Field-induced structures in miscible ferrofluid suspensions with and without latex spheres

Abstract

We explore magnetic-field-induced ordering and microphase separation of aqueous ferrofluid and of aqueous mixtures of ferrofluid with nonmagnetic latex spheres. The ferrofluid is a surfactant stabilized aqueous suspension of magnetite (Fe3O4) particles with average diameter 20 nm (including the approximately 2.5-nm thick surfactant layer); the nonmagnetic latex spheres are charge stabilized polymethylmethacrylate (PMMA) particles with diameters of 42 nm, 108 nm, and 220 nm. In the presence of a uniform magnetic field, needlelike ferrofluid droplets formed that eventually grew to sample-traversing columns at fields of approximately 600 G; the two-dimensional structure of these columns was, however, glassy rather than hexagonal. In higher fields, approximately 1000 G, the columns stretched and coalesced into sheetlike striped liquids, but a true lamellar phase was not observed. The addition of nonmagnetic latex spheres to the ferrofluid suspension lowered substantially the critical field for the formation of columns, and induced lamellar (stripe) phases at relatively low applied fields. Image analysis was used to determine the spatial correlation functions, the average needle or column spacing, and the average lamellae spacing of these samples as a function of latex sphere size and concentration.

Diffusion and magnetotransport in ferrofluids containing chain-shaped aggregates

“…The bulk aggregates have sizes of about 1 µm; they have repeatedly been observed with ordinary micro scopes (see, e.g., [5][6][7]). Since the sizes of particles in common ferrofluids are smaller than the visible light wavelength, their linear chains cannot be registered by the optical methods.…”

“…At the same time, it is well known that, when the energy of the magnetic interparticle interactions in ferrofluids markedly exceeds thermal energy kT, the particles can form linear chains or dense bulk droplet shaped aggregates (see, e.g., [5][6][7][8][9]). …”

Structurization of ferrofluids in the absence of an external magnetic field

“…In order to deter mine the latter quantity, expressions (3) have to be substituted into conditions (2). The first relation (3) readily yields (4) Denoting the total number of particles in a fork by n = i + j + k + 3, we obtain the following expression for the volume fraction of forks: (5) Analogous considerations lead to the relation…”

“…Numerous experiments have shown that particles in ferrofluids are capable of forming various heteroge neous structures such as linear chains, closed rings, Y like forks, and dense droplets [1][2][3][4][5][6]. These struc tures have a typical size of several microns, and they can be readily observed with common optical micro scopes.…”

Microdynamics of Magnetic Particles Dispersed in Complex Media

“…It is well known (see, for example, refs. [11][12][13][14][15][16][17]) that magnetic field stimulates the phase condensation in ferrofluids, although only the biggest particles of polydisperse ferrofluid interact strongly enough to undergo the phase condensation.…”