Modification of adhesion between microparticles and engineered silicon surfaces

TL;DR

This work tackles the challenge of adhesion between microparticles and silicon substrates to enable reliable levitation of superconducting particles. It combines physical, chemical, and physio-chemical surface modifications with quantitative detachment-force measurements, showing that hydrophobic surfaces, particularly PTFE membranes, drastically reduce adhesion. A gamma-distribution Bayesian framework is used to characterize detachment forces, revealing substantial reductions in both the mean detachment force and the force at which 50% of particles detach. The findings offer a practical route to lower adhesion in levitation experiments and other applications requiring minimal particle–surface sticking, with open data facilitating replication and broader adoption.

Abstract

A key challenge in performing experiments with microparticles is controlling their adhesion to substrates. For example, levitation of a microparticle initially resting on a surface requires overcoming the surface adhesion forces to deliver the microparticle into a mechanical potential acting as a trap. By engineering the surface of silicon substrates, we aim to decrease the adhesion force between a metallic microparticle and the silicon surface. To this end, we investigate different methods of surface engineering that are based on chemical, physical, or physio-chemical modifications of the surface of silicon. We give quantitative results on the detachment force, finding a correlation between the water contact angle and the mean detachment force, indicating that hydrophobic surfaces are desired for low microparticle adhesion. We develop surface preparations decreasing the mean detachment force by more than a factor of three compared to an untreated silicon surface. Our results will enable reliable levitation of microparticles and are relevant for experiments requiring low adhesion between microparticles and a surface.

Figures (13)

• Figure 1: (a) Schematic of the experiment to determine the adhesion of microparticles on a Si surface. At a vertical distance $z_{\text{eq}}$, attractive ($F_\text{att}$) and repulsive forces ($F_\text{rep}$) are in equilibrium for a particle of radius $R_\text{p}$. An oscillatory motion $z(t)$ is applied to the microparticle by shaking the substrate using a surface transducer imparting an upwards force $F_{\text{vib}}$ onto the particle. (b) SEM image of a 50µm diameter Sn$_{63}$Pb$_{37}$ spherical microparticle.
• Figure 2: Sample preparation and analysis for selected surface modifications. (a,b,c) photographs of samples, (d,e,f) height scans measured by AFM, and (g,h,i) water droplets on samples to determine the water contact angle (WCA).
• Figure 3: Experimental determination of detachment force. Cumulative distribution functions for (a) the reference Si surface, (b) the HSQ treated surface, and (c) the surface with PTFE. The gray lines show the results of individual samples, while the black line shows the average of all samples. The colored shaded area depicts the fitted gamma distribution model. (d,e,f) show the mean and standard deviation of the posterior probability distributions for the detachment force obtained from MCMC sampling, with the cross marking the 50th percentile.
• Figure 4: Correlations between the 50%-detachment force and (a) the water contact angle, (b) the adhesion force measured by the AFM $F_{\text{adh}}^{\text{AFM}}$, and (c) the mean squared roughness $S_{\text{q}}$. We show the data for the untreated sample (green circle), the physical treatments (orange square), the chemical treatments (purple diamond), and the physio-chemical treatment (magenta hexagon). The dashed lines serve as guides to the eye for indicating trends.
• Figure 5: (a) The coil geometry used in the FEM simulation of the magnetic fields. (b) Lifting force at the centre as a function of particle radius, at different vertical planes of the trap. (c) Lifting force for a particle located at the plane of the bottom coil, with the outline of the coil shown.