Field-Induced Layer Formation in Dipolar Nanofilms

Using molecular dynamics simulations, we demonstrate that the layering of confined colloidal particles with dipolar interactions, such as ferrofluids, in slablike geometries can be controlled by homogeneous external fields. For suitable surface separations, strong fields directed perpendicular to the film plane do not only align the particles but create additional layers in the system. The reverse effect occurs with an in-plane field which can induce a collapse of layers. Both effects are accompanied by pronounced particle rearrangements in lateral directions. Our simulation results are consistent with recent experiments of ferrofluids at surfaces.

Figure 1

Simulation snapshots at $L_z^{*}=5$ (a) $H^{*}=0$ (inset: top-view of the contact-layer), (b) $H_z^{*}=74$.

Figure 2

(a) Density profiles at $L_z^{*}=5.0$ and ${\rho }^{*}=0.6$ (in ${\mathbf{H}}_z$). Included is a profile at ${\rho }^{*}=0.2$ and $H_z^{*}=74$ (bottom curve). Inset: Magnetization as function of $H_z^{*}$ at ${\rho }^{*}=0.6$, ${\rho }^{*}=0.4$, and ${\rho }^{*}=0.6$ with $\lambda =1$ (see labels). (b) Inter- and intra-layer correlation functions (${\rho }^{*}=0.6$) at zero field and at $H_z^{*}=74$ yielding five layers 1–5. Top: in-plane (contact layer, 1), middle: intra-layer ($1+2$), bottom: intra-layer ($1+3$).

Figure 3

(a)–(c) Zero-field normal pressure as function of $L_z^{*}$ at three densities. The triangles up (down) indicate where ${\mathbf{H}}_z$ (${\mathbf{H}}_{\parallel }$) can create (destroy) a layer.

Figure 4

(a) Density profiles at ${\rho }^{*}=0.6$ and $L_z^{*}=4.3$ (parallel field). (b) Same for ${\rho }^{*}=0.8$ and $L_z^{*}=5.0$. (c) Snapshots of the contact layers at the two densities and $H_{\parallel }^{*}=74$.

Figure 5

Density profiles at $L_z^{*}=12$ (perpendicular field).