Mapping three-dimensional temperature in microfluidic chip

Abstract

Three-dimensional (3D) temperature mapping method with high spatial resolution and acquisition rate is of vital importance in evaluating thermal processes in micro-environment. We have synthesized a new temperature-sensitive functional material (Rhodamine B functionalized Polydimethylsiloxane). By performing optical sectioning of this material, we established an advanced method for visualizing the micro-scale 3D thermal distribution inside microfluidic chip with down to 10 ms temporal resolution and 2 ~ 6 °C temperature resolution depending the capture parameters. This method is successfully applied to monitor the local temperature variation throughout micro-droplet heat transfer process and further reveal exothermic nanoliter droplet reactions to be unique and milder than bench-top experiment.

Figure 1

(a) Synthetic Route of RAP: (i) PDMS polymerization through platinum-catalyzed hydrosilylation reaction; (ii) The vinyl group of AGE reacted with the silyl (SiH) group of PDMS chain through platinum-catalyzed hydrosilylation reaction; (iii) The epoxy group of AGE-PDMS reacted with carbonyl group of RhB through ring open reaction. The inset is a schematic illustration of RAP molecular chain: PDMS main chain with RhB grafted on the side chain and a snapshot of the RAP. (b) Fluorescence emission of spectra of RAP with different AGE concentration in wt%. (c) FT-IR spectra of PDMS, RP and RAP obtained from optimized conditions.

Figure 2

(a) Top and schematic cross-section views of microfluidic chip for droplet capture. (b) Fluorescence images taken from confocal microscope under the 5.3 nL droplet when laser is off and on.

Figure 3. Temperature distributions along different dimensions.

(a) Temperature distribution along AB line in the inset. Black line is experimental result. Red line is smoothed data by adjacent-average. The inset is temperature map (xy plane) under 5.8 nL droplet when laser is on. (b) Temperature along CD line as a function of distance to 2.8 nL black ink droplet. Black and red lines are experimental and simulation results respectively. The lower inset is thermal map (xz plane) under 2.8 nL droplet. The upper inset is a snapshot of 3D temperature distribution of 2.8 nL in simulation. (c) Variation of central temperature (solid line) and relative fluorescent intensity (dash line) along time for 2.8 nL (blue line) and 5.3 nL (red line) black ink droplet.

Figure 4

(a) Top view of the chip for droplet trapping and merging. (b) Variation of temperature during 3 nL NaOH droplet merging with 3 nL H₂SO₄ droplet. Red line is smoothed data by adjacent-average. Three insets (1 ~ 3) were taken via focusing on the droplet.