Electrohydrodynamic Patterning Research Articles

Compact micro/nano electrohydrodynamic patterning: using a thin conductive film and a patterned template.

The influence of electrostatic heterogeneity on the electric-field-induced destabilization of thin ionic liquid (IL) films is investigated to control spatial ordering and to reduce the lateral dimension of structures forming on the films. Commonly used perfect dielectric (PD) films are replaced with ionic conductive films to reduce the lateral length scales to a sub-micron level in the EHD pattering process. The 3-D spatiotemporal evolution of a thin IL film interface under homogenous and heterogeneous electric fields is numerically simulated. Finite differences in the spatial directions using an adaptive time step ODE solver are used to solve the 2-D nonlinear thin film equation. The validity of our simulation technique is determined from close agreement between the simulation results of a PD film and the experimental results in the literature. Replacing the flat electrode with the patterned one is found to result in more compact and well-ordered structures particularly when an electrode with square block protrusions is used. This is attributed to better control of the characteristic spatial lengths by applying a heterogeneous electric field by patterned electrodes. The structure size in PD films is reduced by a factor of 4 when they are replaced with IL films, which results in nano-sized features with well-ordered patterns over the domain.

Influence of electrode types on the electrohydrodynamic instability patterning process: a comparative study

A polymer film resting on a planar substrate under the influence of a electric field. (A) A conductive patterned electrode. (B) A conductive pattern on a dielectric substrate.

Influence of Processing Conditions and Material Properties on Electrohydrodynamic Direct Patterning of a Polymer Solution

Influence of Processing Conditions and Material Properties on Electrohydrodynamic Direct Patterning of a Polymer Solution

Bioinspired electrohydrodynamic ceramic patterning of curved metallic substrates

Template-assisted electrohydrodynamic atomisation (TAEA) has been used for the first time to pattern curved metallic surfaces. Parallel lines of ceramic titania (TiO2) were produced on titanium substrates, convex and concave with diameters of ~25 mm, at the ambient temperature. Optimal results were obtained with 4 wt% TiO2in ethanol suspension deposited over 300 s during stable cone-jetting at 20 µl/min, 10kV and collection distance 80 mm. A high degree of control over pattern line width, interline spacing and thickness were achieved. Nanoindentation load-displacement curves were continuous for the full loading and unloading cycle, indicating good adhesion between pattern and substrate. At a loading rate of 1 μN/s and a hold time of 1 s, pattern hardness decreased as load increased up to 7 μN and remained at 0·1 GPa up to higher loads. Elastic modulus behaved similarly, and both were not sensitive to loading rate. The effect of heat treatment to further consolidate the patterned deposits was also investigated. Hardness of the patterns was not markedly affected by heating. This work shows that TAEA is highly controllable and compatible on a range of substrate geometries. Extending TAEA capabilities from flat to curved surfaces, enabling the bioactive patterning of different surface geometries, takes this technology closer to orthopaedic engineering applications.

Electrohydrodynamic patterning of ultra-thin ionic liquid films.

In the electrohydrodynamic (EHD) patterning process, electrostatic destabilization of the air-polymer interface results in micro- and nano-size patterns in the form of raised formations called pillars. The polymer film in this process is typically assumed to behave like a perfect dielectric (PD) or leaky dielectric (LD). In this study, an electrostatic model is developed for the patterning of an ionic liquid (IL) polymer film. The IL model has a finite diffuse electric layer which overcomes the shortcoming of assuming infinitesimally large and small electric diffuse layers inherent in the PD and LD models respectively. The process of pattern formation is then numerically simulated by solving the weakly nonlinear thin film equation using finite difference with pseudo-staggered discretization and an adaptive time step. Initially, the pillar formation process in IL films is observed to be the same as that in PD films. Pillars initially form at random locations and their cross-section increases with time as the contact line expands on the top electrode. After the initial growth, for the same applied voltage and initial film thickness, the number of pillars on IL films is found to be significantly higher than that in PD films. The total number of pillars formed in 1 μm(2) area of the domain in an IL film is almost 5 times more than that in a similar PD film for the conditions simulated. In addition, the pillar structure size in IL films is observed to be more sensitive to initial film thickness compared to PD films.

Electrical perturbations of ultrathin bilayers: role of ionic conductive layer.

The effect of electrostatic force on the dynamics, morphological evolution, and drainage time of ultrathin liquid bilayers (<100 nm) are investigated for perfect dielectric-perfect dielectric (PD-PD) and ionic liquid-perfect dielectric (IL-PD) bilayers. The weakly nonlinear "thin film" equation is solved numerically to obtain spatiotemporal evolution of the liquid-liquid interface responses to transverse electric field. In order to predict the electrostatic component of conjoining/disjoining pressure acting on the interface for IL-PD bilayers, an analytical model is developed using the nonlinear Poisson-Boltzmann equation. It is found that IL-PD bilayers with electric permittivity ratio of layers (lower to top), εr, greater than one remain stable under an applied electric field. An extensive numerical study is carried out to generate a map based on εr and the initial mean thickness of the lower layer. This map is used to predict the formation of various structures on PD-PD bilayer interface and provides a baseline for unstable IL-PD bilayers. The use of an ionic liquid (IL) layer is found to reduce the size of the structures, but results in polydispersed and disordered pillars spread over the domain. The numerical predictions follow similar trend of experimental observation of Lau and Russel. (Lau, C. Y.; Russel, W. B. Fundamental Limitations on Ordered Electrohydrodynamic Patterning; Macromolecules 2011, 44, 7746-7751).

Steady State of Electrohydrodynamic Patterning of Micro/Nanostructures on Thin Polymer Films

This paper presents a nonlinear numerical study of the electrohydrodynamic (EHD) patterning process as a micro/nanostructuring approach for a thin polymer film. Special interest is focused on the steady state (or equilibrium state) in this study. Numerical results show that the polymer nanostructure height depends strongly on the applied voltage, rising higher with an increasing electric field. Moreover, a critical electric field intensity is discovered, above which the deformed film can grow continuously into a final contact with the template to form a nanostructure conformal the pattern premade on the template. An electric field intensity larger than the critical value is necessary for a faithful replica in EHD patterning; however, breakdown may occur for polymer film under a strong electric field and thus fail the process. In this study, the scale limitation is investigated and some methods are then proposed for scaling down the feature size of the structure.

Faithful replication of grating patterns in polymer through electrohydrodynamic instabilities

Electrohydrodynamic instability patterning (EHDIP) as an alternative patterning method has attracted a great deal of attention over the past decade. This article demonstrates the faithful transfer of patterns with a high aspect ratio onto a polymer film via electrohydrodynamic instabilities for a given patterned grating mask. We perform a simple mathematical analysis to determine the influence of process parameters on the pressure difference ▵P. Through numerical simulation, it is demonstrated that thick films subject to large electric fields are essential to realize this faithful replication. In particular, the influence of the material properties of the polymer on pattern replication is discussed in detail. It is found that, to achieve the smaller periodic patterns with a higher resolution, film with a larger value of the dielectric constant and smaller value of the surface tension should be chosen. In addition, an ideal replication of the mask pattern with a short evolution time is possible by reducing the viscosity of the polymer liquid. Finally, the experiments of the pattern replication with and without defects are demonstrated to compare with the numerical simulation results. It is found that experiments are in good agreement with the simulation results and prove that the numerical simulation method provides an effective way to predict faithful replication.

Characterization of flexible temperature sensor fabricated through drop-on-demand electrohydrodynamics patterning

The fabrication and characterization of a temperature sensor on flexible poly(ethylene terephthalate) (PET) substrate is discussed. The device was printed by drop-on-demand (DOD) electrohydrodynamic (EHD) Patterning method using silver nanoparticles ink on a roll-to-roll (R2R) system on a mass scale. EHD patterning was performed at atmospheric pressure and room temperature in a single step. The ink viscosity was 300 cps and it contained 55 wt % of silver nanoparticles. The parameters for printing head speed, and DOD were optimized to create connected lines. The printed temperature sensor has the resistivity of 25.35 µΩ·cm and the temperature coefficient of resistance (TCR) is 0.0007687 °C−1. The micro sensor can be applied to many applications to measure temperature accurately of flat or curved surfaces.

Formation of arbitrary patterns in ultraviolet cured polymer film via electrohydrodynamic patterning.

Electrohydrodynamic patterning of arbitrary patterns is achieved by optimizing the critical parameters (applied voltage and spacer height). The applied voltage has a great influence on the fidelity of L-shaped line structures with different sizes. The L-shaped line structures with high fidelity are obtained by using the moderate applied voltage. The spacer height has a great influence on the fidelity of square structures with different sizes. The square structures with high fidelity are obtained by using the low height spacer. The multi-field coupling transient finite element simulation demonstrates that the lack of polymer owing to the high height spacer leads to the formation of defects.

Simulation of the electrohydrodynamic instability process used in the fabrication of hierarchic and hollow micro/nanostructures

This article demonstrates that the electrohydrodynamic patterning process, a novel technique for the manufacturing of micro- and nano-scale structures, also allows the one-step realization of hierarchical structures and hollow structures. Through numerical simulation, it is shown that multilevel structures can be obtained if process time and applied electric voltage are optimized. As an example, the growth of structures with a width of around 187 nm and depth of 95 nm has been successfully simulated alongside structures with width of around 0.4 μm and depth of 0.8 μm. The width of the protrusive mask patterns is shown to determine whether hollow structures with single or multiple shapes can be formed using electric field assisted capillarity. The numerical simulation process effectively demonstrates that the realization of micro/nano-structures with hierarchic and multilevel shapes can be considered as an innovative manufacturing process for MEMS or micro/nanofluidic structures.

Combined thermal and electrohydrodynamic patterning of thin liquid films

Both electric fields and temperature gradients can destabilize the surface of a thin liquid film and lead to the self-assembly of patterns composed of pillar-like structures. Such instabilities offer a relatively simple way to tailor the surface topography of coatings, which in turn can be used to influence coating appearance, texture, and functionality. The present work explores how the simultaneous application of an electric field and temperature gradient can be used to further influence thin-liquid-film instabilities. Both perfect and leaky dielectric materials are considered, and lubrication theory is applied to develop a system of nonlinear partial differential equations for the interfacial height and charge. Linear stability analysis of the lubrication equations shows that in perfect dielectric films, thermal effects tend to dominate the process, often rendering the electric field unimportant. However, in leaky dielectric films, both the thermal and electric fields play important roles and together can produce an increase in the growth rate and a reduction in the dominant wavelength of the instability. Nonlinear simulations of the lubrication equations show that the predictions of the linear theory hold even when the interfacial perturbations are no longer small. The effects of viscoelasticity are considered within the linear theory, and it is found that the growth rate of the instability, but not the length scale, depends on the rheological parameters. The findings of this work suggest that the simultaneous use of an electric field and temperature gradient will allow thin films to be patterned at length scales not accessible when only one of these destabilizing forces is used.

Fine metal line patterning on hydrophilic non-conductive substrates based on electrohydrodynamic printing and laser sintering

We present a electrohydrodynamic (EHD) fine metal line patterning on a hydrophilic non-conductive substrate for the repair of flat panel displays. There are two crucial problems to solve for fine metal line patterning: 1) The scattering effect caused by the charges accumulated on the printed lines, 2) the unwanted thermal effect around patterned lines during sintering. We found that the scattering can be curbed by optimizing head parameters, and the unwanted thermal effect around patterned lines by directly applying laser sintering to printed lines. We achieved a 3 μm width of metal line patterns with the electrical resistivity of 17.48 μΩ cm.

Investigation of Electrohydrodynamic Patterning of Micro Structures at Room Temperature

EHD (Electrohydrodynamic) patterning forming micro structures with UV light curable materials at room temperature is investigated in this paper. It is a process for fabricating patterns on polymer film by applying electrical field to the template and the substrate as an electrode pair. The maintaining time of EHD patterning is reduced from several hours to tens of minutes by using UV curable polymer with low viscosity. The flat template generates the periodic pillars with micro scale. The periodic pillars with different period are obtained by varying the applied voltage in the different phase of experiment. The micro scale replication with good fidelity is produced by using patterned template made of doped silicon wafer.

ChemInform Abstract: Directing Convection to Pattern Thin Polymer Films

AbstractReview: 95 refs.

3D phase field modeling of electrohydrodynamic multiphase flows

A 3D phase field model is developed to investigate the electrohydrodynamic (EHD) two phase flows. The explicit finite difference method, enhanced by parallel computing, is employed to solve the coupled nonlinear governing equations for the electric field, the fluid flow field and free surface deformation. Numerical tests indicate that an appropriate interpolation of densities within the interface is critical in ensuring numerical stability for highly stratified flows. The 3D phase field model compares well with the Taylor theory for the deformation of a single dielectric droplet in an electric field. Computed results show that the deformation of a leaky dielectric droplet in an electric field undergoes various stages before it reaches the final oblate shape. This is caused by the free charge relaxation near the fluid–fluid interface. The coalescence of four droplets in an electric field illustrates a truly 3D deformation behavior and a complex evolving fluid flow field associated with the participating droplets. The coalescence is a result of combined actions produced by the global electric force, the circulatory flows generated by the local electrohydrodynamic stress and the electrically-induced deformation. The 3D phase field model is also applied in modeling of an electrohydrodynamic patterning process for manufacturing nanoscaled structures, in which complex 3D flow structures develop as the electrically-induced deformation evolves.

Numerical Characterization of Electrohydrodynamic Micro- or Nanopatterning Processes Based on a Phase-Field Formulation of Liquid Dielectrophoresis

The electrohydrodynamic patterning of polymer is a unique technique for micro- and nanostructuring where an electric voltage is applied to an electrode pair consisting of a patterned template and a polymer-coated substrate either in contact or separated by an air gap to actuate the deformation of the rheological polymer. Depending on the template composition, three processes were proposed for implementing the EHDP technique and have received a great amount of attention (i.e., electrostatic force-assisted nanoimprint, dielectrophoresis-electrocapillary force-driven imprint, and electrically induced structure formation). A numerical approach, which is versatile for visualizing the full evolution of micro- or nanostructures in these patterning processes or their variants, is a desirable critical tool for optimizing the process variables in industrial applications of this structuring technique. Considering the fact that all of these processes use a dielectric and viscous polymer (behaving mechanically as a liquid) and are carried out in ambient air, this Article presents a generalized formulation for the numerical characterization of the EHDP processes by coupling liquid dielectrophoresis (L-DEP) and the phase field of the air-liquid dual phase. More importantly, some major scale effects, such as the surface tension, contact angle, liquid-solid interface slip, and non-Newtonian viscosity law are introduced, which can impact the accuracy of the numerical results, as shown experimentally by our electrical actuation of a dielectric microdroplet as a test problem. The numerical results are in good agreement with or are well explained by experimental observations published for the three EHDP processes.

Directing convection to pattern thin polymer films

AbstractConvection can be harnessed in elegant ways to pattern surfaces, often using uncomplicated equipment and materials, providing an interesting platform for future technological developments in thin film topographic assemblies. This manuscript contains a brief review of thin polymer film patterning methods that rely on directing convection, such as “coffee ring” patterning, lithographically induced self‐assembly, and electrohydrodynamic patterning. These techniques are described in the context of a recent approach explored in our group for generating topographic patterns by photochemically directing Marangoni flow in thin polymer films with subtle gradients in surface energy. Aspects unique to this process are highlighted so that they may facilitate new developments in manufacturing technologically impactful patterned surfaces. For example, the features produced by photochemically directed Marangoni‐driven flow are preprogrammed in the solid state, thermally developed and can be formed in multilayer films. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013

A numerical study of nanoscale electrohydrodynamic patterning in a liquid film

A computational model is developed to study the manufacturing of micro/nanostructures by electrohydrodynamic (EHD) patterning processes. The computational methodology is based on the iterative coupling of the discontinuous boundary element method for electric field with the finite element method for free surface deformation. The model is capable of modeling fully nonlinear free surface deformations induced by an electric field. Geometric discontinuity resulting from the contact of the liquid film with the template is fully accounted for by introducing the short-range molecular forces. The critical voltage for liquid film instability is found by tracing the asymptote of the structural height vs. applied voltage curve, followed by a confirmation by the existence of an internal minimum of the total free energy. Computer codes are verified using the analytical solutions and available measurements. An important finding from the numerical simulations is that steady state structures can be electrohydrodynamically patterned when the applied voltage is either below or above a critical value. While linear analyses are useful, they may significantly over-predict the critical voltage, a crucial parameter for an EHD patterning process. The highly nonlinear phenomena of wetting the template above the critical voltage were considered to be unstable within the framework of linear perturbation. However, these phenomena can be very well predicted by the nonlinear boundary/finite element model enhanced by the short-range molecular forces. Finally, the computational model is flexible and may be modified with ease to analyze an EHD-patterning process for an air–polymer–polymer tri-layered system or other multiple-layered films for fabricating more complex nanostructures.

Dynamic modelling of micro/nano-patterning transfer by an electric field

A computational electrohydrodynamic (EHD) model is presented for the 3D electrically induced fluid motion and free surface morphology evolution during EHD patterning transfer of micro/nano-structures. The model entails a finite difference solution of the electric field equation with a leaky dielectric model to account for polymer behaviour, the Navier–Stokes equations for electrically driven flows and the phase field equation for the free surface deformation. These equations are fully coupled and represent a very large complex numerical system. Once discretized, the intensive computation is alleviated with the use of parallel computing algorithms. Computed results are presented that illustrate the transient development of 3D micro/nano-structures under an electric field. Two classical templates are considered in this study, for both of which the polymeric structures conform well to the template, resulting in the micro/nano-patterning transfer from the template to polymer film. The model is a useful tool to explore optimal conditions for scalable manufacturing of large scale nanostructures using the EHD patterning processes.