Interactions of nanoparticles and surfaces

Abstract

The adhesion forces of micro- to nanoscopic particles on surfaces are the main topic of this dissertation. As a model system, the contact between colloidal particles and smooth silicon and glass substrates are investigated. To achieve information about their adhesion forces, particles are detached from the substrates on the timescale of tens of nanoseconds. For this purpose a laser is focussed on the back side of the sample. There a plasma is generated, which evokes a shock wave that travels through the wafer. The concomitant elongation of the front surface which is of the order of tens of nanometers leads to the detachment of the particles. The particle diameters range from 0.25 µm to 20 µm for silica and from 0.3 µm to 5 µm for polystyrene particles. Since this procedure is a means to clean surfaces, it is called acoustic laser cleaning.For particles larger than 2 ¹m the threshold of the laser intensity is almost independent of the particle diameter. For smaller particles the threshold steeply rises with decreasing diameter. Particles with a diameter of 840 nm adhere to silicon with a force of about 300 nN. All particles lift of with velocities between 10 m/s and 60 m/s. For example 0.9 µm large SiO2 particles stop in air after an estimated flight distance of 27 µm. Thereafter, convection, electric fields and gravitation determine their trajectories. For larger particles gravitation becomes the determinant. It exceeds thevan der Waals forces for spherical glass beads with diameters of about 120 µm, when the silicon substrates are put upside down. Because electrostatic forces influence the adhesion and detachment in all these experiments, the typical charge of particles with a diameter of 1.28 µm was estimated to be 20 elementary charges in a Millikan setup. When a laser hits the front side of a sample, surface acoustic waves (SAW) reveal a characteristic detachment pattern that reflects the symmetry of the surface. Since we did not observe such a pattern on the front side in acoustic laser cleaning, SAW do notseem to play a role there. Instead, for illumination of the back side phonon focussing of bulk acoustic waves that propagate through the silicon wafer can be found. Two theoretical models have been developed to better understand the detachment dynamics. A simple spring model accounts qualitatively for the detachment. A more elaborate model by Martin Schlipf includes the deformation of the particles and derives the force on them numerically. Both models yield particle velocities that are maximally twice the maximum surface velocity. In contrast the experiments result in velocities that are two to ten times higher. This difference has not been explained yet. The second focus of this work are capped colloids in laser tweezers. Half of the surfaceof these particles has been coated with metals. Capped SiO2 particles with diameters of 4.7 µm can be dragged around in water at their transparent half like uncoated particles. Above a critical laser power in the optical tweezers of about 4 mW, they start to rotate spontaneously in front of a surface at a frequency of about 1 Hz. The frequency increases linearly until a threshold of about 7 mW is reached, where the particles jump out of the focus. Light pressure is identified as the driving force.