APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011;
Dallas, Texas
Abstract: X44.00004 : Normal Modes and Density of States of Disordered Colloidal Solids
3:06 PM–3:42 PM
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Author:
The normal modes and the density of states (DOS) of any material
provide a
basis for understanding its thermal and mechanical transport
properties. In
perfect crystals, normal modes take the form of planewaves, but
they can be
complex in disordered systems. I will show our recent experimental
measurements of the normal modes, the DOS and dynamical structure
factor
(DSF) in disordered colloidal solids: disordered colloidal
crystals composed
of thermally sensitive micron-sized hydrogel particles at several
different
particle volume fractions, $\phi $. Particle positions are
tracked over long
times using optical microscopy and particle tracking algorithms
in a single
two dimensional (2D) [111] plane of a 3D face-centered-cubic
single crystal.
The dynamical fluctuations are spatially heterogeneous while the
lattice
itself is highly ordered. At all $\phi $, the DOS exhibits an
excess of low
frequency modes, a so-called boson peak (BP), and the DSF exhibits a
crossover from propagating to non-propagating behavior, a so-called
Ioffe-Regel (IR) crossover, at a common frequency somewhat below
the BP for
both longitudinal and transverse modes. As we tune $\phi $ from
0.64 to 0.56,
the Lindemann parameter grows from \textit{$\sim $}3{\%} to
\textit{$\sim $}8{\%}, however, the shape of the
DOS and DSF remain largely unchanged when rescaled by the Debye
level. This
invariance indicates that the effective degree of disorder and
the structure
of the underlying normal modes remain essentially unchanged even
in the
vicinity of melting. This work was supported by NSF through grants
DMR-0645596 {\&} DMR-0619424, the Sloan Foundation and American
Chemical
Society Petroleum Research Fund.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2011.MAR.X44.4