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IFDL at WSUV investigates a variety of surface-tension dominated fluid phenomena including inkjet, droplet impact, wetting, coating. We use both theoretical modeling and experimental methods to understand various types of interfacial flows. IFDL is equipped with state-of-the-art equipment and simulation software to carry out research in microscopic flow area.

CFD Modeling of Microfluidic Flows

In this research, we are investigating the complex interfacial fluid flows through high-speed imaging system and numerical simulation. The outcome of study can help understand the fundamental fluid physics involved in various microfluidic devices and improve the performace of these devices. A Cartesian grid based adaptive-mesh-refinement (AMR) CFD code based on open source code Gerris has been developed to simulate the free surface flows in inkjet devices.  If you would like to know more details, please contact us.

The comparison of droplet ejection from experimental images and simulation
The comparison of droplet ejection from experimental images and simulation

Jet ejection following drop impact on textured surface

Droplet-wall interactions are well-known to generate a wide variety of outcomes such as spreading, splashing, receding, jetting, and rebounding. In this study, we focus on the evolution of jets that form during the recoil of impinging drops on partially wetting hydrophilic substrates composed of cylindrical micro-pillars. We find that the jetting phenomenon arises for certain ranges of Weber We and Ohnesorge Oh numbers. Within these ranges, the jet ejects one droplet at low We and multiple droplets at high We. Our study reveals that the jets observed in our experiments are generated by pure inertial focusing of radial flow at the point of the air cavity collapse.

Images of a typical jetting event for a droplet of water-glycerol mixture (55% of Glycerol, mass fraction)  impact on the hydrophilic micropillars substrate.
Jet regime map

Puddle Jumping from Hydrophobic Surface

The nearly step reduction in gravity arising in routine drop tower tests leads to numerous interesting large-length-scale capillary flow phenomena. For example, a liquid puddle at equilibrium on a hydrophobic substrate is observed to spontaneously jump from the substrate during such tests. We numerically investigate such puddle jump phenomenon for a variety of water puddles on flat substrates. We quantify a range of puddle jump characteristics including jump time, jump velocity, and free puddle oscillation modes for an unearthly range of drop volumes between 0.001 and 15 mL and substrate contact angles between 60° and 175°. A numerical regime map is constructed identifying no jump, standard jump, bubble ingestion, geyser formation, drop fission, and satellite puddle jump regimes. Favourable agreement is found between the simulations, experiments, simple theoretical models, and scaling laws.

20 Hz images of 2 mL distilled water puddle jump from substrate with θ = 150°: top row shows experiment results from drop tower test along with inset equilibrium top view. Bottom row provides numerical results.
Numerical puddle jump regime map: 0.001 ≤ V ≤ 15 mL and 60° ≤ q ≤ 175°. For V ≥ 7 mL the events of bubble ingestion, geyser formation, and fission occur simultaneously.

Self-powered Microfluidic Pump Using Diatom Films

In recent years, researchers have successfully applied diatom biosilica to molecular detection platforms including Surface-Enhanced Raman Scattering (SERS) optofluidic sensors that are currently capable of detecting a variety of biological and chemical molecules at concentrations as low as 10-10 M. This study investigates the feasibility of a SERS device that couples the sensing and pumping capabilities of diatom biosilica thin films by determining flow rate limitations and stability. In this study, we quantify the ability of porous diatom biosilica thin films to continuously pump deionized (DI) water from a reservoir via wicking flow by utilizing the strong capillary forces of the porous film coupled with evaporation. We demonstrated the capability of diatom biosilica thin films to produce steady and continuous flow over 48 hours due to their hydrophilic nature and that the flow rate can be controlled by manipulating the biosilica film area and heating temperature.

Microfluidic pump overview: (a) schematic, (b) pipette and capillary tube used in experiment (zoom-in), (c) diatom biosilica porous thin film.
Comparison of experimentally measured flow rate against theoretic predictions for different film areas and heating temperature

Droplet-Powder Interaction

Liquid droplet interaction with powder substrates is a ubiquitous phenomenon in nature and engineering applications. In recent years, various drop-on-demand inkjet technologies capable of precisely delivering pico-amounts of liquid have been adopted in a number of novel powder-based additive manufacturing processes. The interaction between micrometer-sized droplets and powder plays a significant role in the quality of products made by these 3D printing technologies. The task aims to gain fundamental knowledge of the influences of the interaction between micron-sized droplets and powders on the powder-based 3D printing process.

Figure: (a) Profiles of Droplet of DI water and 7.5 % isopropyl alcohol (drop diameter ~4 mm) on nylon powder substrate with porosity є = 0.28, 0.34and 0.4. (The equilibrium contact angle is affected by both surface tension and powder compaction). (b) droplet absorption by powders due to capillary force.

Figure: Numerical simulation of a 20-micron droplet impact on powders.

High Speed Imaging System for Microscopic Flows

In this project, we have developed a stroboscopic high speed imaging system that can be used to observe the droplet ejection of inkjet technology as well as fluid physics at micrometer scale. The stroboscopic high-speed photography relies on the repeatability of the fluid physical events. The key elements in the system will include a high resolution digital camera with a lens system capable of imaging a field of view of a few millimeters, connected to a PC for control and image storage, an inkjet printhead with drive electronics and data source, a very short duration (~20ns) flash light source with delivery optics, and delay generator to delay the flash relative to the initiation of the printing event and to measure that delay time accurately.

In-house high-speed imaging for inkjet process

Left: Formation of an oscillating satellite droplet during droplet-ejection process (interframe time of 5 μs). Right: The evolution of the droplet profile during droplet impact on the silicon wafer surface.