Question

Answer the following questions
⦁   What is the parameter through which we define wettability?
⦁   What do we mean with partial or complete wetting?
⦁   How do the wetting and nonwetting phases distribute themselves in the pore space?
⦁   Explain why water is naturally imbibed into a paper tissue?
⦁   Capillary pressure may be explained as the pressure required to displace the wetting fluid with a nonwetting fluid in horizontal capillary? Do you agree explain.

Answer 1

Q 1. What is the parameter through which we define wettability?

A. The parameter generally used to define wettability is Contact Angle. Contact angle or Wetting angle is a function of surface tension of the wetting fluid. The contact angle is measured through the aqueous phase and ranges from 0° to 180°, for perfectly water‐wet to perfectly oil‐wet, respectively. Wettability is inversely related to Wetting angle as shown in the figure below:

  

Oil Oil Water Water Mineral Mineral Neutrally Wet Wetting Angle - 90 degrees Oil Wet Wetting Angle > 90 degrees Oil Oil Water Water Mineral Mineral Totally Water Wet Wetting Angle =0 degrees Water Wet Wetting Angle < 90 degrees

Q 2. What do we mean with partial or complete wetting?

A. Partial wetting corresponds to thin liquid layers and liquid droplets with a finite contact angle whereas complete wetting is characterized by vanishing contact angle and thick liquid layers. As one moves the system along the liquid vapor coexistence line, it may undergo a transition from partial to complete wetting and, thus, from a thin to a thick wetting layer.

Q 3. How do the wetting and non wetting phases distribute themselves in the pore space?

A. Principally, because capillary pressure plays a positive role to drive the flow of wetting phase (e.g., water), the flow in an imbibition process (e.g., water displacing oil) should be easier (less pressure cost) than in a drainage (e.g., oil displacing water) process.

However, as shown by many laboratory core-scale test results, an imbibition usually needs a much higher pressure (e.g., 100 kPa) to drive the flow than a drainage (e.g., 20 kPa) under the same injection rate (e.g., 0.5 mL/min).

The high pressure during imbibition should be not due to the viscosity difference between fluids. When injecting either one of the phases alone in the media, the pressure drop is quite low.

Q 4.  Explain why water is naturally imbibed into a paper tissue?

A. Paper towels/tissue are made of ground-up plant material. If you look through a microscope at a torn-up piece of paper (or look up some images on the Internet), you will see a web of tiny plant fibers. Magnifying your paper further will reveal that the fibers are made of long chains of linked sugar molecules, called cellulose. Water is attracted to cellulose and likes to be soaked up and stick to the cellulose in paper. Paper is made of cellulose, which water molecules like to cling to. As a result, paper readily absorbs water. These many tiny holes and gaps between the towel's fibers can hold water due to surface tension, also known as wicking action or capillary action. Capillary action is the small amount of elasticity that naturally occurs between molecules of water, holding them together.

Paper towels are especially absorbent because their cellulose fibers have empty spaces—tiny air bubbles—between them. Water molecules, which like to stay together, follow the one another as they are absorbed by the cellulose and fill the empty spaces. Layering the paper towel creates more spaces for water to fill, which explains why your layered paper towel could hold more water and was more efficient at drying your hands.