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Colloids in External Fields
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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:
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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. |
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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 use of this difference to probe particle interactions in colloidal suspensions, selectively addressing one particle species in a binary colloid mixture. The superposition of a magnetostatic field in parallel or normal direction allows to manipulate the motion of the charged/magnetic probe colloids within the bath. This permits us to probe forces on the level of individual particles (active microrheology) and provides information on the non-linear, non-equilibrium response. |
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Chain of magnetic particles dispersed in a PMMA matrix |
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| Contact: Doris Vollmer, Günter Auernhammer | ||
