Repository: DepositOnce – institutional repository for research data and publications of TU Berlin https://depositonce.tu-berlin.de
TY - THES
AU - Schön, Sebastian
PY - 2020
TI - Oscillatory structural forces
T2 - Technische Universität Berlin
DO - 10.14279/depositonce-10994
UR - http://dx.doi.org/10.14279/depositonce-10994
PB - Technische Universität Berlin
M3 - Doctoral Thesis
CY - Berlin
LA - en
AB - The focus of this thesis is on the investigation of the ordering of nanoparticles in a geometric confinement between two outer surfaces at variable separations and the resulting forces acting on said surfaces, namely the oscillatory structural forces.
At the beginning salt free aqueous suspensions of silica nanoparticles at varying concentrations are investigated via colloidal probe atomic force microscopy (CPAFM). Application of the commonly used fit equation, a decaying harmonic function, shows a persistent under-fitting of the measured data, especially at small separations. This sparked an in-depth analysis of the fit parameters in dependance of the fit starting point. Systematic deviations of the fit parameters are revealed leading to the addition of a second repulsive term to the fit equation. The main fit parameters benefit significantly from this new extended fit equation, their systematic deviations vanish and they become independent both of each other and of the starting point of the fit. Furthermore it is shown that the causal repulsion described by the additional term and thus its significance increases with the nanoparticle concentration.
In the following, the additional repulsion is examined, with regard to its possible physical origin, at different approach speeds and salt concentrations. Although a kinetic contribution is identified, it cannot, due to its low magnitude, account for the observed additional repulsion. The comparison of the calculated Debye lengths with the decay-length of the additional repulsion in various experiments allows to relate the latter to the double-layer forces of the confining surfaces. On the other hand, it is not possible to identify the additional repulsion with the double-layer forces alone, since the former increases with increasing nanoparticle concentration and the associated increase in ion strength. It turns out that the special feature of the present system is a synergistic superposition of the double-layer forces with the oscillatory structural forces, which results in an enhanced ordering of the nanoparticles.
In the last part of the thesis a new system for the investigation of oscillatory structural forces is presented. To this purpose, the physical properties of smart materials in the form of Poly(N-isopropylacrylamide) (PNIPAM) gels are combined with those of nanoparticles. The resulting aqueous PNIPAM nanogel suspensions allow to reversibly switch on and off the oscillatory structural forces by means of an external parameter, in this case the temperature.
KW - oscillatory structural forces
KW - nanoparticles
KW - confinement
KW - colloidal probe
KW - AFM
KW - silica
KW - PNIPAM
KW - oszillierende Strukturkräfte
KW - Nanopartikel
KW - geometrische Einschränkung
KW - kolloidale Sonden
KW - Siliziumdioxid
ER -