137.195.116.222: /* Shape descriptors */

2024-09-04T09:24:26Z

Shape descriptors

New page

In [[physics]], the '''gyration tensor''' is a [[tensor]] that describes the second [[moment (mathematics)|moment]]s of position of a collection of [[Elementary particle|particle]]s

:<math>
S_{mn} \ \stackrel{\mathrm{def}}{=}\ \frac{1}{N}\sum_{i=1}^{N} r_{m}^{(i)} r_{n}^{(i)}
</math>

where <math>r_{m}^{(i)}</math> is the <math>\mathrm{m^{th}}</math>
[[Cartesian coordinate system|Cartesian coordinate]] of the position [[vector (geometric)|vector]] <math>\mathbf{r}^{(i)}</math> of the
<math>\mathrm{i^{th}}</math> particle. The [[origin (mathematics)|origin]] of the [[coordinate system]] has been chosen such that

:<math>
\sum_{i=1}^{N} \mathbf{r}^{(i)} = 0
</math>

i.e. in the system of the [[center of mass]] <math>r_{CM}</math>. Where

:<math>
r_{CM}=\frac{1}{N}\sum_{i=1}^{N} \mathbf{r}^{(i)}
</math>

Another definition, which is mathematically identical but gives an alternative calculation method, is:

:<math>
S_{mn} \ \stackrel{\mathrm{def}}{=}\ \frac{1}{2N^2}\sum_{i=1}^{N}\sum_{j=1}^{N} (r_{m}^{(i)} - r_{m}^{(j)}) (r_{n}^{(i)} - r_{n}^{(j)})
</math>

Therefore, the x-y component of the gyration tensor for particles in Cartesian coordinates would be:

:<math>
S_{xy} = \frac{1}{2N^2}\sum_{i=1}^{N}\sum_{j=1}^{N} (x_{i} - x_{j}) (y_{i} - y_{j})
</math>

In the [[continuum limit]],

:<math>
S_{mn} \ \stackrel{\mathrm{def}}{=}\ \dfrac{\int d\mathbf{r} \ \rho(\mathbf{r}) \ r_{m} r_{n}}{\int d\mathbf{r} \ \rho(\mathbf{r})}
</math>

where <math>\rho(\mathbf{r})</math> represents the number density of particles at position <math>\mathbf{r}</math>.

Although they have different units, the gyration tensor is related to the
[[moments of inertia|moment of inertia tensor]]. The key difference is that the particle positions are weighted by [[mass]] in the inertia tensor, whereas the gyration tensor depends only on the particle positions; mass plays no role in defining the gyration tensor.

==Diagonalization==

Since the gyration tensor is a symmetric 3x3 [[matrix (mathematics)|matrix]], a [[Cartesian coordinate system]] can be found in which it is diagonal

:<math>
\mathbf{S} = \begin{bmatrix}
\lambda_{x}^{2} & 0 & 0 \\
0 & \lambda_{y}^{2} & 0 \\
0 & 0 & \lambda_{z}^{2}
\end{bmatrix}
</math>

where the axes are chosen such that the diagonal elements are ordered <math>\lambda_{x}^{2} \leq \lambda_{y}^{2} \leq \lambda_{z}^{2}</math>.
These diagonal elements are called the '''principal moments''' of the gyration tensor.

==Shape descriptors==

The principal moments can be combined to give several parameters that describe the distribution of particles. The squared [[radius of gyration]] is the sum of the principal moments:

:<math>
R_{g}^{2} = (\lambda_{x}^{2} + \lambda_{y}^{2} + \lambda_{z}^{2})
</math>

The [[asphericity]] <math>b</math> is defined by

:<math>
b \ \stackrel{\mathrm{def}}{=}\ \lambda_{z}^{2} - \frac{1}{2} \left( \lambda_{x}^{2} + \lambda_{y}^{2} \right) = \frac{3}{2} \lambda_{z}^{2} - \frac{R_{g}^{2}}{2}
</math>

which is always non-negative and zero only when the three principal moments are equal, λx = λy = λz. This zero condition is met when the distribution of particles is spherically symmetric (hence the name ''asphericity'') but also whenever the particle distribution is symmetric with respect to the three coordinate axes, e.g., when the particles are distributed uniformly on a [[cube]], [[tetrahedron]] or other [[Platonic solid]].

Similarly, the [[acylindricity]] <math>c</math> is defined by

:<math>
c \ \stackrel{\mathrm{def}}{=}\ \lambda_{y}^{2} - \lambda_{x}^{2}
</math>

which is always non-negative and zero only when the two principal moments are equal, λx = λy.
This zero condition is met when the distribution of particles is cylindrically symmetric (hence the name, ''acylindricity''), but also whenever the particle distribution is symmetric with respect to the two coordinate axes, e.g., when the particles are distributed uniformly on a [[prism (geometry)|regular prism]].

Finally, the relative shape anisotropy <math>\kappa^{2}</math> is defined

:<math>
\kappa^{2} \ \stackrel{\mathrm{def}}{=}\ \frac{b^{2} + (3/4) c^{2}}{R_{g}^{4}} = \frac{3}{2} \frac{\lambda_{x}^{4}+\lambda_{y}^{4}+\lambda_{z}^{4}}{(\lambda_{x}^{2}+\lambda_{y}^{2}+\lambda_{z}^{2})^{2}} - \frac{1}{2}
</math>

which is bounded between zero and one. <math>\kappa^{2}</math> = 0 only occurs if all points are spherically symmetric, and <math>\kappa^{2}</math> = 1 only occurs if all points lie on a line.

==References==
*{{cite book |last1=Mattice |first1=WL |last2=Suter |first2=UW |year=1994 |title=Conformational Theory of Large Molecules |publisher=Wiley Interscience |isbn=0-471-84338-5}}
*{{cite journal |last1=Theodorou |first1=DN |last2=Suter |first2=UW |year=1985 |title=Shape of Unperturbed Linear Polymers: Polypropylene |journal=Macromolecules |doi=10.1021/ma00148a028 |bibcode=1985MaMol..18.1206T |volume=18 |issue=6 |pages=1206–1214}}

[[Category:Polymer physics]]
[[Category:Tensors]]

Gyration tensor - Revision history

137.195.116.222: /* Shape descriptors */