Adhesion of Colloidal Particles on Modified Electrodes

Abstract

The adhesion between colloidal silica particles and modified electrodes has been studied by direct force measurements with the colloidal probe technique based on the atomic force microscope (AFM). The combination of potentiostatic control of gold electrodes and chemical modification of their surface with self-assembled monolayers (SAMs) allows for the decoupling of forces due to the electrical double layers and functional groups at the solid/liquid interface. Adhesion on such electrodes can be tuned over a large range using the externally applied potential and the aqueous solution's ionic strength. By utilizing cantilevers with a high force constant, it is possible to separate the various contributions to adhesion in an unambiguous manner. These contributions comprise diffuse-layer overlap, van der Waals forces, solvent exclusion, and electrocapillarity. A quantitative description of the observed adhesion forces is obtained by taking into account the surface roughness of the silica particle. The main component of the adhesion forces originates from the overlap of the electrical double layers, which is tuned by the external potential. By contrast, effects due to electrocapillarity are of only minor importance. Based on our quantitative analysis, a new approach is proposed that allows tuning of the adhesion force as a function of the externally applied potential. We expect this approach to have important applications for the design of microelectromechanical systems (MEMS), the development of electrochemical sensors, and the application of micro- and nanomanipulation.

The Next Generation of Colloidal Probes: A Universal Approach for Soft and Ultra‐Small Particles

“…The resulting normalization of the experimental adhesion forces F exp by the average radius of the colloidal particles R (see inset in Figure b) is shown in the first column of Table 1 . These values are in good quantitative agreement with estimations based on literature values for the interfacial energies γ within the framework of the JKR‐theory . With the values for the interfacial energies reported by Sinniah et al and Warszyński et al (see compilation in the Supporting Information), one obtains F JKR / R .…”

“…With the values for the interfacial energies reported by Sinniah et al and Warszyński et al (see compilation in the Supporting Information), one obtains F JKR / R . At pH 4, the electrostatic interactions are largely suppressed and adhesion is dominated by solvent exclusion . We assume in the following that γ CML/COOH = 0 mN m −1 , as no hydrogen bonding or other chemical bonds should be formed.…”

“…The resulting adhesion forces normalized to the radius F JKR / R are compiled in the second column of Table . However, these values largely overestimate the adhesion forces in assuming a perfectly planar surface, while surface roughness has to be taken into account . Here, the model of Rabinovich et al has been applied .…”

“…Preparation of Samples by µCP and Their Characterization : Chemically heterogenous samples were prepared on uniformly flat gold substrates fabricated by a template‐assisted process as previously described . A laterally defined surface modification was obtained by µCP of alkanethiolates using an elastomeric stamp .…”

“…The colloidal probe technique is utilized in many laboratories worldwide and typical applications range from determining adhesion behavior, to mechanical properties, or colloidal interaction forces . Nevertheless, the essential preparation procedure for colloidal probes has not seen significant changes in the last 30 years: Shortly, a colloidal particle is placed by means of a micromanipulator (or the AFM) at the end of a tipless cantilever, where it is immobilized permanently, either by glue or by a sintering procedure .…”