PDMS Replica Molding and Bonding | Fluid Mechanics Lab Tutorial

Micro PIV (Micro Particle Image Velocimetry) is a non-invasive optical measurement technique for characterizing microscale flow features, utilizing seeded fluorescent particles illuminated by laser pulses with nanosecond/microsecond durations and processed through cross-correlation algorithms to quantify flow velocity and provide qualitative flow visualization in microfluidic systems.

Droplet microfluidics enables nanoliter-to-picoliter reactions by using water-in-oil droplets as isolated reaction containers, overcoming limitations of traditional microplates (evaporation, handling precision, surface absorption). Key applications include protein crystallization, PCR, and digital PCR. Multiphase CFD extends single-phase modeling by incorporating interfacial dynamics: surface tension forces, interfacial pressures, and moving boundaries. The level set method tracks interfaces using a continuous function φ that varies from 0 to 1 across the interface. Four primary interface tracking methods exist: Front Tracking, Volume of Fluid, Conservative Level Set, and Phase Field, each with trade-offs between accuracy, computational cost, and topology handling. Core droplet operations include: (1) Generation via squeezing/dripping regimes—squeezing uses geometry for monodisperse drops, dripping depends on capillary number; (2) Mixing through T-junctions or cross-junctions with bidirectional shear; (3) Merging using hydrodynamic pillars where continuous phase drains away before coalescence; (4) Splitting via geometrically mediated forks with nonlinear resistance (resistance ∝ length³ for rectangular channels).

Particle Image Velocimetry (PIV) is a powerful experimental technique for characterizing microfluidic systems by tracking particle motion to infer fluid velocity fields. The methodology involves illuminating seeded particles within the flow field using laser light sheets and capturing images with high-speed CCD cameras (typically 0.4 microsecond timing resolution). Image processing algorithms analyze particle displacement between consecutive frames to calculate velocity vectors. For microfluidic applications, PIV can resolve droplet diameters ranging from 4 to 100 micrometers and accurately measure droplet velocities in complex flow configurations. This technique enables detailed analysis of droplet breakup, coalescence, and impingement behaviors that are difficult to observe with conventional visualization methods.

Microfluidic chips generate water droplets in oil through precise flow control using syringe pumps. Oil fills lateral channels while water enters centrally, with surfactants preventing droplet coalescence. High-speed imaging reveals oil pinching water streams hundreds of times per second. Microdroplets offer significant advantages: they provide small enclosed volumes for easy manipulation, enable rapid chemical reactions due to high surface-to-volume ratios, allow concentration variation between droplets via flow rate adjustments, and serve as carriers for cells or particles. These features make microdroplets valuable for research experiments and industrial applications requiring precise control of small-volume reactions.

Microfluidics manipulates fluids at the micro scale (1-100s micrometers) to enable precise deterministic control over individual droplet formation. Three main geometries exist: T-junction (dispersed flows perpendicular to continuous, pinching off uniform droplets), flow focusing (continuous flows from two sides squeeze dispersed phase through narrow orifice for smaller, more uniform droplets), and co-flowing (both phases flow coaxially for core-shell structures). Droplet formation physics involves competition between interfacial tension (minimizing surface area) and viscous drag forces, quantified by the capillary number Ca = μv/γ. Droplet diameter is proportional to the flow rate ratio of dispersed to continuous phase. The coefficient of variation (CV = σ/μ × 100%) quantifies monodispersity, with microfluidics achieving CV < 2% versus 20-50% for traditional methods.

Complete microfluidic systems require integrated hardware components including color sensors, sampling bottles, and syringe pumps. TCS 34725 RGB color sensors with infrared filters detect chemical reactions through color changes, communicating via I²C protocol. Sampling bottles are constructed from 50ml amber glass bottles with drilled caps, stainless steel tube inserts, and silicone seals connected to aquarium one-way valves. Syringe pumps use 12V linear actuators with 100mm stroke, sliding potentiometers for feedback, and 3D-printed mechanical structures. L298N motor drivers provide cost-effective control for these pump mechanisms.

3D printing enables fabrication of functional microfluidic components including membrane valves (46 micron diameter), squeeze valves (15 micron active area), and displacement pumps. Membrane valves use elastic deflection for fluid control, while squeeze valves employ collapsing membranes. High-aspect-ratio channels solve laminar flow mixing challenges through diffusion across short distances (15 microns). Squeeze pumps achieve flow rates comparable to human sweat glands (8 nanoliters/minute). These integrated components demonstrate the transformative potential of 3D printing for creating sophisticated microfluidic systems.

Microfluidic point of care devices can efficiently monitor population immunity status to determine booster vaccination needs, detecting pathogens like MRSA and influenza. Despite this potential, microfluidics faced 20-year barriers including perceived inaccessibility, lack of scalability, and limited know-how. The fluidic periphery was historically bulky and expensive, preventing realization of microfluidic benefits. Today, components including piezo pumps, shape memory alloy valves, fluidic chips, and sensors are available and ready for integration. Components from multiple manufacturers can be combined into compact systems the size of a 96-well plate, assembled within 30 minutes. These systems enable smart, automated, wireless-controlled fluid handling with low power consumption, reagent savings, and low cost (hundreds of euros).

A complete microfluidic emulsion generation platform integrates multiple components: pressure controllers for pressurizing continuous, shell, and core phases; inline filters (10 μm for continuous phase, 2 μm for shell/core phases) to prevent clogging; flow meters for measuring flow rates; valves for starting/stopping the system and switching between reservoirs; and the Raydrop device itself. Optional integration with microphotonic chips (such as Chip Chop or Micronite) enables further analysis. This integrated approach simplifies laboratory setup compared to traditional spaghetti-like tubing arrangements.

Organ-on-a-chip is an innovative biomedicine technology consisting of devices approximately the size of a USB drive that imitate human organ structure and function. These devices contain microchannels and chambers populated with living human cells through which nutrient medium flows to simulate blood circulation. The technology reproduces key physiological conditions including controlled nutrient flow, intercellular interactions in three-dimensional space, and dynamic processes like respiratory movements, cardiac contractions, and intestinal peristalsis. This technology addresses three major problems: the 90% failure rate of drugs in human clinical trials due to species differences, limitations of traditional two-dimensional cell cultures, and ethical concerns of animal testing. The 2023 FDA Modernization Law approved alternative methodologies, with the first liver-on-a-chip application accepted in 2024. Main applications include preclinical research, toxicological testing, academic disease research, and personalized medicine. Specific organ models include lung-on-a-chip for respiratory diseases, liver-on-a-chip for hepatotoxicity and drug metabolism, gut-on-a-chip for absorption and inflammatory diseases, and placenta-on-a-chip for pregnancy complications.

Organ-on-a-chip technology can be extended to create multiple organ chips including liver, kidney, and heart chips. These can be combined to create a comprehensive human body-on-a-chip system. Such platforms have applications in drug screening, toxicology testing, and pharmaceutical development, potentially revolutionizing how new drugs are tested and approved.

A laboratórium-on-a-chip (LOC) eszközök olyan hordozható diagnosztikai eszközök, amelyek a hagyományos laboratóriumi vizsgálatokat valósítanak meg egy nagyon kicsi csipen (általában néhány négyzetcentiméter méretű). Ezek az eszközök lehetővé teszik, hogy a laboratóriumi vizsgálatokat bármilyen körülmények között el lehessen végezni, nem csak a központi laborokban. A LOC eszközök alapvetően laboratórium egy csipen eszköznek tekinthetők, amelyek kémiai vagy biológiai vizsgálatokat végezhetnek. A csip mérete néhány négyzetcentiméter, de az emberi laboratóriumi műveleteket lekicsinyítik erre a méretre, hordozhatóvá és gyorsabbá téve a laboratórium meghatározást.

Lab-on-a-chip (LOC) integrates various laboratory functions onto a single microchip for chemical or biological analysis, enabling rapid and precise testing of small sample volumes with high sensitivity and specificity. LOC devices consist of channels, chambers, and valves etched or fabricated on chips using microfabrication techniques. Advantages include small size, low cost, and ability to automate laboratory processes. Applications span biomedical research (drug screening, cellular analysis, genomics), clinical diagnostics (point-of-care testing, infectious disease detection, personalized medicine), environmental monitoring (water quality, air pollution, soil contamination), and food safety testing (pathogen and contaminant detection).

Organ-on-chip technology adapts semiconductor fabrication methods used to manufacture computer chips (like those in iPhones) to create integrated biochemical circuits. These microfabricated chips are dime-sized and can accommodate structures ranging from nanometers to microns (similar to cell sizes). The technology enables precise placement of features, scalability for mass production, and integration of multiple functions including nutrient delivery, waste removal, mechanical stimulation, and real-time monitoring capabilities.