878 research outputs found
Mechanism for Spontaneous Growth of Nanopillar Arrays in Ultrathin Films Subject to a Thermal Gradient
Several groups have reported spontaneous formation of periodic pillar-like
arrays in molten polymer nanofilms confined within closely spaced substrates
maintained at different temperatures. These formations have been attributed to
a radiation pressure instability caused by acoustic phonons. In this work, we
demonstrate how variations in the thermocapillary stress along the nanofilm
interface can produce significant periodic protrusions in any viscous film no
matter how small the initial transverse thermal gradient. The linear stability
analysis of the interface evolution equation explores an extreme limit of
B\'{e}nard-Marangoni flow peculiar to films of nanoscale dimensions in which
hydrostatic forces are altogether absent and deformation amplitudes are small
in comparison to the pillar spacing. Finite element simulations of the full
nonlinear equation are also used to examine the array pitch and growth rates
beyond the linear regime. Inspection of the Lyapunov free energy as a function
of time confirms that in contrast to typical cellular instabilities in
macroscopically thick films, pillar-like elongations are energetically
preferred in nanofilms. Provided there occurs no dewetting during film
deformation, it is shown that fluid elongations continue to grow until contact
with the cooler substrate is achieved. Identification of the mechanism
responsible for this phenomenon may facilitate fabrication of extended arrays
for nanoscale optical, photonic and biological applications.Comment: 20 pages, 9 figure
Frictional Duality Observed during Nanoparticle Sliding
One of the most fundamental questions in tribology concerns the area
dependence of friction at the nanoscale. Here, experiments are presented where
the frictional resistance of nanoparticles is measured by pushing them with the
tip of an atomic force microscope. We find two coexisting frictional states:
While some particles show finite friction increasing linearly with the
interface areas of up to 310,000nm^2, other particles assume a state of
frictionless sliding. The results further suggest a link between the degree of
surface contamination and the occurrence of this duality.Comment: revised versio
Formation of Nanopillar Arrays in Ultrathin Viscous Films: The Critical Role of Thermocapillary Stresses
Experiments by several groups during the past decade have shown that a molten
polymer nanofilm subject to a large transverse thermal gradient undergoes
spontaneous formation of periodic nanopillar arrays. The prevailing explanation
is that coherent reflections of acoustic phonons within the film cause a
periodic modulation of the radiation pressure which enhances pillar growth. By
exploring a deformational instability of particular relevance to nanofilms, we
demonstrate that thermocapillary forces play a crucial role in the formation
process. Analytic and numerical predictions show good agreement with the pillar
spacings obtained in experiment. Simulations of the interface equation further
determine the rate of pillar growth of importance to technological
applications.Comment: 5 pages, 4 figure
Integration of a microprobe into a CMM
Various microprobes have been developed in the last decade to address the
needs of micrometrology. However, most microprobes are only employed in specialized
measuring machines located in a few research institutes and are not widespread in the
industry. This work aims to extend the capabilities of conventional coordinate
measuring machines (CMMs) towards measuring microgeometries through the low-cost
integration of a tactile microprobe. In order to demonstrate this, a gear measuring
instrument (GMI), which is a commercial CMM not specialized for measurements at the
microscale, has been equipped with a recently developed silicon-membrane-based
microprobe. In the first part of this work, the working principle of the microprobe, its
assembly and its integration into the GMI are described. Two different mounting setups
of the microprobe onto the GMI were evaluated and tested. Measurements on the GMI
were performed solely with the microprobe or by combining the microprobe and the
measurement system already present on the GMI. This combination makes it possible to
use the microprobe advantageously and to exchange it in a comfortable semi-automatic
way. To test these two mounting setups, a new involute scanning artifact (SAFT) with
superimposed waviness was measured
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