Physics at Interfaces

Research - Colloids


Colloids in external fields

In colloidal suspensions subjected to electric fields, a variety of responses can be observed depending on the nature of the field (frequency, field gradients), the particles (polarizability, charge), and the solvent (dielectric constant, mechanical properties). To investigate particle transport and structure formation in colloidal systems under electric fields, we are using different experimental approaches:


Dielectrophoretic trapping:
Uncharged to weakly charged particles in organic solvents accumulate in an AC field generated between two glass slides with transparent conducting stripes on the outside. Electric field gradients exert forces on colloids due to the dielectric contrast between the colloids and the surrounding carrier fluid (dielectrophoretic force). We used the dielectrophoretic force to confine colloids in suspension in a so-called electric bottle, consisting of a flat glass chamber whose bottom and top plate are locally covered with transparent ITO electrodes. This allows us to study particle migration in response to field gradients and dielectric properties of the solvent. For sufficient high field gradients the colloid motion becomes unstable, and a wave-like instability travels through a colloid depleted zone.

  • J. Zhao, D. Vollmer, H.-J. Butt, and G. K. Auernhammer;
    Localized instabilities of colloidal motion in ac electric field gradients;
    J. Phys.: Condens. Matter 20, 404212 (2008).

Colloids in crossed electric and magnetic fields:
Whereas electric fields exert weak forces on both magnetic and non-magnetic colloids,magnetic fields act strongly and exclusively on magnetic colloids. We will make useof this difference to probe particle interactions in colloidal suspensions in an electricbottle, selectively addressing one particle species in a binary colloid mixture. Thesuperposition of a second, either electrostatic or magnetostatic, field in parallel or normal direction allows to manipulate the motion of the charged/magnetic probe colloids within the bath.