Non-encapsulated thermo-liquid crystals for digital particle tracking thermography/velocimetry in microfluidics

Rodrigo Segura,Sonja Joseph,Massimiliano Rossi,Christian J Kähler,Christian Cierpka,Heike Bunjes

doi:10.1007/s10404-012-1063-y

Abstract

The ever accelerating state of technology has powered an increasing interest in heat transfer solutions and process engineering innovations in the microfluidics domain. In order to carry out such developments, reliable heat transfer diagnostic techniques are necessary. Thermo-liquid crystal (TLC) thermography, in combination with particle image velocimetry, has been a widely accepted and commonly used technique for the simultaneous measurement and characterization of temperature and velocity fields in macroscopic fluid flows for several decades. However, low seeding density, volume illumination, and low TLC particle image quality at high magnifications present unsurpassed challenges to its application to three-dimensional flows with microscopic dimensions. In this work, a measurement technique to evaluate the color response of individual non-encapsulated TLC particles is presented. A Shirasu porous glass membrane emulsification approach was used to produce the non-encapsulated TLC particles with a narrow size distribution and a multi-variable calibration procedure, making use of all three RGB and HSI color components, as well as the proper orthogonally decomposed RGB components, was used to achieve unprecedented low uncertainty levels in the temperature estimation of individual particles, opening the door to simultaneous temperature and velocity tracking using 3D velocimetry techniques.

Highlights

Heat transfer problems are commonplace in modern science and engineering and their solutions have led to a long list of technological developments, many of which have played a major role in transportation, communications, consumer electronics and personal computing, amongst many other areas that affect our lives and society
The uncertainty of the temperature estimation increases with the RGB calibration approach for temperatures beyond the nominal temperature response range of the Thermo-liquid crystal (TLC) material (35 °C), surpassing the uncertainty values provided by the proper orthogonal decomposition (POD) approach, and the HSI data yields uncertainties of 1-7 % across a range of 25-37 °C
The only other reports of temperature measurements in individual tracers were written by Park et al (2001) and Basson and Pottebaum (2012) where they reported uncertainty values more than twice as high as those obtained in this study, though the confidence interval is unknown for Park et al (2001) results

Summary

Introduction

Heat transfer problems are commonplace in modern science and engineering and their solutions have led to a long list of technological developments, many of which have played a major role in transportation, communications, consumer electronics and personal computing, amongst many other areas that affect our lives and society. The major drawback of this method is that in order to be able to reliably measure Brownian motion, the particles have to be very small, substantially decreasing their signal. Their size distribution must be very narrow and the viscosity of the fluid should not change if a consistent relationship between Brownian motion and temperature is to be established. Besides these techniques, liquid-crystal thermography has received great attention over the last several decades due to its capability to quickly reconstruct temperature fields in surfaces and flow planes/ volumes. Thermochromic liquid crystals (TLCs) are substances whose molecular structure has both solid and liquid properties, the name liquid crystal, that change their color as a function of temperature (Fergason 1966; Adams et al 1969; Parsley 1991)

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Conclusion