O***e
发帖数: 89 | 1 In liquid, viscosity has the unit of Pa.s, in glasses, there is also
viscosity with a much larger number, but there is also internal friction,
whose unit is Hz to characterize the life time of phonons, some papers
mentioned they are the same thing and I am confused, what is the connection
between them?
In liquid, my boss mentioned Debye model for relaxation time t changing with
temperature,when wt=1, viscosity is the maximum. Anyone know a textbook
about this?
Thanks very much! | w********h
发帖数: 12367 | 2 how could two quantities with different units be the same thing?
connection
with
【在 O***e 的大作中提到】
: In liquid, viscosity has the unit of Pa.s, in glasses, there is also
: viscosity with a much larger number, but there is also internal friction,
: whose unit is Hz to characterize the life time of phonons, some papers
: mentioned they are the same thing and I am confused, what is the connection
: between them?
: In liquid, my boss mentioned Debye model for relaxation time t changing with
: temperature,when wt=1, viscosity is the maximum. Anyone know a textbook
: about this?
: Thanks very much!
| a********0
发帖数: 34 | 3 Conceptually, viscosity and internal friction is the same thing. But you can
measure it by different means, resulting in different quantities and units.
Viscosity is a measure of stress at a certain strain rate. Glass's viscosity
may be very high but because it can still flow its viscosity is still
finite. You can directly measure viscosity based on its definition.
I don't know much about "phonon lifetime" but I guess you are talking about
some mechanical relaxation test, probably using acoustic range of frequency.
This is called a 'dynamical mechanical spectrum' where a sinusoidally
oscillating deformation of a range of frequency is applied on the materials.
The responsive stress gives us stress amplitudes and phase angle, or in a
different representation, storage and loss modulus/susceptibility. When the
(reciprocal of) applied frequency matched the intrinsic relaxation time the
dissipation is the maximum, resulting maximum loss modulus or viscosity.
Because dissipation (viscosity) is resulted from internal friction.
The wt=1 rule may be not of Debye but Deborah. A dimensionless number called
Deborah number, De, characterizes the relationship between the experiment
timescale and the material relaxation time.
There is no book about these. | w********h
发帖数: 12367 | 4 what you said is very familiar to a polymer scientist like me.
there are indeed several good books on that if one wants to know that better.
Rubinstein and Colby's Polymer Physics and Ferry's Viscoelastic Properties
of Polymers.
I have to point out two things I won't agree:
(1) Maximum G" does not mean maximum viscosity in the spectrum of dynamic
frequency sweep.
(2) Even some items can be correlated, for example, viscosity and relaxation
time,
but they are different quantities with different units. Or, if two
parameters have two different units without possibility to reconcile, they
are different in physics.
can
units.
viscosity
about
frequency.
materials.
【在 a********0 的大作中提到】
: Conceptually, viscosity and internal friction is the same thing. But you can
: measure it by different means, resulting in different quantities and units.
: Viscosity is a measure of stress at a certain strain rate. Glass's viscosity
: may be very high but because it can still flow its viscosity is still
: finite. You can directly measure viscosity based on its definition.
: I don't know much about "phonon lifetime" but I guess you are talking about
: some mechanical relaxation test, probably using acoustic range of frequency.
: This is called a 'dynamical mechanical spectrum' where a sinusoidally
: oscillating deformation of a range of frequency is applied on the materials.
: The responsive stress gives us stress amplitudes and phase angle, or in a
| O***e
发帖数: 89 | 5 Thanks very much for up floorers and your discussions, here is my thoughts:
definition of viscosity as shear stress/velocity gradient is clear in liquid
and glass, but the internal friction is not that cleat to me. In soilds
like glass, some papers mention it as the ratio of acoustic phonon full
width of half maximum(FWHM) and acoustic phonon frequency(f, f is at GHz
level) IF=FWHM/f; in liquid, it should be the same definition and from the
unit, I think FWHM(inverse of the phonon lifetime) is related to the
relaxation time(t). I hope to get a textbook to see this.
My bossed mentioned that in liquid, t is decribed by Debye exponential
relaxation, I also hope to see a book of it-I only found Debye relaxation of
dielectrics, not acoustics; My purpose is to check the temperature effect,
which should also be in books.
Please note, I only discussed t in liquids, it is more completed in solids
like glasses, where strucutral relaxation need integrals of t to describe;
in general situations of liquid, it is simple, only one t is needed. | T********k
发帖数: 94 | 6 There are two types of viscosity: shear and bulk.
shear viscosity applys to fluids and has units Poise or Pa s
bulk viscosity relates to the energy lost in internal frictions. it should
be used in solid case.
thoughts:
liquid
of
【在 O***e 的大作中提到】
: Thanks very much for up floorers and your discussions, here is my thoughts:
: definition of viscosity as shear stress/velocity gradient is clear in liquid
: and glass, but the internal friction is not that cleat to me. In soilds
: like glass, some papers mention it as the ratio of acoustic phonon full
: width of half maximum(FWHM) and acoustic phonon frequency(f, f is at GHz
: level) IF=FWHM/f; in liquid, it should be the same definition and from the
: unit, I think FWHM(inverse of the phonon lifetime) is related to the
: relaxation time(t). I hope to get a textbook to see this.
: My bossed mentioned that in liquid, t is decribed by Debye exponential
: relaxation, I also hope to see a book of it-I only found Debye relaxation of
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