MICRO SURFACE FUNCTIONALIZATION TO IMPROVE MEMBRANE DISTILLATION

As water scarcity becomes an increasing problem around the world,
water desalination is becoming more important as it can provide an
alternative source of water in regions with access to seawater. Among
other technologies, membrane distillation is a promising approach
because it can be powered by low–grade heat and operates below the
boiling point. In this thesis, I studied membranes typically used for
membrane distillation in an unprecedented detail, using a multi phase
and multi component Lattice Boltzmann method. We investigated the
liquid entry pressure and the interface shape of pressurized liquids in
contact with various rough structures. We have shown that the D3Q27
Shan–Chan like multiphase model is capable of accurately predicting
liquid entry pressures for realistic membrane geometries.
In our investigation, we found that surface functionalization, either
through hydrophilic–hydrophobic zoning or a pillar structure, can
increase the liquid–gas interface area of a liquid film in contact with
such surfaces and increases the magnitude of the evaporation flux by
5% to 6%. For an untreated membrane surface the liquid–gas interface
was limited by the porosity of the membrane.
Moreover we observed an agglomeration of water droplets in mem-
brane regions of high porosity when water vapor condenses inside
the membrane structure.
We also conducted a theoretical analysis of the constant mean cur-
vature problem in cases of rotational symmetry. This analysis allowed
us to determine an analytical expression for the liquid entry pressure
of a hydrophobic pillar–pore structure, as well as the shape of the
liquid–gas interface and the location of its minimum.
Furthering our research, we analyzed the stability and energy barrier
of droplets in the Cassie–Baxter state on periodically pillared surfaces.
Based on this, we further develop a transition criterion and derive an
improved version which allows predicting for which pillar geometries,
equilibrium contact angles and droplet volumes the Cassie–Baxter
state switches from a meta stable to an unstable state. This enabled
a comparison with existing experiments and 3D multi phase Lattice
Boltzmann simulations for different pillar geometries and equilibrium
contact angles, where a good agreement has been found.