Spin-coating of dilute magnetic colloids in a magnetic field

Spin-coating of colloids is a versatile method in fabricating films of colloidal particles at a short span of time. Controlling key parameters like the rate of rotation, the initial concentration and the nature of the continuum phase is easy to perform experimentally. However, the effects of these parameters are not fully understood yet. To enhance the understanding of the coating process, we study the spin-coating of dilute magnetic colloids. The coating process is controlled externally by means of a magnetic field and we investigate its effects by studying the morphology of the dried coating obtained after the experiments. Morphological transitions are explained through the mean area and the number density of clusters, together with anisotropy properties. Further, we relate the occupation factor of clusters to the non-planarization conditions observed in this kind of experiments

Effect of External Fields on the Dynamics of Colloidal Phase Transitions

In this experimental work, the effects of the external fields on the colloidal phase transition from liquid phase to solid deposits (evaporative colloidal phase transition) have been investigated. The external fields are applied while the transition is in process. The experiments are performed with two different transition duration: (a) In the experiments of long duration, the fluid is allowed to evaporate by exposing the colloidal dispersion (negatively charged polystyrene particles of diameter 1.3 μm dispersed in ultra pure water) to an environment at high temperature (63 degree C) and low humidity (below 2% RH). The colloidal dispersion is placed between the two vertical conducting substrates. Electric fields (DC) are of the order of 1 V/mm and they are applied perpendicularly to the substrates while the phase transition is in process. When the continuous phase evaporates, the contact line recedes. We measure the speed of the receding contact line for different initial concentrations (0.1%, 0.3% and 0.5% w/w, respectively) as well as for varying electric field. The dried deposits of colloidal particles are then correlated with the initial conditions and electric field strength of the respective experiment. To deepen the understanding of the three phase contact line in vertical deposition of colloids, the meniscus of a colloid is observed while the weak external field (AC) of the order of 1 V/mm and 1 Hz is applied. In this case, the working temperature is relatively low (room temperature) when compared to the previous set of experiments explained above and consequently the contact line does not recede during the measurement time. The applied field generates flows near the meniscus through electrokinetic and electrowetting mechanisms resulting in the formation of clusters of colloidal particles in the fluid matrix along the horizontal contact line. The clusters are separated by a well defined characteristic length and in our experimental conditions, they remain between 5 and 15 minutes. (b) In the experiments of short duration (spin-coating), the fluid phase of the colloidal dispersion is made to evaporate in fractions of a second by pouring a volume of the dispersion over the spinning substrate (spinning rate is of the order of 1000 rpm). In the absence of the magnetic field, equivalent film thickness for different kinds of colloids (superparamagnetic and nonmagnetic, respectively) are compared. After that, external fields are applied while the dispersion is pipetted onto the spinning substrate. On the one hand, external magnetic fields up to 0.066 T are applied while spin-coating the dilute superparamagnetic colloidal dispersion (polystyrene coated magnetite of diameter 1 to 2 μm and SiO2 coated magnetite of diameter 1.51 μm dispersed in ultra pure water, respectively). A spin-coating model is constructed by considering the evaporation of the fluid and the particulate characteristics of the spin-coated deposits. Morphological transition from sparse to submonolayer deposits (clusters) of superparamagnetic particles occurs. The magnetic field increases the effective viscosity of the dispersion through magnetic dipole interactions. On the other hand, to overcome the axial symmetry imposed by the spin-coating (in experiments with nonmagnetic particles of high initial concentration, 40% w/w), nonuniform alternating electric fields of the order of 0.1 kV/mm; frequency of the order of 1 kHz are applied while spin-coating the colloidal dispersion (SiO2 particles of diameter 458 nm dispersed in 2-Pentanone) over the patterned conducting substrate. We conclude that the dielectrophoretic confinement of the dispersion affects the hydrodynamic flows resulting in a predefined direction for the colloidal deposits. Thus, the electric field breaks the axial symmetry

Pattern formation in spin-coating of hybrid colloids in different magnetic field configurations

We report experimental results on the patterns that are formed during spin-coating of magnetic colloids at moderate concentrations and compare them with results obtained in diluted colloids. We show that, for moderate concentrations, the magnetic interaction between the (ferro)magnetic particles and with the external field is strong enough to overcome the centrifugal force. We study two different configurations for the magnetic field. The first one consists on an axial uniform field, where we obtain spikes perpendicular to the substrate with a well defined order which decreases as rotation rate increases. The second one consists on a radial non-uniform field, where we obtain elongated deposits radially disposed on the substrate. The effect of magnetic fields at moderate concentrations on the effective viscosity is confirmed to be much more important in the case of a uniform magnetic field, by increasing the hydrodynamic time-scale which gives the ferromagnetic particles enough time to strongly interact to form the spikes

Breath figures of two immiscible substances on a repellent surface

The understanding of the competition between different substances while condensing on a cold surface is of high interest in situations in which it is desirable to control their condensation rates and the formed morphologies. We do the experiments for mixtures of water and hexamethyldisiloxane vapors at several concentrations. The dropwise condensation of the vapors forms breath figures on a substrate that is repellant to both substances. We report the average radius of the drops for each specie as a function of time. Also, we pay attention to the evolution of the corresponding morphologies and the appearance of hybrid clusters

Spin-coating of moderately concentrated superparamagnetic colloids in different magnetic field configurations

Spin-coating technique is very fast, cheap, reproducible, simple and needs less material to fabricate films of particulate systems/colloids. Their thickness and uniformity may be controlled by means of external fields. We apply magnetic fields during the spin-coating of a moderately concentrated superparamagnetic colloid (made of silica coated magnetite particles). We study the influence of different magnetic field configurations (homogeneous and inhomogeneous) on the resulting spin-coated deposits and compare experimental results under various conditions. Superparamagnetic colloids behave as, non-Newtonian, magnetorheological fluids. Their viscosity vary significantly under applied magnetic fields. We measure and compare the effect of uniform and non-uniform magnetic fields on their relative effective viscosity, using the spin-coated deposits and a previously existing model for simple colloids. The mechanisms involved in the deposits formation under different experimental conditions are also discussed. In particular, we show that the magnetophoretic effect plays an important role in the spin-coating of magnetic colloids subjected to non-uniform magnetic fields. We characterize an effective magnetoviscosity in non-uniform magnetic fields that is largely influenced by the magnetophoretic effect that enhances the flow of the magnetic fluid

Impact of electric fields on the speed of contact line in vertical deposition of diluted colloids

We report experimental results on the influence of electric fields on the contact line dynamics of the vertical deposition of water-based diluted colloidal suspensions. We measure the speed of macroscopically receding contact line for different initial concentrations and applied voltages. We explain the observed behavior via the electrophoretic effect in the region near the contact line. The electrophoretic effect induces a concentration gradient along the direction of the applied field which influences the morphology of the dried deposit of colloidal particles. Thus the applied field has an effect on the receding contact line through morphological formation and its transition

Magnetorheology from surface coverage of spin-coated colloidal films

In magnetorheological fluids, the viscosity usually increases with the field and the non-Newtonian character of these complex fluids may vary significantly. We provide a new method to measure the relative viscosity of a superparamagnetic colloid, by applying a magnetic field during a spin-coating process, which involves evaporation of the solvent. We define the compact equivalent height to take into account the discrete nature of the suspension, and we compare experimental results under different conditions. We extend the model of Cregan et al. (J. Colloid Interface Sci., 2007, 314, 324) to turn it into an evaporation rate independent one. The generality of the resulting model facilitates measurement of the magnetic field dependent viscosity. We also discuss the morphologies of the final dried colloidal deposits and the possible mechanisms involved in their formation

Contact-line speed and morphology in vertical deposition of diluted colloids

We report experimental results on the speed of a receding contact line, in a vertical deposition configuration, during the deposition of polystyrene colloidal particles. We study the effects of the initial concentration of the suspension and compare the measured speeds with the corresponding dried deposit. It is observed that multilayer structures are associated with high speeds. This result is explained through a region in the bulk of the suspension where the concentration of particles increases until multilayer is formed depending on the porosity of the previously deposited structures

The Kibble-Zurek mechanism in a subcritical bifurcation

We present the study of the freezing dynamics of topological defects in a subcritical system by testing the Kibble-Zurek (KZ) mechanism while crossing a tri-stable region in a one-dimensional quintic complex Ginzburg-Landau equation. The critical exponents of the KZ mechanism and the horizon (KZ-scaling regime) are predicted from the quasistatic study, and are in full accordance with the quenched study. The correlation length, in the KZ freezing regime, is corroborated from the number of topological defects and from the spatial correlation function of the order parameter. Furthermore, we characterize the dynamics to differentiate three out-of-equilibrium regimes: the adiabatic, the impulse and the free-relaxation. We show that the impulse regime shares a common temporal domain with a fast exponential increase of the order parameter