Defense Date


Document Type


Degree Name

Doctor of Philosophy


Mechanical and Nuclear Engineering

First Advisor

Hooman V Tafreshi


Water droplet mobility on a hydrophobic surface cannot be guaranteed even when the droplet exhibits a high contact angle with the surface. Droplet mobility is defined as the required force to move a droplet on the surface. In fact, droplet mobility on a surface, especially a fibrous surface, has remained an unsolved empirical problem. As the earth gravity may not be strong enough to initiate water droplet mobility or penetration into some hydrophobic fibrous coatings (electrospun polystyrene), a novel test method was designed. In the experiment, an aqueous ferrofluid droplets rather than water are used so that the body force on the droplets could be enhanced using a magnet, and droplet detachment or penetration could be induced. Our combined experimental-computational revealed the role of microstructure on droplet mobility and penetration into a fibrous coating. It was found that a coating made of aligned fibers could have a droplet mobility less than a random coating. A fiber level force calculation showed that fibers in the middle of the surface do not play a significant role in keeping the droplet on the surface (negligible relative contribution in resisting droplet detachment). Using the balance of forces acting on the detaching or penetrating droplet, novel easy-to-use expressions are developed to estimate droplet detachment (or penetration) force from (or into) a fibrous surface. This circumvents the need for running CPU-intensive simulations for each and every droplet–coating combinations of interest, and provides a means for designing nonwoven materials with low droplet mobility, e.g., self-cleaning fabrics. In addition, a new technique to study a nonmagnetic droplet (e.g., water) adhesion on hydrophobic surfaces is developed. The nonmagnetic droplet is partially cloaked with a high-surface tension oil-based ferrofluid and a permanent magnet is used to detach the resulting droplet (i.e. compound droplet). At the end, an insightful analysis into the complex nature of this multiphase problem is also provided, and thereby a general-purpose plot that extends the application of our work to other oil–water–solid combinations is presented.


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