducted a systematic study including theory development, acoustofluidic device design/fabrication, and flow system implementation. Moreover, we have explored the multidisciplinary expansion combining the acoustofluidic zebrafish phenotyping device with the computer-vision-based 3D model reconstruction and characterization. With this method, we can obtain substantial information from a single zebrafish sample, including the 3D model, volume, surface area, and deformation ratio. Moreover, with the design of the continuous flow system, a flow-cytometry-like system was developed for zebrafish larvae morphological phenotyping. In this study, a standard workflow is established which can directly transfer the groups of samples to a statistical digital readout and provide a new guideline for applying acoustofluidic techniques to biomedical applications. This work represents a complete fusion of acoustofluidic theory, experimental function, and practical application implementation.

Fiber-based coalescers are widely used to accumulate droplets from aerosols and emulsions, where the accumulated droplets are typically removed by gravity or shear. This thesis investigate self-propelled removal of droplets from a hydrophobic fiber, where the surface energy released upon drop coalescence overcomes the drop-fiber adhesion toward the spontaneous departure. The self-propelled removal occurs above a threshold drop-to-fiber radius ratio, disrupting the power-law accumulation on a fibrous coalescer. The departure velocity approaches the capillary-inertial one at large radius ratios.

In experiments, the condensation process including self-propelled removal phenomenon was captured on Teflon-coated fibers with radius of 13~$mu$m and 40~$mu$m. The power law of condensation is obtained by plotting time and averaged radius of droplets condensed on fibers in 2-dimensional (2D) image at that time. Then, to better understand the mechanism resulting such self-propelled removal, droplet-pairs with equal size were manipulated on different radius of fibers (on cones with slowly varied radius). To simplify analysis, two droplets were aligned so that their center connection line was perpendicular to the axis of fiber. By using two high-speed cameras, two views of this removal process were captured simultaneously. Based on information obtained in those video-pairs, the velocity immediate after removal and drop-to-fiber radius ratio were extracted for every case. In plotting those velocity against the radius ratio, data-sets of different size of fibers were collapsed on a single curved, implying critical radius-ratio (at which the removal starts) and asymptotic removal velocity (when radius ratio is very large). In understanding the fluid field in this dynamic process, a 2D phase-field simulation were qualitatively compared with experimental observation, which help to explain why such self-propelled removal can happen on highly curved hydrophobic surface (micro-fiber), but not on flat hydrophobic surface. Understanding to this phenomenon can be useful in chemical industry, ventilation system, and oil separation, in all of which fibrous beds are used to separate aerosol from immiscible flow.

Thin liquid films are often studied by reducing the Navier-Stokes equations

using Reynolds lubrication theory, which leverages a small aspect ratio

to yield simplified governing equations. In this dissertation a plate

coating application, in which polydimethylsiloxane coats a silicon substrate,

is studied using this approach. Thermal Marangoni stress

drives fluid motion against the resistance of gravity, with the parameter

regime being chosen such that these stresses lead to a stable advancing front.

Additional localized thermal Marangoni stress is used to control the thin film;

in particular, coating thickness is modulated through the intensity of such

localized forcing. As thermal effects are central to film dynamics, the dissertation

focuses specifically on the effect that incorporating temperature dependence

into viscosity, surface tension, and density has on film dynamics and control.

Incorporating temperature dependence into viscosity, in particular,

leads to qualitative changes in film dynamics.

A mathematical model is developed in which the temperature dependence

of viscosity and surface tension is carefully taken into account.

This model is then

studied through numerical computation of solutions, qualitative analysis,

and asymptotic analysis. A thorough comparison is made between the

behavior of solutions to the temperature-independent and

temperature-dependent models. It is shown that using

localized thermal Marangoni stress as a control mechanism is feasible

in both models. Among constant steady-state solutions

there is a unique such solution in the temperature-dependent model,

but not in the temperature-independent model, a feature that

better reflects the known dynamics of the physical system.

The interaction of boundary conditions with finite domain size is shown

to generate both periodic and finite-time blow-up solutions, with

qualitative differences in solution behavior between models.

This interaction also accounts for the fact that locally perturbed solutions,

which arise when localized thermal Marangoni forcing is too weak

to effectively control thin film thickness, exist only for a discrete

set of boundary heights.

Modulating the intensity of localized thermal Marangoni forcing is

an effective means of modulating the thickness of a thin film

for a plate coating application; however, such control must be initiated before

the film reaches the full thickness it would reach in the absence of

such localized forcing. This conclusion holds for both the temperature-independent

and temperature-dependent mathematical models; furthermore, incorporating

temperature dependence into viscosity causes qualitative changes in solution

behavior that better align with known features of the underlying physical system.