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

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

Weak ac field-induced patters in vertical deposition of colloids at various evaporation rates

Pattern formation in colloids by weak ac fields in vertical deposition-like configuration at different temperatures has been studied experimentally. At low evaporation (room temperature), the effect of the field leads to the evolution of a one-dimensional array of clusters along the contact line and columnar colloidal dried deposits are obtained at higher evaporation. We investigate the flow dynamics involved in this pattern formation. Homogeneous variation of the contact angle by electrowetting effect becomes unstable and breaks the translational symmetry at the meniscus. Electrokinetic forces together with capillary forces result in the accumulation of particles for pattern formation. The movement of electrically charged colloidal particles is controlled by weak ac electric field even at higher temperatures. We observe the effect of increasing initial particle concentration on the behavior of the clusters for various field frequencies. The average distance between clusters increase monotonically with an increase in the initial particle concentration. We also observe that the average width of columns increases according to the applied field strength

Magnetorheology of hybrid colloids obtained by spin-coating and classical rheometry

Hybrid colloids composed of micron-sized ferromagnetic (carbonyl iron) and diamagnetic (silica) particles suspended in cyclohexanone, behave as, non-Newtonian, magnetorheological fluids. We measure and compare the magnetic field-dependent viscosity of hybrid diluted colloids using spin-coating and conventional magnetorheometry. We extend a previously developed model for simple colloids to this kind of hybrid colloids. As in the previous model, the viscosity of the colloidal suspension under applied fields can be derived from the surface coverage of the dry spin-coated deposits for each type of particles, and from the viscosity of the colloid at zero field. Also, our results allow us to obtain the evaporation rate of the solvent as a function of the rotation speed. Finally, we also measure the viscosity of the same suspension with a torsional parallel plate magnetorheometer under uniaxial DC magnetic fields aligned in the velocity gradient direction of a steady shearing flow. The experimental results under different conditions and the effect of each component on the magnetorheological properties of the resulting colloid are discussed. Standard spin-coating technique can be used both to characterize smart materials - complex fluids as well as to fabricate films with arbitrary solvents by tuning their viscosity by means of external fields

Quantifying disorder in colloidal films spin-coated onto patterned substrates

Polycrystals of thin colloidal deposits, with thickness controlled by spin-coating speed, exhibit axial symmetry with local 4-fold and 6-fold symmetric structures, termed orientationally correlated polycrystals (OCPs). While spin-coating is a very facile technique for producing large-area colloidal deposits, the axial symmetry prevents us from achieving true long-range order. To obtain true long-range order, we break this axial symmetry by introducing a patterned surface topography and thus eliminate the OCP character. We then examine symmetry- independent methods to quantify order in these disordered colloidal deposits. We find that all the information in the bond-orientational order parameters is well captured by persistent homology analysis methods that only use the centers of the particles as input data. It is expected that these methods will prove useful in characterizing other disordered structures

Colloidal Phase Transition Dynamics

In this work, we present experimental observations on the colloidal phase transitions (from liquid phase to solid phase) involving evaporation at different time scales. Two experimental setups have been used: vertical deposition configuration (large duration and low evaporation) and spincoating (short duration and high evaporation) to carry out various research objectives. (a) We study patterns formation in vertical deposition of colloids on applying weak AC fields at various temperatures. Diluted suspension of negatively charged polystyrene particles of 1.3 µm in diameter suspended in ultra pure water are used to perform several experiments at different evaporation in external weak AC fields (0.8 V/mm to 1.2 V/mm and of frequency from 1 Hz to 3 Hz). At room temperature the application of AC field leads to the formation of one dimensional clusters array at the interface. The dynamical flows of the system during clusters formation in external AC fields have been studied by PIV analysis. Also, we demonstrate the effect of increasing initial particle concentration (0.5% to 1.1% (w/w)) on the clusters evolution. At higher temperature (63 ◦C) we obtain ordered columnar deposits of these colloidal suspensions in AC fields. We study the effect of field strengths on the behavior of columns formation. (b) We obtain polycrystals of mono-layer on photo-patterned substrates by spin-coating using concentrated colloidal suspension of 458 nm diameter silica spheres dispersed in ethanol 95% of 20% (V/V) concentration. We break the axial symmetry imposed by spin axis and eliminate the emerging OCP character (6– or 4–arms). The effect of scaling spacing of surface patterning on the different structures formation of the spin-coated deposits has been studied extensively. We analyze the different structures in depth with bond order parameters and with Minkowski structure metrics. Also, we investigate the influence of external magnetic fields on the flow dynamics of mixed colloidal particles suspension (carbonyl iron and silica particles of micrometer size) by spin-coating and to manipulate the magneto-rheology in terms of occupation factor of the resulting deposits. We develop a generalized model by modifying Cregan and O’Brien model [85] to measure the change in relative viscosity of the mixed colloids in magnetic fields.En este trabajo experimental, se presentan observaciones experimentales en transiciones de fase coloidales (de estado líquido a sólido) donde la evaporación a diversas escalas es relevante. Se han usado dos sistemas experimentales bien diferenciados: una configuración tipo deposición vertical (evaporación relativamente baja y larga duración del experimento) y “spin-coating” (alta evaporación y corta duración del experimento) para alcanzar los objetivos propuestos en esta tesis. (a) Se estudia la formación de patrones en la deposición vertical de coloides a varias temperaturas con campo eléctrico alterno aplicado. Se utiliza una suspensión acuosa diluida de partículas de poliestireno de 1.3 µm de diámetro cargadas negativamente para llevar a cabo experimentos con campos débiles y muy lentamente variables (0.8 V/mm a 1.2 V/mm y con frecuencias de 1 Hz a 3 Hz). A temperatura ambiente, la aplicación de los campos eléctricos citados da lugar a la formación de un arreglo unidimensional de agregados de partículas (“clusters”) cerca de la linea de contacto. Se han estudiado los flujos que aparecen en el sistema durante la formación y evolución de los agregados anteriormente nombrados mediante análisis de PIV. Además, se demuestra el efecto que tiene la concentración de partículas inicial (0.5 % a 1.1 % (m/m)) en dicha evolución. A mayor temperatura (63 ◦C), pero con campo se obtienen depósitos de tipo columnata. Se ha estudiado el efecto de la intensidad del campo en el comportamiento de los depósitos obtenidos. (b) Se obtienen monocapas policristalinas, sobre sustratos fotolitografiados, de suspensiones etanólicas (95 %) coloidales de partículas de sílice con un diámetro de 458 nm a concentración media (20 % (V/V)). El patrón que forma el sustrato rompe la simetría axial del sistema y evita la aparición de la estructura policristalina correlacionada orientacionalmente (OCP) que da lugar a la aparición de cuatro o seis brazos por reflexión de luz. El efecto de las distancias características del patrón sobre los depósitos por “spin-coating” ha sido estudiado extensivamente. Se caracterizan las estructuras a través del parámetro de orden de ligadura (“bond”) con una métrica estructural de Minkowski. Además se estudia la magnetohidrodinámica de una suspensión coloidal híbrida (Hierro carbonilo y sílice de tamaños micrométricos) por “spin-coating” y se obtienen las propiedades magnetorreológicas en función del factor de ocupación en los depósitos resultantes. Se desarrolla un modelo generalizado modificando el obtenido por Cregan y O’Brien [85] para medir el efecto de la viscosidad relativa de coloides híbridos con campos magnéticos

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

Colloidal Phase Transition Dynamics

In this work, we present experimental observations on the colloidal phase transitions (from liquid phase to solid phase) involving evaporation at different time scales. Two experimental setups have been used: vertical deposition configuration (large duration and low evaporation) and spincoating (short duration and high evaporation) to carry out various research objectives. (a) We study patterns formation in vertical deposition of colloids on applying weak AC fields at various temperatures. Diluted suspension of negatively charged polystyrene particles of 1.3 µm in diameter suspended in ultra pure water are used to perform several experiments at different evaporation in external weak AC fields (0.8 V/mm to 1.2 V/mm and of frequency from 1 Hz to 3 Hz). At room temperature the application of AC field leads to the formation of one dimensional clusters array at the interface. The dynamical flows of the system during clusters formation in external AC fields have been studied by PIV analysis. Also, we demonstrate the effect of increasing initial particle concentration (0.5% to 1.1% (w/w)) on the clusters evolution. At higher temperature (63 ◦C) we obtain ordered columnar deposits of these colloidal suspensions in AC fields. We study the effect of field strengths on the behavior of columns formation. (b) We obtain polycrystals of mono-layer on photo-patterned substrates by spin-coating using concentrated colloidal suspension of 458 nm diameter silica spheres dispersed in ethanol 95% of 20% (V/V) concentration. We break the axial symmetry imposed by spin axis and eliminate the emerging OCP character (6– or 4–arms). The effect of scaling spacing of surface patterning on the different structures formation of the spin-coated deposits has been studied extensively. We analyze the different structures in depth with bond order parameters and with Minkowski structure metrics. Also, we investigate the influence of external magnetic fields on the flow dynamics of mixed colloidal particles suspension (carbonyl iron and silica particles of micrometer size) by spin-coating and to manipulate the magneto-rheology in terms of occupation factor of the resulting deposits. We develop a generalized model by modifying Cregan and O’Brien model [85] to measure the change in relative viscosity of the mixed colloids in magnetic fields.En este trabajo experimental, se presentan observaciones experimentales en transiciones de fase coloidales (de estado líquido a sólido) donde la evaporación a diversas escalas es relevante. Se han usado dos sistemas experimentales bien diferenciados: una configuración tipo deposición vertical (evaporación relativamente baja y larga duración del experimento) y “spin-coating” (alta evaporación y corta duración del experimento) para alcanzar los objetivos propuestos en esta tesis. (a) Se estudia la formación de patrones en la deposición vertical de coloides a varias temperaturas con campo eléctrico alterno aplicado. Se utiliza una suspensión acuosa diluida de partículas de poliestireno de 1.3 µm de diámetro cargadas negativamente para llevar a cabo experimentos con campos débiles y muy lentamente variables (0.8 V/mm a 1.2 V/mm y con frecuencias de 1 Hz a 3 Hz). A temperatura ambiente, la aplicación de los campos eléctricos citados da lugar a la formación de un arreglo unidimensional de agregados de partículas (“clusters”) cerca de la linea de contacto. Se han estudiado los flujos que aparecen en el sistema durante la formación y evolución de los agregados anteriormente nombrados mediante análisis de PIV. Además, se demuestra el efecto que tiene la concentración de partículas inicial (0.5 % a 1.1 % (m/m)) en dicha evolución. A mayor temperatura (63 ◦C), pero con campo se obtienen depósitos de tipo columnata. Se ha estudiado el efecto de la intensidad del campo en el comportamiento de los depósitos obtenidos. (b) Se obtienen monocapas policristalinas, sobre sustratos fotolitografiados, de suspensiones etanólicas (95 %) coloidales de partículas de sílice con un diámetro de 458 nm a concentración media (20 % (V/V)). El patrón que forma el sustrato rompe la simetría axial del sistema y evita la aparición de la estructura policristalina correlacionada orientacionalmente (OCP) que da lugar a la aparición de cuatro o seis brazos por reflexión de luz. El efecto de las distancias características del patrón sobre los depósitos por “spin-coating” ha sido estudiado extensivamente. Se caracterizan las estructuras a través del parámetro de orden de ligadura (“bond”) con una métrica estructural de Minkowski. Además se estudia la magnetohidrodinámica de una suspensión coloidal híbrida (Hierro carbonilo y sílice de tamaños micrométricos) por “spin-coating” y se obtienen las propiedades magnetorreológicas en función del factor de ocupación en los depósitos resultantes. Se desarrolla un modelo generalizado modificando el obtenido por Cregan y O’Brien [85] para medir el efecto de la viscosidad relativa de coloides híbridos con campos magnéticos