Access Type

Open Access Dissertation

Date of Award

January 2015

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Mechanical Engineering

First Advisor

Trilochan Singh

Second Advisor

Hassan M. Farhat

Abstract

SPECIALIZED INTER-PARTICLE INTERACTION LBM FOR PATTERNED SUPERHYDROPHOBIC SURFACES

by

AMAL S. YAGUB

ABSTRACT:

Superhydrophobic surface characteristics are important in many industrial applications, ranging from the textile to the military. It was observed that surfaces fabricated with nano/micro roughness can manipulate the droplet contact angle, thus providing an opportunity to control the droplet wetting characteristics. The Shan and Chen (SC) lattice Boltzmann model (LBM) is a good numerical tool, which holds strong potentials to qualify for simulating droplets wettability. This is due to its realistic nature of droplet contact angle (CA) prediction on flat smooth surfaces. But SC-LBM was not able to replicate the CA on rough surfaces because it lacks a real representation of the physics at work under these conditions. By using a correction factor to influence the interfacial tension within the asperities, the physical forces acting on the droplet at its contact lines were mimicked. This approach allowed the model to replicate some experimentally confirmed Wenzel and Cassie wetting cases. Regular roughness structures with different spacing were used to validate the study using the classical Wenzel and Cassie equations. This work highlights the strength and weakness of the SC model and attempts to qualitatively conform it to the fundamental physics, which causes a change in the droplet apparent contact angle, when placed on nano/micro structured surfaces.

In the second part of this work, the model is used also to analyze the sliding of droplets in contact with flat horizontal surfaces. This part identifies the main factors, which influence the multiphase fluids transport in squared channels. Effects of dimensionless radius, Weber number, Reynolds number and static contact angles are evaluated by calculating the power required for moving single droplets in comparison to the power needed for moving the undisturbed flow in the channel. Guidelines for optimizing the design of such flow are presented.

In last part of work, the sliding of droplets on sloped surfaces with and without roughness is numerically investigated. The Shan and Chen (SC) Lattice Boltzmann model (LBM) is used to analyze the effect of pinning on the movement of droplets placed on sloped surfaces. The model is checked for conformance with the Furmidge equation which applies to tilted unstructured surfaces. It is shown that a droplet sliding on a perfectly smooth surface requires very minimal slope angle and that pinning due to the inhomogeneous nature of manufactured smooth surfaces is the key factor in determining the minimal slope angle. The model is also used on sloped rough surfaces to check the effects of roughness on the movement of single droplets. The numerical outcomes are compared with published experimental results for validation and a dimensionless number is suggested for quantifying the degree of pinning needed to control the behavior of sliding droplets on sloped surfaces.

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