Thesis (Ph.D)--Mechanical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Salant, Richard F.; Committee Member: Bair, Scott; Committee Member: Cowan, Richard S; Committee Member: Danyluk, Steven; Committee Member: Scott, Waymond R. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Thesis (Ph. D.)--Duke University, 2008. / Includes bibliographical references.

In the recent years, there has been a growing interest in droplet-based (digital) microfluidic systems due to their ability to handle multiple discrete samples in a self-contained configuration compared to continuous flow systems. Various methods for droplet manipulation are currently available based on hydrodynamic, electrostatic, chemical, photonic and thermal interactions. High speed, controlled response and minimal thermal loading with least contamination are required in practical applications, especially in chemistry and biology. Although, thermal actuation of droplets has been recognized as an attractive choice due to a wide range of thermomechanical properties that can be exploited, the previous studies yielded limited success in addressing issues such as droplet evaporation, contamination, pinning, hysteresis and irreversibility that are associated with using solid substrate platforms In order to overcome shortcomings of traditional approaches, a novel thermally-actuated droplet manipulation platform based on using an inert liquid film was proposed and its working mechanisms were studied. Droplets at the air-liquid interface of immiscible liquids usually form partially submerged lens shapes (e.g. water on oil). In the thermally-induced motion of droplets on the free surface of immiscible liquid films, lens-shaped droplets move from warm toward cooler regions. In addition to this structure, we showed that droplets released from critical heights above the target liquid can sustain the impact iv and at the end maintain a spherical ball-shape configuration above the surface, despite undergoing large deformation. It was discovered in this study that such spherical droplets migrate in the opposite direction to lens droplets when subject to a thermal gradient; i.e. direction of increasing temperatures. The existence of this metastable spherical state above the free surface and its transition into more stable lens configuration was investigated using optical diagnostic tools and theoretical analysis. Opposite direction of motion observed for droplets at the free surface of immiscible liquids was explained based on droplet shape at the interface and the dynamics of thin liquid films subject to lateral thermal gradients: mainly 1) deformation of the free surface and 2) development of an outward moving flow (hot to cold) at the free surface due to surface tension gradients caused by thermal gradients. A lens droplet moves due to the free surface flow caused by Marangoni convection which is from hot to cold. On the other hand, the spherical droplet moves towards the maximum depression on the free surface, occurring at the hottest region as a result of the balance between gravity and drag forces from the opposing free surface flow. The proposed theoretical models predict experimental observations of droplet motion due to thermal gradients satisfactorily. Opposite responses of thermally-induced motion of lens and spherical droplets on a thin liquid layer, were characterized experimentally and compared to theory by v studying droplet motion in an exponentially-decaying temperature field maintained across the length of a shallow liquid layer. The effect of droplet size and magnitude of thermal gradient (slope) on drop velocity were investigated. The down-scaling effect is prominent, which shows that the proposed concept of droplet manipulation could be used favorably in miniaturized platforms. Based on the theoretical development and measurements obtained from meso-scale experiments, a silicon-based droplet transportation platform with embedded metal film micro heaters was developed. A thin layer of a chemically-inert and thermally stable liquid was chosen as the carrier liquid. Heaters were interfaced with control electronics and driven through a computer graphical user interface. By creating appropriate spatio-temporal thermal gradient maps, transport of droplets on predetermined pathways was demonstrated with a high level of controllability and speed.

No description available.

No description available.

Two-phase heat transfer techniques such a boiling make use of the high latent heat of fluids to enable dissipation of higher heat fluxes from surfaces compared to conventional single-phase cooling methods. To meet the increasing heat flux dissipation requirements of high-power electronic devices, modifications to the surface properties and roughness are often considered as a means to enhance two-phase heat transfer processes. Although surface roughness of varying length scales has been observed experimentally to enhance boiling heat transfer performance, the physical mechanisms that govern this improvement are not widely accepted. Correlations can be developed to map the behavior of specific surface structure geometries, but a broader investigation of the fundamental forces affecting evaporation at the three-phase contact line, which is critically important to the two-phase heat transfer process, may provide more widely applicable insights. In this thesis, an experimental setup was developed to investigate the effect of small scale surface roughness, with feature sizes below 1 micron, on the liquid film profile of a meniscus formed on a surface. This physical film profile can provide insight into how the surface roughness affects disjoining pressure, an important force that affects the phase change heat transfer process at the contact line. Using an interferometry technique to measure the liquid film profile for a model system of octane on silicon substrate with varying roughness, the change in disjoining pressure in the liquid film was observed. We found that the strength of disjoining pressure in the liquid film increases with increasing surface roughness feature depth.<br>

The impact dynamics and drying of paint films sprayed on steel were experimentally investigated. The rupture of sprayed liquid films was first photographed on different substrates. The critical film thickness, below which a film would break, was observed to increase with increasing advancing liquid-solid contact angle, and was unaffected by liquid viscosity for a given substrate. For viscous paint, it was observed that there is no rupture or splashing from a paint droplet impacting a solid substrate or another paint drop or film. For paint films drying at room temperature, mass fluxes were measured and correlated with a simple analytical model based on transient diffusion, and showed good agreement. The mass flux of sprayed paint films decreased slightly with time, and the volatile concentration decreased appreciably. For sprayed paint films cured with heat, there is a minimum stand-time in order to cure a film without any entrapped bubbles.

The impact dynamics and drying of paint films sprayed on steel were experimentally investigated. The rupture of sprayed liquid films was first photographed on different substrates. The critical film thickness, below which a film would break, was observed to increase with increasing advancing liquid-solid contact angle, and was unaffected by liquid viscosity for a given substrate. For viscous paint, it was observed that there is no rupture or splashing from a paint droplet impacting a solid substrate or another paint drop or film. For paint films drying at room temperature, mass fluxes were measured and correlated with a simple analytical model based on transient diffusion, and showed good agreement. The mass flux of sprayed paint films decreased slightly with time, and the volatile concentration decreased appreciably. For sprayed paint films cured with heat, there is a minimum stand-time in order to cure a film without any entrapped bubbles.

Thesis (Ph. D.)--Mechanical Engineering, Georgia Institute of Technology, 2004. / Abdel-Khalik, Said, Committee Chair. Vita. Includes bibliographical references (leaves 376-384).

Thesis (M.S. in M.E.)--School of Mechanical Engineering, Georgia Institute of Technology, 2004. Directed by Minami Yoda. / Includes bibliographical references (leaves 93-96).