# The Naked Scientists Forum

### Author Topic: Why does a rough surface encourage water to form droplets?  (Read 1432 times)

#### Roju

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##### Why does a rough surface encourage water to form droplets?
« on: 24/01/2016 22:21:24 »
Lately I have been reading about this topic and one of the things I found out is that if you have a rough, textured, surface, this will make a hydrophobic interaction more hydrophobic and a hydrophilic interaction more hydrophilic.

I have found many equations describing different types of wetting, but they seem to be relating the contact angle, roughness and amount of surface contact. What I dont understand is WHY does making a surface rough make it water form into a sphere (in the case of a hydrophobic coating) but make it spread out more (in the case of a hydrophilic coating)?. How is it different from simply a flat polar or non-polar surface?

Im sure I have read an explaination but totally missed it because I dont understand a lot of the terminology used. Can someone explain in a way I understand?
« Last Edit: 24/01/2016 23:38:06 by chris »

#### Atomic-S

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##### Re: Why does a rough surface encourage water to form droplets?
« Reply #1 on: 25/01/2016 05:42:59 »
The following may be involved:  The more synclastically curved a surface is, the lesser area of it is required to accumulate a given force per unit length of periphery because the lay of the surface deviates from flatness over a shorter distance. Therefore, if a sphere of liquid encounters a hydrophobic surface that is flat, that deforms the surface the same as does placing a basketball on the floor, resulting in tighter curvature in certain areas that repels it from the floor enough to stabilize it there. However, if you take the same basketball and place it on a surface having irregularities that are substantial but smaller than the diameter of the ball, and try to force the ball into the same amount of contact that it had with the flat surface, it will resist more vigorously because the curvature required to make it  conform to the surface is greater.  That suggests that hydrophobic liquids will tend to have less contact with a rough surface than a smooth one at any given pressure, because the surface tension resists deforming so as to match the roughness.

As for hydrophilic surfaces: Because they tend to attract the liquid, the liquid will tend to cover the surface. It is energetically favorable for the liquid to contact the entire surface rather than just the high points of it, which it will try to do. But there is also the issue of the surface tension on the exposed side of the liquid. The liquid will try to keep that surface flat.   The volume of the available region is determined by the thickness of the region of roughness. If the amount of liquid is less than this volume, it will not be able to assume a fully flat surface on the outside, and will develop a negative pressure (relative to the environment) because of the conflict, and if additional liquid  becomes available, it will be pulled into the system as the system tries to flatten its outside surface. Thus, the rough surface will tend to adsorb liquid until its roughness is filled.  On a flat (smooth) hydrophilic surface, of course there is no roughness and therefore no roughness volume, so there is no energy advantage for a thin layer of liquid more than several molecules thick to become thicker.

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#### Roju

• Jr. Member
• Posts: 23
##### Re: Why does a rough surface encourage water to form droplets?
« Reply #2 on: 25/01/2016 16:09:02 »
The following may be involved:  The more synclastically curved a surface is, the lesser area of it is required to accumulate a given force per unit length of periphery because the lay of the surface deviates from flatness over a shorter distance. Therefore, if a sphere of liquid encounters a hydrophobic surface that is flat, that deforms the surface the same as does placing a basketball on the floor, resulting in tighter curvature in certain areas that repels it from the floor enough to stabilize it there. However, if you take the same basketball and place it on a surface having irregularities that are substantial but smaller than the diameter of the ball, and try to force the ball into the same amount of contact that it had with the flat surface, it will resist more vigorously because the curvature required to make it  conform to the surface is greater.  That suggests that hydrophobic liquids will tend to have less contact with a rough surface than a smooth one at any given pressure, because the surface tension resists deforming so as to match the roughness.

As for hydrophilic surfaces: Because they tend to attract the liquid, the liquid will tend to cover the surface. It is energetically favorable for the liquid to contact the entire surface rather than just the high points of it, which it will try to do. But there is also the issue of the surface tension on the exposed side of the liquid. The liquid will try to keep that surface flat.   The volume of the available region is determined by the thickness of the region of roughness. If the amount of liquid is less than this volume, it will not be able to assume a fully flat surface on the outside, and will develop a negative pressure (relative to the environment) because of the conflict, and if additional liquid  becomes available, it will be pulled into the system as the system tries to flatten its outside surface. Thus, the rough surface will tend to adsorb liquid until its roughness is filled.  On a flat (smooth) hydrophilic surface, of course there is no roughness and therefore no roughness volume, so there is no energy advantage for a thin layer of liquid more than several molecules thick to become thicker.

I like the basketball analogy, thinking of the basketball shell as the surface tension holding in the liquid. Quite intuitive :)

#### alancalverd

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##### Re: Why does a rough surface encourage water to form droplets?
« Reply #3 on: 25/01/2016 19:46:06 »
Within a given boundary, a rough surface will have a greater area of exposed material than a smooth surface.

Hydrophilic and hydrophobic behaviors are surface interactions.

More area = more interaction.

#### Roju

• Jr. Member
• Posts: 23
##### Re: Why does a rough surface encourage water to form droplets?
« Reply #4 on: 25/01/2016 21:47:28 »
Within a given boundary, a rough surface will have a greater area of exposed material than a smooth surface.

Hydrophilic and hydrophobic behaviors are surface interactions.

More area = more interaction.
I dont see how that explains the lotus effect, where the droplets barely touch the surface. They kind of float along on the air bubbles inbetween the microstructures.

#### The Naked Scientists Forum

##### Re: Why does a rough surface encourage water to form droplets?
« Reply #4 on: 25/01/2016 21:47:28 »