I know liquid crystallinity isn't required for shear thinning, but the ensuing increase in stability (which is part of the definition of thixotropy) is a feature of liquid crystallinity, at least in some systems (e.g. gels). Unfortunately there aren't any S-S bonds in cuticle . . .
Julian
On 13 Jul 2014, at 15:36, Michael Ellison <[log in to unmask]> wrote:
> Thixotropic Fluids: Viscosity decreases with increased duration of shear.
>
> Examples: Melts of high molecular weight polymers; greases, margarine and
> shortening; some printing inks, and paints.
>
> {One source: Polymer Mixing and Extrusion Technology By Nicholas P.
> Cheremisinoff}
>
>
> The obvious commonality is a polymeric molecular structure. The more time
> the material is subjected to a shear flow field, the more alignment of the
> polymer chains is possible, resulting in less entanglement and
> interaction, thereby enabling more facile motion, hence lower viscosity.
>
> Liquid crystallinity is not required for this.
>
> Intermolecular bond breaking and subsequent "self-healing² (labile bonds
> such as disulfide bonds for example) could account for the observed
> cuticle behavior.
>
> Michael Ellison
>
>
>
>
> On 7/13/14, 8:23 AM, "Julian Vincent" <[log in to unmask]> wrote:
>
>> Just been reading a paper in which the recovery of stiffness in an
>> overstrained bit of cuticle was not, as I have always thought, ascribed
>> to 'stress-softening' (strain amplification in a rubber with carbon black
>> (or other) filler, caused by the rubber molecules going around the stiff
>> particles) but to 'thixotropy' which could also be typified as
>> recoverable shear-thinning, and is a characteristic of (certain?) liquid
>> crystalline systems. Since many (all?) biological materials are liquid
>> crystals at some time in their life, and many (all?) show
>> 'stress-softening', I wonder if this really says that a (significant?)
>> part of recovery from injury is some sort of recovery of a
>> previous-to-injury liquid crystalline order? Would this recovery of
>> order require less energy than the sort of recovery that we normally
>> think of in technology, which requires external sources of glue, weld,
>> etc?
>>
>> Julian Vincent
>
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