# Word Net

wetting See wetwet adj

1 covered or soaked with a liquid such as water;
"a wet bathing suit"; "wet sidewalks"; "wet paint"; "wet weather"
[ant: dry]

2 supporting or permitting the legal production
and sale of alcoholic beverages; "a wet candidate running on a wet
platform"; "a wet county" [ant: dry]

3 producing or secreting milk; "a wet nurse"; "a
wet cow"; "lactating cows" [syn: lactating] [ant: dry]

4 consisting of or trading in alcoholic liquor;
"a wet cargo"; "a wet canteen"

5 very drunk [syn: besotted, blind drunk,
blotto, crocked, cockeyed, fuddled, loaded, pie-eyed, pissed, pixilated, plastered, potty, slopped, sloshed, smashed, soaked, soused, sozzled, squiffy, stiff, tiddly, tiddley, tight, tipsy] n : wetness caused by
water; "drops of wet gleamed on the window" [syn: moisture]

### Verb

1 cause to become wet; "Wet your face" [ant:
dry]

2 make one's bed or clothes wet by urinating;
"This eight year old boy still wets his bed" [also: wetting, wetted, wettest, wetter]

wetting

### Noun

1 the act of making something wet

## Noun

- The act of making something wet
- The act of accidental urination on or in something.

### Derived terms

## Adjective

- That makes (something) wet.

### Derived terms

Wetting is the contact between a liquid and a
solid surface, resulting from intermolecular interactions when the
two are brought together. The amount of wetting depends on the
energies (or surface tensions) of the interfaces involved such that
the total energy is minimized. The degree of wetting is described
by the contact
angle, the angle at which the liquid-vapor interface meets the
solid-liquid interface. If the wetting is very favorable, the
contact angle will be low, and the fluid will spread to cover a
larger area of the surface. If the wetting is unfavorable, the
fluid will form a compact droplet on the surface. Regardless of the
amount of wetting, the shape of a drop wetted to a rigid surface is
roughly a truncated
sphere. Various degrees of wetting are depicted in Figure
1.

A contact angle of 90° or greater generally
characterizes a surface as not-wettable, and one less than 90° as
wettable. In the context of water, a wettable surface may also be
termed hydrophilic
and a non-wettable surface hydrophobic.
Superhydrophobic surfaces have contact angles greater than 150°,
showing almost no contact between the liquid drop and the surface.
This is sometimes referred to as the "Lotus
effect". Wetting is also important in the bonding or adherence of two materials.
Wetting and the surface forces that control wetting are also
responsible for other related effects, including so-called capillary
effects.

# Minimization of energy, three phases

Consider the line of contact where three phases
meet, as shown in Figure 2. In equilibrium, the net force per unit
length acting along the boundary line between the three phases must
be zero. The components of net force in the direction along each of
the interfaces are given by:

- \gamma_+\gamma_\cos+\gamma_\cos\ = 0
- \gamma_\cos+\gamma_+\gamma_\cos\ = 0
- \gamma_\cos+\gamma_\cos+\gamma_\ = 0

where \alpha, \beta, and \theta are the angles
shown and \gamma_ is the surface energy between the two indicated
phases. These relations can also be expressed by an analog to a
triangle known as Neumann’s triangle, shown in Figure 3. Neumann’s
triangle is consistent with the geometrical restriction that
\alpha+\beta+\theta=2\pi, and applying the law of sines and law of
cosines to it produce relations that describe how the interfacial
angles depend on the ratios of surface energies.

Because these three surface energies form the
sides of a triangle, they are constrained by the triangle
inequalities, \gamma_ meaning that no one of the surface tensions
can exceed the sum of the other two. If three fluids with surface
energies that do not follow these inequalities are brought into
contact, no equilibrium configuration consistent with Figure 2 will
exist.

## Simplification to planar geometry, Young's relation

If the \beta phase is replaced by a flat rigid
surface, as shown in Figure 4, then \beta=\pi, and the second net
force equation simplifies to the Young equation,

- \gamma_\ =\gamma_+\gamma_\cos

which relates the surface tensions between the
three phases solid, liquid and gas, and which predicts the contact
angle of a liquid droplet on a solid surface from knowledge of the
three surface energies involved. This equation also applies if the
"gas" phase is another liquid, immiscible with the droplet of the
first "liquid" phase.

The Young–Dupre equation dictates that neither
\gamma_ nor \gamma_ can be larger than the sum of the other two
surface energies. The consequence of this restriction is the
prediction of complete wetting when \gamma_ > \gamma_+\gamma_
and zero wetting when \gamma_ > \gamma_+\gamma_. The lack of a
solution to the Young–Dupre equation is an indicator that there is
no equilibrium configuration with a contact angle between 0 and 180
degrees for those situations.

A useful parameter for gauging wetting is the
spreading parameter S,

- S\ = \gamma_-(\gamma_+\gamma_)

When S > 0, the liquid wets the surface
completely (complete wetting). When S < 0, there is partial
wetting.

Combining the spreading parameter definition with
the Young relation, we obtain the Young-Dupre equation:

- S\ = \gamma_(\cos\theta-1)

which only has physical solutions for \theta when
S < 0.

# Dynamic wetting

The above derivations all apply only to the state
in which the interfaces are not moving and the phase boundary line
exists in equilibrium. When a phase boundary is in motion, such as
in the case of a spreading droplet or advancing contact edge,
different mechanics apply. Many aspects of dynamic wetting are not
fully understood, and the subject is an area of great interest to
many scientists.

When a contact line such as the one in figure 4
is displaced, by either expansion or retraction of the droplet,
there is a hysteresis observed in the contact angle. The static
contact angle that results after expansion of a droplet is higher
than that observed after a contraction. It is also often observed
that the contact line does not move smoothly at the microscale.
Rather, it is seen to jump abruptly in increments, by an apparent
stick-slip mechanism. This has often been attributed to
imperfections in the surface causing the contact line to be
momentarily pinned, but this description is not complete.

When a contact line advances, covering more of
the surface with liquid, the contact angle is increased and
generally is related to the velocity of the contact line.A receding
interface likewise has a contact angle that is reduced from the
static contact angle. The limits of contact angle as velocity
approaches zero in the forward and backward directions are not
equal, and the range between them defines a range of contact angles
that are observed as static contact angles in hysteresis
experiments.

If the velocity of a contact line is increased
without bound, the contact angle increases, and as it approaches
180° the gas phase will become entrained in a thin layer between
the liquid and solid. This is a kinetic non-equilibrium effect
which results from the contact line moving at such a high speed
that complete wetting cannot occur.

Dynamic wetting is of great importance in
industrial processes, where surfaces often must be coated uniformly
and quickly with a liquid. Entrainment of air is unacceptable for
the quality of products, but the volume demanded necessitates
coating at as high a speed as possible.

## Molecular theories

Several molecular theories of dynamic wetting
have been proposed. The determination of a theory that describes
dynamic wetting observations is complicated by the apparent
contradiction with established theories of wetting. For example, in
the standard model of viscous flow, there is no slippage of the
surface layer of liquid atoms along the surface, but in the
immediate vicinity of a progressing contact line, it is necessary
to relax this restriction to prevent the prediction of infinite
shear.

# See also

# References

wetting in German: Benetzung

wetting in Dutch: Bevochtigen

wetting in Russian: Смачивание

wetting in Chinese: 浸润