4.2 Interfacial Gaseous Layers
93
Temporary formations of metastable gaseous layers are heterogeneous nucleation
processes. Heterogeneous nucleation of bubbles on a solid surface is always energetically favorable to homogeneous nucleation of bubbles in the bulk of a liquid [23].
Of course, such heterogeneous nucleation or cavitation could also occur on small,
sub-micrometer sized contaminant particles in the liquid which would be hard to
distinguish from homogeneous nucleation experimentally. In liquid water, homogeneous nucleation of bubbles or cavitation has been known to become virtually
irrelevant at normal temperatures that are far below the critical point [23]. Formation
of such thermodynamically metastable interfacial gaseous states is the subject of this
section.
4.2.2 Experimental Observations
There are several established methods of inducing a thermodynamically metastable
interfacial gaseous state and, as might be expected, each of them involves building
up of supersaturation of a gas next to a foreign solid substrate [1, 2, 24–35]. One
such method involves displacement of an organic solvent that has a higher solubility
of a gas of interest than water (like ethanol) with water. This method is called the
“solvent exchange” method.
We consider a setting in which water displaces ethanol in a channel bounded
by hydrophobic surfaces. A typically large advancing contact angle of water and a
small receding contact angle of ethanol on a hydrophobic surface means that the
displacement will initially be incomplete and would leave a thin layer of ethanol
on the hydrophobic surface. Ethanol and water are fully miscible, so the ethanol
initially left on the hydrophobic surface will then quickly dissolve into the water that
has displaced the ethanol. A gas (like nitrogen) is more soluble in ethanol than in
water so the dissolution of ethanol into the surrounding water will induce a local
supersaturation of the gas on the hydrophobic surface. This phenomenon can be
regarded as a special case of spontaneous emulsification we covered in Sect. 1.3 in
that the “solute” here is a gas instead of an oil and the nucleation of the gas is assisted
by the presence of a solid wall (unlike in Sect. 1.3, heterogeneous nucleation takes
place here).
Another method of inducing a thermodynamically metastable interfacial gaseous
state involves displacement of cold water (≈0 °C) with warm water (≈40 °C) that has
a lower solubility of a gas than the cold water [20, 30]. Yet another method involves
generation of a temperature gradient by pre-heating or in situ heating of the substrate
[20, 30]. For an electrically conducting surface, an application of an appropriate bias
voltage can generate interfacial nanobubbles on the substrate by an electrochemical
reaction [20, 27]. In each case, the generated interfacial gaseous states were flat in
their shapes; their lateral dimensions were sub-micrometers and only their height
were in the nanometer scale [20, 34]. The induced gaseous entities can be removed
from the substrate by injection of pre-degassed water or by direct degassing using a
moderate vacuum (≈0.1 atm) [20, 33].
93
Temporary formations of metastable gaseous layers are heterogeneous nucleation
processes. Heterogeneous nucleation of bubbles on a solid surface is always energetically favorable to homogeneous nucleation of bubbles in the bulk of a liquid [23].
Of course, such heterogeneous nucleation or cavitation could also occur on small,
sub-micrometer sized contaminant particles in the liquid which would be hard to
distinguish from homogeneous nucleation experimentally. In liquid water, homogeneous nucleation of bubbles or cavitation has been known to become virtually
irrelevant at normal temperatures that are far below the critical point [23]. Formation
of such thermodynamically metastable interfacial gaseous states is the subject of this
section.
4.2.2 Experimental Observations
There are several established methods of inducing a thermodynamically metastable
interfacial gaseous state and, as might be expected, each of them involves building
up of supersaturation of a gas next to a foreign solid substrate [1, 2, 24–35]. One
such method involves displacement of an organic solvent that has a higher solubility
of a gas of interest than water (like ethanol) with water. This method is called the
“solvent exchange” method.
We consider a setting in which water displaces ethanol in a channel bounded
by hydrophobic surfaces. A typically large advancing contact angle of water and a
small receding contact angle of ethanol on a hydrophobic surface means that the
displacement will initially be incomplete and would leave a thin layer of ethanol
on the hydrophobic surface. Ethanol and water are fully miscible, so the ethanol
initially left on the hydrophobic surface will then quickly dissolve into the water that
has displaced the ethanol. A gas (like nitrogen) is more soluble in ethanol than in
water so the dissolution of ethanol into the surrounding water will induce a local
supersaturation of the gas on the hydrophobic surface. This phenomenon can be
regarded as a special case of spontaneous emulsification we covered in Sect. 1.3 in
that the “solute” here is a gas instead of an oil and the nucleation of the gas is assisted
by the presence of a solid wall (unlike in Sect. 1.3, heterogeneous nucleation takes
place here).
Another method of inducing a thermodynamically metastable interfacial gaseous
state involves displacement of cold water (≈0 °C) with warm water (≈40 °C) that has
a lower solubility of a gas than the cold water [20, 30]. Yet another method involves
generation of a temperature gradient by pre-heating or in situ heating of the substrate
[20, 30]. For an electrically conducting surface, an application of an appropriate bias
voltage can generate interfacial nanobubbles on the substrate by an electrochemical
reaction [20, 27]. In each case, the generated interfacial gaseous states were flat in
their shapes; their lateral dimensions were sub-micrometers and only their height
were in the nanometer scale [20, 34]. The induced gaseous entities can be removed
from the substrate by injection of pre-degassed water or by direct degassing using a
moderate vacuum (≈0.1 atm) [20, 33].