Why doesn't water actually perfectly wet glass?
According to every high school textbook ever written, water perfectly wets glass. That is, the adhesion between water and glass is so strong that it is energetically favorable for a drop of water on glass to spread out and coat the entire surface. Students are often made to memorize this fact, and many physics questions use it as an assumption.
However, it's perfectly obvious that this doesn't actually happen. If you put a drop of water on glass, it might spread out a little, but it doesn't remotely coat the whole thing. In fact, I've never seen anything like the phenomenon described as "perfect wetting".
Can perfect wetting actually be observed for everyday materials? If not, what are the main additional factors preventing it from happening, as the textbooks say?
water surface-tension
asked 2 hours ago
knzhou
40.8k11114196
add a comment |
According to every high school textbook ever written, water perfectly wets glass. That is, the adhesion between water and glass is so strong that it is energetically favorable for a drop of water on glass to spread out and coat the entire surface. Students are often made to memorize this fact, and many physics questions use it as an assumption.
However, it's perfectly obvious that this doesn't actually happen. If you put a drop of water on glass, it might spread out a little, but it doesn't remotely coat the whole thing. In fact, I've never seen anything like the phenomenon described as "perfect wetting".
Can perfect wetting actually be observed for everyday materials? If not, what are the main additional factors preventing it from happening, as the textbooks say?
water surface-tension
asked 2 hours ago
knzhou
40.8k11114196
I have a feeling "every high school textbook ever written" might reflect a certain bias. I, for one, do not recall being taught that water "perfectly" wets glass, just that it wets it pretty well. That physics questions might assume perfect wetting strikes me as an assumption of much the same kind as neglecting friction. Lastly, doesn't the kind of wetting depend on mechanical properties of the surface (e.g. polished vs. roughened) at least as much as whether it's glass or not?
– ACuriousMind♦
1 hour ago
@ACuriousMind Maybe your education system is better! Indeed, I would imagine that viscosity and imperfections on the surface play a role, but there may be many other factors. I'm interested in what the actually most important factors are in practice.
– knzhou
1 hour ago
perfect wetting kind of occurs, if you drop a oil on a lake, this oil will spread the surface of a lake such that it will cover it in a single molecular layer of oil sheet. youtube.com/watch?v=f2H418M3V6M
– physshyp
41 mins ago
add a comment |
According to every high school textbook ever written, water perfectly wets glass. That is, the adhesion between water and glass is so strong that it is energetically favorable for a drop of water on glass to spread out and coat the entire surface. Students are often made to memorize this fact, and many physics questions use it as an assumption.
However, it's perfectly obvious that this doesn't actually happen. If you put a drop of water on glass, it might spread out a little, but it doesn't remotely coat the whole thing. In fact, I've never seen anything like the phenomenon described as "perfect wetting".
Can perfect wetting actually be observed for everyday materials? If not, what are the main additional factors preventing it from happening, as the textbooks say?
water surface-tension
asked 2 hours ago
knzhou
40.8k11114196
According to every high school textbook ever written, water perfectly wets glass. That is, the adhesion between water and glass is so strong that it is energetically favorable for a drop of water on glass to spread out and coat the entire surface. Students are often made to memorize this fact, and many physics questions use it as an assumption.
However, it's perfectly obvious that this doesn't actually happen. If you put a drop of water on glass, it might spread out a little, but it doesn't remotely coat the whole thing. In fact, I've never seen anything like the phenomenon described as "perfect wetting".
Can perfect wetting actually be observed for everyday materials? If not, what are the main additional factors preventing it from happening, as the textbooks say?
water surface-tension
water surface-tension
asked 2 hours ago
knzhou
40.8k11114196
asked 2 hours ago
knzhou
40.8k11114196
asked 2 hours ago
knzhou
40.8k11114196
asked 2 hours ago
knzhou
40.8k11114196
asked 2 hours ago
knzhou
40.8k11114196
40.8k11114196
I have a feeling "every high school textbook ever written" might reflect a certain bias. I, for one, do not recall being taught that water "perfectly" wets glass, just that it wets it pretty well. That physics questions might assume perfect wetting strikes me as an assumption of much the same kind as neglecting friction. Lastly, doesn't the kind of wetting depend on mechanical properties of the surface (e.g. polished vs. roughened) at least as much as whether it's glass or not?
– ACuriousMind♦
1 hour ago
@ACuriousMind Maybe your education system is better! Indeed, I would imagine that viscosity and imperfections on the surface play a role, but there may be many other factors. I'm interested in what the actually most important factors are in practice.
– knzhou
1 hour ago
perfect wetting kind of occurs, if you drop a oil on a lake, this oil will spread the surface of a lake such that it will cover it in a single molecular layer of oil sheet. youtube.com/watch?v=f2H418M3V6M
– physshyp
41 mins ago
add a comment |
I have a feeling "every high school textbook ever written" might reflect a certain bias. I, for one, do not recall being taught that water "perfectly" wets glass, just that it wets it pretty well. That physics questions might assume perfect wetting strikes me as an assumption of much the same kind as neglecting friction. Lastly, doesn't the kind of wetting depend on mechanical properties of the surface (e.g. polished vs. roughened) at least as much as whether it's glass or not?
– ACuriousMind♦
1 hour ago
@ACuriousMind Maybe your education system is better! Indeed, I would imagine that viscosity and imperfections on the surface play a role, but there may be many other factors. I'm interested in what the actually most important factors are in practice.
– knzhou
1 hour ago
perfect wetting kind of occurs, if you drop a oil on a lake, this oil will spread the surface of a lake such that it will cover it in a single molecular layer of oil sheet. youtube.com/watch?v=f2H418M3V6M
– physshyp
41 mins ago
I have a feeling "every high school textbook ever written" might reflect a certain bias. I, for one, do not recall being taught that water "perfectly" wets glass, just that it wets it pretty well. That physics questions might assume perfect wetting strikes me as an assumption of much the same kind as neglecting friction. Lastly, doesn't the kind of wetting depend on mechanical properties of the surface (e.g. polished vs. roughened) at least as much as whether it's glass or not?
– ACuriousMind♦
1 hour ago
I have a feeling "every high school textbook ever written" might reflect a certain bias. I, for one, do not recall being taught that water "perfectly" wets glass, just that it wets it pretty well. That physics questions might assume perfect wetting strikes me as an assumption of much the same kind as neglecting friction. Lastly, doesn't the kind of wetting depend on mechanical properties of the surface (e.g. polished vs. roughened) at least as much as whether it's glass or not?
– ACuriousMind♦
1 hour ago
@ACuriousMind Maybe your education system is better! Indeed, I would imagine that viscosity and imperfections on the surface play a role, but there may be many other factors. I'm interested in what the actually most important factors are in practice.
– knzhou
1 hour ago
@ACuriousMind Maybe your education system is better! Indeed, I would imagine that viscosity and imperfections on the surface play a role, but there may be many other factors. I'm interested in what the actually most important factors are in practice.
– knzhou
1 hour ago
perfect wetting kind of occurs, if you drop a oil on a lake, this oil will spread the surface of a lake such that it will cover it in a single molecular layer of oil sheet. youtube.com/watch?v=f2H418M3V6M
– physshyp
41 mins ago
perfect wetting kind of occurs, if you drop a oil on a lake, this oil will spread the surface of a lake such that it will cover it in a single molecular layer of oil sheet. youtube.com/watch?v=f2H418M3V6M
– physshyp
41 mins ago
add a comment |
1 Answer
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In everyday life glass surfaces are always covered by a layer of, well, crud. Glass surfaces are exceedingly high energy surfaces due to the high density of polar hydroxyl groups and they attract pretty much anything. This means that outside of a colloid science laboratory you will never encounter a clean glass surface.
I spent many years carrying out experiments involving interactions with glass surfaces, and to get the surface clean we had to clean it with chromic acid. A quick Google found instructions for doing this here, but if you ever feel tempted to try this at home do note the comment in that article:
The dichromate should be handled with extreme care because it is a powerful corrosive and carcinogen.
If you survive the cleaning process then you will find a water drop placed on the glass does have an effectively zero contact angle and the drop will spread out almost completely.
But it's only under these extreme conditions that you will see this. Just leaving the glass exposed to the air for a few hours is enough to coat it with a monolayer of whatever organic detritus if floating around (which if humans are present is quite a lot :-). Once this happens you aren't measuring the contact angle on glass, you are measuring it on whatever organic film is coating the glass.
answered 1 hour ago
John Rennie
270k42528780
add a comment |
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1 Answer
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1 Answer
1
active
oldest
votes
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oldest
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oldest
votes
In everyday life glass surfaces are always covered by a layer of, well, crud. Glass surfaces are exceedingly high energy surfaces due to the high density of polar hydroxyl groups and they attract pretty much anything. This means that outside of a colloid science laboratory you will never encounter a clean glass surface.
I spent many years carrying out experiments involving interactions with glass surfaces, and to get the surface clean we had to clean it with chromic acid. A quick Google found instructions for doing this here, but if you ever feel tempted to try this at home do note the comment in that article:
The dichromate should be handled with extreme care because it is a powerful corrosive and carcinogen.
If you survive the cleaning process then you will find a water drop placed on the glass does have an effectively zero contact angle and the drop will spread out almost completely.
But it's only under these extreme conditions that you will see this. Just leaving the glass exposed to the air for a few hours is enough to coat it with a monolayer of whatever organic detritus if floating around (which if humans are present is quite a lot :-). Once this happens you aren't measuring the contact angle on glass, you are measuring it on whatever organic film is coating the glass.
answered 1 hour ago
John Rennie
270k42528780
add a comment |
In everyday life glass surfaces are always covered by a layer of, well, crud. Glass surfaces are exceedingly high energy surfaces due to the high density of polar hydroxyl groups and they attract pretty much anything. This means that outside of a colloid science laboratory you will never encounter a clean glass surface.
I spent many years carrying out experiments involving interactions with glass surfaces, and to get the surface clean we had to clean it with chromic acid. A quick Google found instructions for doing this here, but if you ever feel tempted to try this at home do note the comment in that article:
The dichromate should be handled with extreme care because it is a powerful corrosive and carcinogen.
If you survive the cleaning process then you will find a water drop placed on the glass does have an effectively zero contact angle and the drop will spread out almost completely.
But it's only under these extreme conditions that you will see this. Just leaving the glass exposed to the air for a few hours is enough to coat it with a monolayer of whatever organic detritus if floating around (which if humans are present is quite a lot :-). Once this happens you aren't measuring the contact angle on glass, you are measuring it on whatever organic film is coating the glass.
answered 1 hour ago
John Rennie
270k42528780
add a comment |
In everyday life glass surfaces are always covered by a layer of, well, crud. Glass surfaces are exceedingly high energy surfaces due to the high density of polar hydroxyl groups and they attract pretty much anything. This means that outside of a colloid science laboratory you will never encounter a clean glass surface.
I spent many years carrying out experiments involving interactions with glass surfaces, and to get the surface clean we had to clean it with chromic acid. A quick Google found instructions for doing this here, but if you ever feel tempted to try this at home do note the comment in that article:
The dichromate should be handled with extreme care because it is a powerful corrosive and carcinogen.
If you survive the cleaning process then you will find a water drop placed on the glass does have an effectively zero contact angle and the drop will spread out almost completely.
But it's only under these extreme conditions that you will see this. Just leaving the glass exposed to the air for a few hours is enough to coat it with a monolayer of whatever organic detritus if floating around (which if humans are present is quite a lot :-). Once this happens you aren't measuring the contact angle on glass, you are measuring it on whatever organic film is coating the glass.
answered 1 hour ago
John Rennie
270k42528780
In everyday life glass surfaces are always covered by a layer of, well, crud. Glass surfaces are exceedingly high energy surfaces due to the high density of polar hydroxyl groups and they attract pretty much anything. This means that outside of a colloid science laboratory you will never encounter a clean glass surface.
I spent many years carrying out experiments involving interactions with glass surfaces, and to get the surface clean we had to clean it with chromic acid. A quick Google found instructions for doing this here, but if you ever feel tempted to try this at home do note the comment in that article:
The dichromate should be handled with extreme care because it is a powerful corrosive and carcinogen.
If you survive the cleaning process then you will find a water drop placed on the glass does have an effectively zero contact angle and the drop will spread out almost completely.
But it's only under these extreme conditions that you will see this. Just leaving the glass exposed to the air for a few hours is enough to coat it with a monolayer of whatever organic detritus if floating around (which if humans are present is quite a lot :-). Once this happens you aren't measuring the contact angle on glass, you are measuring it on whatever organic film is coating the glass.
answered 1 hour ago
John Rennie
270k42528780
answered 1 hour ago
John Rennie
270k42528780
answered 1 hour ago
John Rennie
270k42528780
answered 1 hour ago
John Rennie
270k42528780
270k42528780
add a comment |
add a comment |
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I have a feeling "every high school textbook ever written" might reflect a certain bias. I, for one, do not recall being taught that water "perfectly" wets glass, just that it wets it pretty well. That physics questions might assume perfect wetting strikes me as an assumption of much the same kind as neglecting friction. Lastly, doesn't the kind of wetting depend on mechanical properties of the surface (e.g. polished vs. roughened) at least as much as whether it's glass or not?
– ACuriousMind♦
1 hour ago
@ACuriousMind Maybe your education system is better! Indeed, I would imagine that viscosity and imperfections on the surface play a role, but there may be many other factors. I'm interested in what the actually most important factors are in practice.
– knzhou
1 hour ago
perfect wetting kind of occurs, if you drop a oil on a lake, this oil will spread the surface of a lake such that it will cover it in a single molecular layer of oil sheet. youtube.com/watch?v=f2H418M3V6M
– physshyp
41 mins ago