Characterization of Droplet Injection Process of a MicroinjectorF.-G. Tseng, C.-J. Kim and C.-M. HoMechanical and Aerospace Engineering DepartmentUniversity of California, Los Angeles C. Shih Mechanical Engineering DepartmentFAMU-FSU College of EngineeringFlorida A&M University and Florida State University

This Work is Partially Supported by the Office of Naval Research

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Outline

• MEMS/Micro-injector System

• Design Issues

• Virtue Chamber Neck Concept

• Droplet Characterization
  • Visualization, PIV and PDPA

• Summary

Micro-Electro-Mechanical Systems

• Mechanical/Electrical Devices made by microfabrication techniques

• Miniature in Size (< 1mm)

• Readily Integrable with electronic control system

• Can be massively produced using batch process to lower per unit cost

• Successful examples
  • Air-bag release sensor
  • Inkjet printer

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Applications of a Micro-Droplet Injector

• Inkjet Printing

• Distributed Micro-injector Array

• Inject micro fuel droplets

• Reduce evaporation time

• Improve fine scale mixing

• Spatial & temporal perturbations

• Modify large scale vortices

• Large array

• High frequency response

• High spatial resolution

• High droplet quality

• Micro Drug Delivery Device

• Precise dosage control

• Integrated with MEMS

• Local treatment

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Operational Principle of a Thermal Bubble Injector

ejected droplet

heater

bubble

Chamber neck

liquid

• Electric current pulse vaporizes liquid to form bubble

• Bubble functions as a pump ejecting droplets

• Bubble collapses and chamber is refilled by capillary force

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Design Issues of a Thermal Bubble Injector

• Frequency response is restricted by the presence of chamber neck

• Heat loss to substrate

• Cross talk and overflow

• Satellite droplet formation

• Ink Puddle formation

• Multi-layer packaging is required

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Satellite Droplets

Droplet ejection sequence

Sample straight-line printing

HP Printhead

Puddle Formation

HP Printhead

Microinjector with 30 mm Nozzle

Puddle interferes droplet ejection

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Fabrication Process Comparison

• Monolithic fabrication,

accuracy ~ 1mm

• No need of bonding process,

• Can be fabricated in high

spatial resolution, 1200 dpi

• 3 different substrates,

accuracy > 5mm

• Needs bonding Process,

• Spatial resolution depends on

bonding process~ now 300 dpi

Microinjector

Commercial Inkjet Printhead

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Packaging

Front view of a microinjector array

with temperature sensors

Back side liquid entrance slot

Microinjector array packaged

on a PC board

Pipe connection at the backside

of PC board

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MEMS Fabricated Micro-injector Array

Top view of a completed

micro-injector array

Chamber inner structure

• 300 nozzles in an array

• Integrated circuits for spatial and temporal injection sequence control

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Virtue Chamber Neck Concept

(patent pending)

Virtual chamber neck

formation sequence

(d)

Bubble collapses

& liquid refills

Liquid

refill

• Reduced cross talk

• No satellite droplets

• High frequency response (ᡖ kHz)

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Droplet Visualization

CCD

Camera

Microscope

LED

Microinjector

signalA

signalB

To VCR or computer

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Satellite Droplet Formation

Nozzle: 40 mm

Droplet: 45mm

Frequency: 1 kHz

Speed: 6 m/s

Nozzle: 60 mm

Droplet: 50 mm

Frequency: 1 kHz

Speed: 10 m/s

MEMS Micro-injector

HP 51626A Printhead

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Droplet Trajectory Visualization

(using strobe-light back-illumination)

No Satellite

Droplets

HP 51626A

Microinjector with 30 mm nozzle

Two streams

One stream

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Laser

Laser sheets

Digital

Camera

Instantaneous

Velocity Field

Multiply exposed

Digital Images

D

x

Time between two consecutive

laser pulses:

D

t

u =

D

x /

D

t

FFT

Auto/Cross

Correlation

Bragg cell

Particle Image Visualization/Velocimetry Setup

Micro-injector

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Multiple Droplet Injection

Splash ejection due to

puddle formation?

Multiply-exposed Particle Images

Main Droplet Stream

Satellite

Droplets

Primary

Droplet

Dt=0.5 ms, DT=15.2 ms

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Real-time

Signal Processor

PMT-A

PMT-B

PMT-C

f: phase shift

T=1/f, f:Doppler shift

Particle velocity V ~ fDoppler

Particle diameter. D ~ f

PMT-A

PMT-B

Micro-injector

Phase Doppler Particle Analyzer (PDPA)

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PDPA Size/Velocity Correlation

X=4 mm

X=4.8 mm

Main droplet

Satellite droplet

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PDPA Data (Main droplet only, x=4.8 mm)

Size/Velocity Correlation

Velocity time variation

• Velocity distribution shows bi-stable mode

• Particle sizing distribution shows greater but random scatter

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Summary

• No satellite droplets

• Fast frequency response (ᡖ kHz)

• Reduced cross talk

• Reduced heat loss to substrate

Microinjector with Virtual Neck & Embraced Heater Design

Characterization of the droplet injection process

• Injection splash near nozzle

• Bi-stable velocity distribution

• Detailed comparison between the inkjet printhead and the MEMS microinjector