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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.

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.

Colloidal particles provide an efficient means of building multiple scale structured materials from colloidal dispersions. In this Brief Report we account for xperimental evidence on the formation of a colloidal cluster array at a three-phase contact line. We study the influence of low frequency external alternating electric fields on a diluted colloidal dispersion opened to the air. We focus on the cluster formation and their evolution in the meniscus by measuring characteristic times and lengths. We observe that the clusters are separated by a well-defined length and that, in our xperimental conditions, they survive between 5 and 15 min. These new results could be of technological relevance in building tailored colloidal structures in nonpatterned substrates.

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.