3 research outputs found
Critical Casimir effect for colloids close to chemically patterned substrates
Colloids immersed in a critical or near-critical binary liquid mixture and
close to a chemically patterned substrate are subject to normal and lateral
critical Casimir forces of dominating strength. For a single colloid we
calculate these attractive or repulsive forces and the corresponding critical
Casimir potentials within mean-field theory. Within this approach we also
discuss the quality of the Derjaguin approximation and apply it to Monte Carlo
simulation data available for the system under study. We find that the range of
validity of the Derjaguin approximation is rather large and that it fails only
for surface structures which are very small compared to the geometric mean of
the size of the colloid and its distance from the substrate. For certain
chemical structures of the substrate the critical Casimir force acting on the
colloid can change sign as a function of the distance between the particle and
the substrate; this provides a mechanism for stable levitation at a certain
distance which can be strongly tuned by temperature, i.e., with a sensitivity
of more than 200nm/K.Comment: 27 pages, 14 figure
Tunability of Critical Casimir Interactions by Boundary Conditions
We experimentally demonstrate that critical Casimir forces in colloidal
systems can be continuously tuned by the choice of boundary conditions. The
interaction potential of a colloidal particle in a mixture of water and
2,6-lutidine has been measured above a substrate with a gradient in its
preferential adsorption properties for the mixture's components. We find that
the interaction potentials at constant temperature but different positions
relative to the gradient continuously change from attraction to repulsion. This
demonstrates that critical Casimir forces respond not only to minute
temperature changes but also to small changes in the surface properties.Comment: 4 figures;
http://www.iop.org/EJ/article/0295-5075/88/2/26001/epl_88_2_26001.htm
Normal and lateral critical Casimir forces between colloids and patterned substrates
We study the normal and lateral effective critical Casimir forces acting on a
spherical colloid immersed in a critical binary solvent and close to a
chemically structured substrate with alternating adsorption preference. We
calculate the universal scaling function for the corresponding potential and
compare our results with recent experimental data [Soyka F., Zvyagolskaya O.,
Hertlein C., Helden L., and Bechinger C., Phys. Rev. Lett., 101, 208301
(2008)]. The experimental potentials are properly captured by our predictions
only by accounting for geometrical details of the substrate pattern for which,
according to our theory, critical Casimir forces turn out to be a sensitive
probe.Comment: 6 pages, 3 figure