Characterization of Dilute and Concentrated Microgel Suspensions

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To gain physical insight on the particle dynamics, we transform the correlation function into the particle mean squared displacement (MSD), $$<\Delta r^2(\tau)>$$, which we plot as a function of the lag time in Figure 2(c). Since the chosen q-vector nearly corresponds to the structure factor peak position, the msqd describes single-particle dynamics. It is characterized by sub-diffusive behavior at short lag times followed by a plateau reminiscent of kinetic arrest. However, for even larger lag times, the msqd increases with time again. The sub-diffusive behavior corresponds to $$<\Delta r^2(\tau)>\sim \tau^{0.62}$$, while the long time behavior is consistent with $$<\Delta r^2(\tau)>\sim \tau$$, which corresponds to diffusion. The plateau in the msqd at intermediate times is typical of systems approaching a kinetically arrested state, such as super cooled liquids on the approach to the glass or a transient colloidal gel [10, 11]. We find a plateau value, $$\delta ^2\sim 0.1 R_h^2$$. The diffusive dynamics at long lag time dynamics results from particles escaping from the surrounding cage formed by their neighbors [12]. Finally, we note that the dynamic measurements were performed for 14 hours with absence of significant aging. This could be interpreted as suggesting that within the length of the experiment, cage escape results from the break-up of the associative structures suggested by the low-q increase of the effective structure factor. However, more experiments are under way to fully explore the behavior of this soft-particle system.

References

[1]    Pusey P.N: Introduction  to Scattering  Experiments, in  Neutrons, X-Rays and Light: Scattering Methods Applied to

Soft Condensed Matter. 2002, Amsterdam: Elsevier p. 3-21.

[2]    A. Guinier and G. Fournet: Small-Angle Scattering of X-Rays. 1955, New York: Wiley Interscience.

[3]    P. N. Pusey: Dynamic Light Scattering, in Neutrons, X-Rays and Light: Scattering Methods Applied to Soft Condensed

Matter. 2002, Amsterdam: Elsevier p. 203-220.

[4]    B.J. Berne and R. Pecora: Dynamic Light Scattering. 1976, New York: Wiley

[5]    H. Senff and W. Richtering: Temperature sensitive microgel suspensions: Colloidal phase behavior and rheology of

soft spheres. Journal of Chemical Physics, 1999. 111(4): p. 1705-1711.

[6]    A. Fernandez-Nieves, F.J. de las Nieves, A. Fernandez-Barbero: Static light Scattering from microgel particles:

Model of variable dielectric permittivity. Journal of Chemical Physics, 2004, 120(1): p. 374-378.

[7]    G. Romeo, A. Fernandez-Nieves, H. W. Hyss, D. Acierno, D. A. Weitz: Temperature-Controlled Transitions

Between Glass, Liquid, and Gel States in Dense p-NIPA Suspensions, Advanced Materials (accepted).

[8]    C. Urban and P. Schurtenberger: Characterization of Turbuid Colloidal Suspensions Using Light Scattering

Techniques Combined with Cross-Correlation Methods. Journal of Colloid and Interface Science, 1998, 207,

p. 150-158

[9]    G. Nägele, O. Kellerbauer, R. Krause and R. Klein: Hydrodynamic effects in polydisperse-charged colloidal

suspensions at short times. Physical Review E, 1993, 47, p. 2562-2574

[10]  A.H. Krall and D.A. Weitz: Internal Dynamics and Elasticity of Fractal Colloidal Gels. Physical Review Letters,

1998. 80(4): p. 778-781.

[11]  B. R. Dasgupta and D.A. Weitz: Microrheology of cross-linked polylamide networks. Physical Review E, 2005, 71,

p. 021504 1-9

[12]  W. van Megen and S.M. Underwood: Glass Transition in Colloidal Spheres: Mode-Coupling Theory Analysis.

Physical Review Letters, 1993, 70(18), p. 2766-2768