Order parameters

An order parameter is some observable physical quantity that is able to distinguish between two distinct phases. The choice of order parameter is not necessarily unique.

Solid-liquid transition

Possible choices:

• Fourier transform of the density
• Shear modulus

Isotropic-nematic transition

The uniaxial order parameter is zero for an isotropic fluid and one for a perfectly aligned system. First one calculates a director vector

$Q_{\alpha \beta }={\frac {1}{N}}\sum _{j=1}^{N}\left({\frac {3}{2}}{\hat {e}}_{j\alpha }{\hat {e}}_{j\beta }-{\frac {1}{2}}\delta _{\alpha \beta }\right),~~~~~\alpha ,\beta =x,y,z,$

where $Q$ is a second rank tensor, ${\hat {e}}_{j}$ is a unit vector along the molecular long axis, and $\delta _{\alpha \beta }$ is the Kronecker delta. Diagonalisation of this tensor gives three eigenvalues $\lambda _{+}$, $\lambda _{0}$ and $\lambda _{-}$, and Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle n} is the eigenvector associated with the largest eigenvalue (Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \lambda_+} ). From this director vector the nematic order parameter is calculated from

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle S_2 =\frac{d \langle \cos^2 \theta \rangle -1}{d-1}}

where d is the dimensionality of the system.

i.e. in three dimensions

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle S_2 = \lambda _{+}= \langle P_2( n \cdot e)\rangle = \langle P_2(\cos\theta )\rangle =\langle \frac{3}{2} \cos^{2} \theta - \frac{1}{2} \rangle }

where $S_{2}$ is known as the uniaxial order parameter. Here Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle P_2} is the second order Legendre polynomial, Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \theta} is the angle between a molecular axes and the director Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle n} , and the angle brackets indicate an ensemble average.

See also

• Landau theory of second-order phase transitions

References

Related reading

• Joseph P. Straley "Ordered phases of a liquid of biaxial particles", Physical Review A 10 pp. 1881 - 1887 (1974)
• Denis Merlet, James W. Emsley, Philippe Lesot and Jacques Courtieu "The relationship between molecular symmetry and second-rank orientational order parameters for molecules in chiral liquid crystalline solvents", Journal of Chemical Physics 111 pp. 6890-6896 (1999)
• Erik E. Santiso and Bernhardt L. Trout "A general set of order parameters for molecular crystals", Journal of Chemical Physics 134 064109 (2011)