Seyed Reza Mahmoudi

Active surfaces: Ferrofluid-impregnated surfaces for active manipulation of droplets

Droplet manipulation and mobility on non-wetting surfaces is of practical importance for diverse applications ranging from micro-fluidic devices, anti-icing, dropwise condensation, and biomedical devices. The use of active external fields has been explored via electric, acoustic, and vibrational, yet moving highly conductive and viscous fluids remains a challenge. Magnetic fields have been used for droplet manipulation; however, usually, the fluid is functionalized to be magnetic, and requires enormous fields of superconducting magnets when transitioning to diamagnetic materials such as water. Here we present a class of active surfaces by stably impregnating active fluids such as ferrofluids into a textured surface. Droplets on such ferrofluid-impregnated surfaces have extremely low hysteresis and high mobility such that they can be propelled by applying relatively low magnetic fields. Our surface is able to manipulate a variety of materials including diamagnetic, conductive and highly viscous fluids, and additionally solid particles.

Enhancing droplet deposition through in-situ precipitation

Retention of agricultural sprays on plant surfaces is an important challenge. Bouncing of sprayed pesticide droplets from leaves is a major source of soil and groundwater pollution and pesticide overuse. Here we report a method to increase droplet deposition through in-situ formation of hydrophilic surface defects that can arrest droplets during impact. Defects are created by simultaneously spraying oppositely charged polyelectrolytes that induce surface precipitation when two droplets come into contact. Using high-speed imaging, we study the coupled dynamics of drop impact and surface precipitate formation. We develop a physical model to estimate the energy dissipation by the defects and predict the transition from bouncing to sticking. We demonstrate macroscopic enhancements in spray retention and surface coverage for natural and synthetic non-wetting surfaces and provide insights into designing effective agricultural sprays. The extensive use of pesticides in agriculture calls for efficient spraying techniques to reduce pollution of soils and groundwater by toxic chemicals. Damak et al. simultaneously spray liquids containing oppositely charged polyelectrolytes that form defects, pinning droplets on targeted surfaces.

Spreading of a dielectric droplet through an interfacial electric pressure

A corona discharge-assisted technique for spreading of a gently deposited dielectric droplet into a uniform thin film over a dry isothermal conductive substrate is proposed. The surface charge was built up over the droplet interface through ion bombardment using a sharp emitter electrode. Interaction of the surface charge density and intense electric field generates an interfacial electrical pressure and leads to a uniform axisymmetric spreading of the droplet in the radial direction. It was shown that the droplet expansion process can be controlled through variation of the ion injection current and/or discharge exposure time. It is also demonstrated that the proposed technique can be analogous to the classical Stefans squeezing liquid flow between two separated parallel discs. The dynamics of the film spreading owing to the corona discharge can be predicted through a simplified analytical model based on this analogy.

Retraction control of an impacted dielectric droplet through electrical pressure

When a deposited dielectric liquid interface is exposed to a unipolar ion injection in a gaseous medium, the liquid expands over the grounded substrate due to the squeezing force of electric pressure (S. R. Mahmoudi, K. Adamiak, G. S. P. Castle, Spreading of a dielectric droplet through an interfacial electric pressure, Proc. R. Soc., 2011, 467, 3257–3271). A new active method based on the concept is proposed to control the deposition of an impacted dielectric droplet. An electric pressure resulting from the electric surface charge produced by corona discharge squeezes the droplet interface towards the grounded substrate and generates a resistance against the droplet retraction. It is demonstrated that the electrical pressure effectively suppresses the droplet retraction at voltages above the corona discharge threshold.

Study of Electrohydrodynamic Micropumping Through Conduction Phenomenon

In the present paper, a single-stage axisymmetric conduction micropump in the vertical configuration has been proposed. This micropump consists of four components: high voltage ring electrode, grounded disk-shaped electrode, insulator spacer, and inlet/outlet ports. The high-voltage electrode and grounded electrode of the device were patterned on the two separate commercial LCP substrates with 30 μm copper cladding using standard lithographic techniques. The final spacing between two electrodes and the overall size of the device were measured to be 286 μm and 50 mm × 70 mm × 5 mm, respectively. The static pressure generation of the micropump was measured at different applied voltage using three different dielectric liquids, 10-GBN Nynas and Shell Diala AX transformer oils, and N-hexane. The range of applied voltages was between 300 and 1500 VDC, and maximum pressure generation up to 100 Pa was achieved at 1500 VDC applied voltage. To further verify the experimental results, a numerical simulation was also performed. The pressure head generation was predicted numerically and compared with experimental results at different applied voltages.

Prediction of the static pressure generation for an electrohydrodynamic conduction pump

Steady-state 2-D electrohydrodynamic pumping through bipolar hetero-charge conduction phenomenon has been investigated numerically. In order to validate the presented numerical algorithm, the pump geometry was chosen to be identical to the device whose experimental data is available in the literature. The concentrations of the positive and negative ions and the electric field distribution of the pump were calculated. The resulting static pressure generation of the pump was predicted for the refrigerant R-123 at different applied voltages ranging between 2 to 20kV DC. The comparison between the predicted static pressure generation and the previous experimental results in the absence of fluid flow shows a good agreement with a maximum deviation of 12.5% at 20 kV applied voltage. In the present work, the electrical conductivity of R-123 was assumed to be the most recent proposed value of σ=7×10−11 S/m. Assuming this value of electrical conductivity, the numerical model predicts the hetrocharge layer thicknesses in the order of gap spacing even at low applied voltage and weak field regime. By increasing the applied voltage, above ∼15kV, the heterocharge layers of the opposite electrodes extend to the entire gap spacing and create an overlapping region. Assuming three orders of magnitude higher value of electrical conductivity for R-123 in the previous numerical studies, the thickness of the hetrocharge layer and pressure generation was underestimated by almost two orders of magnitude. Through extensive numerical experiments, the electrical conductivity of the working fluid was found to be an important parameter to determine the heterocharge layer thickness, electric body force, and predicted static pressure. The issues related to the scaling of the conduction pump when the heterocharge layers create an overlapping region in the reduced gap spacing are discussed.

Study of Electrohydrodynamic Micropumping through Conduction Phenomenon

Steady-state electrohydrodynamic micropumping through bipolar conduction phenomenon has been investigated experimentally and numerically. A single-stage axisymmetric conduction micropump in the vertical configuration has been proposed in this paper. This micropump consists of four components: high voltage ring electrode, grounded disk-shaped electrode, insulator spacer, and inlet/outlet ports. The packaged micropump was positioned vertically in a bath of working fluid; the generated static pressure and electric current were measured in the absence of fluid flow at different applied voltages. Three different dielectric liquids, 10-GBN Nynas and Shell Diala AX transformer oils, and N-hexane were introduced as the working fluids. The micropump was operated at the range of applied voltages between 300 and 1500 VDC. Maximum pressure generations up to 100 Pa was achieved. In order to estimate the space charge density distribution and the spatial variation of the electric field in the micropump, and to further verify the experimental results, a numerical simulation was also performed. The pressure head generation was predicted numerically and compared with experimental results at different applied voltages. The comparison between the experimental and numerical pressure generations shows good agreement.

Electrostatic precursor films

When a liquid spontaneously spreads over a solid surface, a progressive microscopic structure, conventionally known as a van der Waals driven precursor film, develops ahead of the moving contact line. Here, we report a new class of electrostatically assisted precursors containing microscopic charged particles. This precursor manifests itself at the late stage of forced-spreading of a macroscopic dielectric film subjected to a unipolar ionic discharge in a gas containing particulate. We put a model forward to predict dynamic behaviour of this electrostatic precursor. The spreading of the precursor film is predicted to be proportional to the square root of the exposure time, which is consistent with the ellipsometric measurements.

Two-Phase Cooling Characteristics of Mono-Dispersed Droplets Impacted on a Heated Surface

Droplet impact cooling has been shown to be a promising method for high heat flux removal applications. Recent experimental studies have revealed that even higher heat transfer at low mass fluxes and low Weber number can be achieved with only few degrees of superheat. In the present work, mono-dispersed droplet cooling of a horizontal upward facing heated surface was investigated at low Weber numbers. The impact velocity and frequency of free falling stream of droplets were varied dependently through changing the gap between the heated surface and tip of different capillaries and variation of volumetric flow rate (0.5–4.7 cc/min).The range of impact velocity and droplet frequency was ranged between 0.28 to 1.3 m/s and 0.5 Hz to 5 Hz, respectively using different capillaries size between 17g to 22g. The coolant was 25°C deionized water and all the experiments were performed at atmospheric pressure. The time-averaged two-phase characteristic curves were obtained up to Critical Heat Flux (CHF)-regime. Through the extensive set of experiments, two separate correlations are proposed to predict the average CHFs based on the Weber between 3<We<10, 10<We<100 and Strouhal number ranged and 6.35×10−3 <St<3.88×10−2 1.81×10−3 <St<3.86×10−2 , respectively. The correlation predicts the average CHFs with absolute errors less than 20% and 25%, respectively.Copyright

Entropy Generation Analysis for Mono-Dispersed Droplet Cooling at Critical Heat Flux Regime

In the present work, an analytical expression for entropy generation production for mono-dispersed stream of droplets impacted on the upward heated surface at critical heat flux regime was found. The expression involving the relevant parameters for entropy production contributed to the heat transfer and the entropy flux due to the phase change and the spreading process. The influence of impact velocity and flow rate on entropy generation was explored.Copyright