findings agreed with Fromm whereby ejected volume The dimensionless Ohnesorge number (Oh) describes the increases with decreasing Oh value. relative importance of viscous to surface tension effects, Several studies [3][4][5] had been conducted on the effects of which is commonly used to characterize jet breakup. In this Oh on the mechanics of droplet formation, e.g. capillary study, the experimental results have proven that single break-off length and time, droplet volume and satellite droplets can be jetted for 0.02 < Oh < 1.5, for viscosity > 50 formation. Xu and Basaran [5] stated that these parameters mPas, using different mixture compositions of glycerol in depend weakly on Oh when Weber number (We) is water (0 -80 wt%) as the ink. This is contrary to previous sufficiently large. Schulkes [6] and Dong et al. [7] had also published literature [1, 2]. However, the findings are in good investigated the effects of Oh on the creation of satellite agreement with Reis and Derby [2] that the velocity of the droplets due to end-pinching. Experiments and simulations of ejected droplet exhibits a maximum (i.e. inverted 'U' curve) Chen and Basaran [8] studied the effect of Ohnesorge number and is a function of pulse width. This velocity curve is on the size of droplets produced. They stated that a uni-polar apparently governed by the Ohnesorge numbers. From a waveform could lead to satellite formation rather than a single practical standpoint, with the large variety of inkjet printing droplet. With their tailored waveform, droplets smaller than inks having different properties, these results can be useful for the diameter of the nozzle orifice can be produced at waveform tailoring to achieve optimal performance for droplet intermediate Oh (-. 0.1 -0.2), but not when Oh is too low ('. formation control. 0.02) or too high (z 1.0). However, results of Gohari and Introduction Chandra [9] disagreed with this finding by producing smaller Inkjet printing has received enormous attention and drops from a 204 ptm diameter nozzle (Oh = 0.21 -1.73), interetas a mufactugtechnology with the applications using a pneumatic DOD generator. Their results also stated otintertst manvenuionglgyfice/home use in printed graphics that single droplets could not be produced when Oh was too outside itS conventional office/home use in printed graphics lw ..O .8 and text. These include fabrication of polymer electronics lo,ie O .8 aompondtext.ese o nincLuD fabricationof polymponernelctrani Bogy and Talke [10] defined the optimum pulse width of micrompons, forganical screrics cm pnemnt anr d the applied rectangular pulse at which the ejected droplet has mppicrtioar s for biolo a screnting. Forthedmose manufact ur the maximum velocity, due to the pressure wave in the cavity s te i t p g m o ms c y being optimally enhanced. They concluded that this optimum is the piezoelectric drop-on-demand method (DOD). The basic I w i principle of operation is via the use of pressure waves induced Pu in an ink-filled conduit by piezoelectric sleeve actuation, to ...

In micro-channels, the flow of viscous liquids e.g. water, is laminar due to the low Reynolds number in miniaturized dimensions. An aqueous solution becomes viscoelastic with a minute amount of polymer additives; its flow behavior can become drastically different and turbulent. However, the molecules are typically invisible. Here we have developed a novel visualization technique to examine the extension and relaxation of polymer molecules at high flow velocities in a viscoelastic turbulent flow. Using high speed videography to observe the fluorescein labeled molecules, we show that viscoelastic turbulence is caused by the sporadic, non-uniform release of energy by the polymer molecules. This developed technique allows the examination of a viscoelastic liquid at the molecular level, and demonstrates the inhomogeneity of viscoelastic liquids as a result of molecular aggregation. It paves the way for a deeper understanding of viscoelastic turbulence, and could provide some insights on the high Weissenberg number problem. In addition, the technique may serve as a useful tool for the investigations of polymer drag reduction.

Numerical methods are presented for the simulation of steady and unsteady micro gas¯ows with moving boundaries found in micro scale¯uidic devices. Both steady and unsteady¯ows are calculated by using an implicit real-time discretization and a dual-time stepping scheme implemented in a high-order upwind ®nite-volume unstructured-grid Navier±Stokes solver. For moving boundary problems, a new dynamic mesh method has been developed which is shown to be robust in handling large mesh deformation. Micro-scale¯ows studied with the methods developed include¯ow in micro channels, unsteady¯ow around a micro cylinder in oscillation and transport processes in micro pumps. The simulation is based on the continuum¯uid model (the compressible Navier±Stokes equations) with slip boundary conditions implemented in the context of unstructured grids as the micro¯ows studied are all in the slip¯ow regime. Results are presented to validate the methods and demonstrate their applications to the analysis and design of micrō uidic devices. The implicit dual-time stepping scheme is found to be robust and ef®cient in dealing with both steady and unsteady micro¯ows. The unstructured-grid solver proves to be very¯exible in dealing with complex geometries such as micro pumps. This is the ®rst known report on the use of ®nite-volume unstructured grid solver for studying micro¯ows based on the slip boundary condition with moving boundaries.

At low Reynolds number (Re < 1), the flow of viscous liquids e.g. water, is laminar. An aqueous solution e.g. water becomes viscoelastic when a small amount of polymer additives (< 1 wt%) is added to it; its flow behavior can become drastically different and turbulent i.e. viscoelastic turbulence. This phenomenon has gained increasing attention because it violates the conventional school of thought i.e. high-Re criteria, for creating chaos and disorder in a fluid dynamics system. As the polymer molecules are invisible, the indirect deduction of molecular behavior via motion analyses of tracer additives has been adopted as the main investigative approach in viscoelastic turbulent flows. Based on this approach, reported works attribute viscoelastic turbulence to the release of elastic energy by the polymer molecules, which had been extended due to strong velocity gradients in the flow field. The release of energy occurs over a range of time scales which is dependent on the characteristic time scales of the molecules and bulk viscoelastic liquid. Although the reported characteristic time scales vary significantly, their effects on the structure of the viscoelastic turbulent flow field have not been investigated. Despite the significant number of investigations based on the conventional approach, the underlying mechanisms in viscoelastic turbulence remains elusive; several outstanding questions on viscoelastic flows remain unresolved e.g. the high Weissenberg number problem, and the drag reduction theory debate. "How do the polymer molecules change the flow field so drastically when they are only present in minute amounts?". This fundamental question has yet to be clearly answered. Although fluorescent-tagged DNA molecules are commercially available, because I would like to express my heartfelt gratitude to my supervisor, Professor Lam Yee Cheong. His advice and guidance has been invaluable throughout my Ph.D. study. In addition, I would like to thank NTU for the funding and support, which made this research work possible. The excellent research environment has greatly facilitated the experimental work required in this study. I would also like to thank all staffs of Materials Laboratory 1, Thermal and Fluids Laboratory and Precision Engineering Laboratory for their support and help in the management and usage of facilities. Most importantly, I wish to thank my parents and friends for their unwavering support. Blank

Viscous liquid flow in micro-channels is typically laminar because of the low Reynolds number constraint. However, by introducing elasticity into the fluids, the flow behavior could change drastically to become turbulent; this elasticity can be realized by dissolving small quantities of polymer molecules into an aqueous solvent. Our recent investigation has directly visualized the extension and relaxation of these polymer molecules in an aqueous solution. This elastic-driven phenomenon is known as ‘elastic turbulence’. Hitherto, existing studies on elastic flow instability are mostly limited to single-stream flows, and a comprehensive statistical analysis of a multi-stream elastic turbulent micro-channel flow is needed to provide additional physical understanding. Here, we investigate the flow field characteristics of elastic turbulence in a 3-stream contraction-expansion micro-channel flow. By applying statistical analyses and flow visualization tools, we show that the flow field bares many similarities to that of inertia-driven turbulence. More interestingly, we observed regions with two different types of power-law dependence in the velocity power spectra at high frequencies. This is a typical characteristic of two-dimensional turbulence and has hitherto not been reported for elastic turbulent micro-channel flows.