(161d) A Magnetic Colloidal Force Probe to Measure Nanometer Spacing Via Optical Imaging

Surface force versus distance laws between colloidal particles are of fundamental importance in understanding their physical and chemical properties. Compared with conventional measurements such as surface forces apparatus (SFA) and atomic force microscope (AFM), a relatively simple method, referred to as magnetic chaining technique (MCT), has been developed to measure the repulsive inter-particle force-distance relations.

In the MCT, an ensemble of magnetic colloidal particles forms linear chains of particles due to each particle acquiring an induced dipole in an externally applied uniform magnetic field. Inter-particle forces can be calculated with high precision, as their magnetic dipole interactions have a resolution of 0.2pN. The inter-particle distance is measured by observing the back-scattered Bragg diffraction spectra peak wavelength, which has a resolution of 4nm. This resolution is relatively high considering its easy-to-use nature. However, a large volume of magnetic colloidal particles with a homogeneous particle size distribution are needed to achieve a high resolution.

In this talk, we present a novel approach to overcome these limitations. Specifically, we image a chain of micron-sized magnetic particles via an optical microscope under various magnetic field strengths allowing us to probe the interparticle forces between the particles. In our technique, less than a few micro-liters of sample suspension with low particle concentration (50ppm) is required. Additionally, our technique is not as sensitive to size distribution, as long as all the particles in a chain can be optically resolved. Finally, by proper image processing, our technique brings the distance resolution down to angstrom level. We will demonstrate our extremely simple and versatile force probing technique by measuring electrostatic forces between charged particles and steric repulsions between polymer grafted particle surfaces.