PROBING THE NANOMECHANICAL PROPERTIES OF VIRUSES, CELLS AND CELLULAR STRUCTURES - Life at the Nanoscale: Atomic Force Microscopy of Live Cells - page 331

Actin 2.3
Collagen 2
Tubulin 1.9
Living cells 0.001-0.1
The Poisson's ratio ( ν ) expresses the shrinkage perpendicular to the
elongation direction. More explicitly, it is the ratio between the transverse
( ε trans ) and the axial strain ( ε axial ):

ε trans

_____

ν =

ε axial

(16.4)

Since strain is a dimensionless number ( Δ L/L in Fig. 16.1 ) , it follows that

the Poisson's ratio is likewise dimensionless. Its value usually lies between
0.5 (incompressible rubber) and 0.3 (most metals). Cork has a Poisson's ratio
close to 0, which accounts for its use as a plug for wine bottles.

16.1.3 Shear Deformaon

Another important concept in elasticity is shear stress, which measures
the tendency of one part of a solid to slide past a neighbouring portion, as
depicted in Fig. 16.2 .

Figure 16.2. Shear stress in a solid.

Shear stress ( N ) is deined as the ratio between a force and a surface.

F

__

N =

A

(16.5)

where ).

The shear strain [tan( θ )], as depicted in Fig. 16.3 , is described as the
ratio of

N is shear stress (N/m

2

); F , force (N); and A , area (m

2

. For small deformations, tan( θ )

z

θ and is usually measured in x / h

radians.