However, the apparent generic adequacy of the viscoelastic measurements to the
Maxwell model should not hide the experimental divergences about the terminal
time τ t . Over large time scale relaxations (Boué et al. 1987; Wang 2006),
unexplainable spectacular flow instabilities (spurt effect, “shark-skin” instability,
shear-induced transitions) (Graham 1999; Pujolle-Robic and Noirez 2001), heterogeneous flows, and slippage at the boundaries (Mansard et al. 2014; Metivier et al. 2012)
continue to feed a debate about the pertinence of the viscoelastic relaxation times to
describe molecular dynamics over decades (Rault 1987; Litinov et al. 2013). The
absence of chain deformation under steady-state shear flow in polymer melts at shear
rates exceeding the inverse of the viscoelastic time (Watanabe et al. 2007; Noirez et al.
2009b – the photograph of Fig. 5 snapshotted at shear rates much larger than the inverse
of the relaxation time shows optically a shear-thinning effect while there is no chain
deformation) questions the entanglement/disentanglement concept.
These observations point out the shortcomings of the macroscopic description in
terms of molecular relaxation times while other works highlight nonlinear effects or
highlight collective behavior and multiple intermolecular interactions.
The recent consideration of the boundary conditions between surface and fluid
interactions in rheology measurements has proven that the viscoelastic response is
not universal but strongly influenced by the wetting or the anchoring conditions
(Mendil 2006; Noirez, Baroni 2010). A totally different response, stronger and
exhibiting a solid-like response, is obtained using total wetting conditions. The
boundary conditions thus govern the efficiency of the transmission of the stress to
the sample and play a major role in the quality of the measurements. It will be
demonstrated that the optimization of the interaction between the surface and the
material extends the dynamic relaxation spectrum and that the Maxwell viscoelastic
response is actually only a part of a wider dynamic response.
Optimizing the Stress Transmission in Viscoelastic Measurements
and Scanning the Submillimeter Scale Response
Up-to-date progresses in rheology instrumentation allow the access to the measurement of the shear modulus with a high precision over six decades of magnitude.
These improvements have considerably widened the frequency window and the
access to very low stress moduli. Therefore, the detection of properties that would
not been considered at the time of the first concepts of the viscoelasticity become
accessible.
These technical improvements can be conjugated to an optimization of the
stress transmission by working on interfacial fluid/solid boundary conditions.
The surface parameter is rarely taken into account. However, the validity of a
dynamic relaxation experiment is entirely depending on the efficiency of the stress
transmission which is ensured by the molecular contact forces between the sample
and the surfaces. Because of symmetry reasons, interfacial energy differs from the
3D properties. The ideal boundary conditions correspond to lower the energy gap
between two different media; i.e., the surface energy has to be as high as possible.
256
L. Noirez
Maxwell model should not hide the experimental divergences about the terminal
time τ t . Over large time scale relaxations (Boué et al. 1987; Wang 2006),
unexplainable spectacular flow instabilities (spurt effect, “shark-skin” instability,
shear-induced transitions) (Graham 1999; Pujolle-Robic and Noirez 2001), heterogeneous flows, and slippage at the boundaries (Mansard et al. 2014; Metivier et al. 2012)
continue to feed a debate about the pertinence of the viscoelastic relaxation times to
describe molecular dynamics over decades (Rault 1987; Litinov et al. 2013). The
absence of chain deformation under steady-state shear flow in polymer melts at shear
rates exceeding the inverse of the viscoelastic time (Watanabe et al. 2007; Noirez et al.
2009b – the photograph of Fig. 5 snapshotted at shear rates much larger than the inverse
of the relaxation time shows optically a shear-thinning effect while there is no chain
deformation) questions the entanglement/disentanglement concept.
These observations point out the shortcomings of the macroscopic description in
terms of molecular relaxation times while other works highlight nonlinear effects or
highlight collective behavior and multiple intermolecular interactions.
The recent consideration of the boundary conditions between surface and fluid
interactions in rheology measurements has proven that the viscoelastic response is
not universal but strongly influenced by the wetting or the anchoring conditions
(Mendil 2006; Noirez, Baroni 2010). A totally different response, stronger and
exhibiting a solid-like response, is obtained using total wetting conditions. The
boundary conditions thus govern the efficiency of the transmission of the stress to
the sample and play a major role in the quality of the measurements. It will be
demonstrated that the optimization of the interaction between the surface and the
material extends the dynamic relaxation spectrum and that the Maxwell viscoelastic
response is actually only a part of a wider dynamic response.
Optimizing the Stress Transmission in Viscoelastic Measurements
and Scanning the Submillimeter Scale Response
Up-to-date progresses in rheology instrumentation allow the access to the measurement of the shear modulus with a high precision over six decades of magnitude.
These improvements have considerably widened the frequency window and the
access to very low stress moduli. Therefore, the detection of properties that would
not been considered at the time of the first concepts of the viscoelasticity become
accessible.
These technical improvements can be conjugated to an optimization of the
stress transmission by working on interfacial fluid/solid boundary conditions.
The surface parameter is rarely taken into account. However, the validity of a
dynamic relaxation experiment is entirely depending on the efficiency of the stress
transmission which is ensured by the molecular contact forces between the sample
and the surfaces. Because of symmetry reasons, interfacial energy differs from the
3D properties. The ideal boundary conditions correspond to lower the energy gap
between two different media; i.e., the surface energy has to be as high as possible.
256
L. Noirez