Optomicrofluidic measurement of particle-encapsulated droplet system

Abstract

Droplet microfluidics combined with optical detection has become a powerful approach for high-throughput single-cell assays, but these systems often face limited sensitivity and signal heterogeneity due to optical and geometrical constraints. We investigate how key operating parameters influence the performance of a droplet-based optomicrofluidic platform. Experiments examine optical interactions between guided light and aqueous droplets containing fluorescent (FL) particles flowing in oil. Geometrical optics simulations model light-droplet interactions, while FL simulations quantify signal variations caused by particle size and position. Two refracted signals are observed experimentally: a droplet-refracted signal (DRS) that scales with droplet diameter and a particle-refracted signal (PRS) produced by light interaction with encapsulated particles. Both experiments and simulations show that PRS becomes prominent when the particle-to-droplet size ratio $D^*_\text{p}$ lies between 0.23-0.33, enabling label-free detection. Particles near the droplet center ($r^*_\text{p} < 0.4$) display reduced angular dependence and more uniform FL signals. Simulations further show that FL intensity increases with $D^*_\text{p}$, rising sharply from 0.33 to 0.5 and more gradually up to 0.66. Additionally, reducing the oil layer thickness enhances fluorescence by minimizing optical losses at the droplet-channel interface. These results demonstrate that controlling $D^*_\text{p}$, particle position, and oil layer thickness improves FL strength and uniformity, providing a framework for optimizing droplet-based fluorescence detection in microflow cytometry and single-cell assays.

Figures (12)

• Figure 1: (a) Schematic of the optomicrofluidic experimental setup showing the droplet encapsulation and optical detection regions, (b) Experimental image of the flow focusing junction used for droplet generation, (c) Photograph of the optomicrofluidic device with the excitation and collection fibers.
• Figure 2: (a) Model geometry of the optomicrofluidic device with a droplet, fluid channel, and excitation/collection optical fibers used for the numerical simulations, (b) Model showing the positions of ray release from a hexapolar grid within the core of a singlemode fiber (SMF), (c) Geometry of the excitation beam profile which is measured to be 7.82 µ m, (d). Excitation profile plotted across the optical beam, fit to a Gaussian $f(x) = 0.78~e^{-r^2/(2\sigma^2)}$ where $\sigma = 3.49$ with $R^2 = 0.88$.
• Figure 3: (a) Images of the droplets generated at varying $Q_\text{r}$, (b) corresponding droplet refracted signal acquired. The dashed line indicates missing data due to real-time data acquisition limitations, (c) Correlation plot for droplet refracted signal height with droplet diameter. Inset: Oil layer thickness decreases with increasing droplet diameter. Error bars indicate ±SD. (d) Simulation studies show the particle-refracted signal increases with increasing $D^*_{\text{p}}$, i.e., smaller droplet size at $\theta_{\text{p}} = 90\degree$ with fixed particle diameter of 15 µ m and a fixed 1.5 µ m gap between the particle surface and the droplet boundary.
• Figure 4: Comparison of experimental and simulated droplet-refracted signals at varied positions. (a-i) Ray trajectory of the excitation source at droplet position $g_\text{o}$ = -50 µ m, (a-ii) Ray trajectory at $g_\text{o}$ = 0 µ m, (a-iii) Ray trajectory at $g_\text{o}$ = 50 µ m, (b) Average accumulated intensity collected at 45$\degree$ (solid line) and 315$\degree$ MMF (dash line) for different droplet sizes, (c) Comparison of droplet refracted signals for varying droplet sizes between simulation and experiments. Experimental data points and simulated values exhibit linear and parallel trends indicating consistent scaling behavior. A calibration constant $k_\text{PD} = 2.4 \times 10^7$ was applied using the relation ${S_{\text{dr}}} = k_\text{PD} {I_{{\text{sim}}}}$. The blue line represents a linear fit to the simulated data using the equation $y = Ax + B$, where $A = 0.087$ and $B = -4.2$. Error bars indicate ±SD.
• Figure 5: Effect of droplet size and particle confinement on fluorescence.(a) Particle encapsulated droplets of varying size and corresponding droplet refracted signal (green) and particle fluorescence signal (red), (b) Normalized fluorescence intensity values for varying particle-to-droplet size ratio, (c) Comparison of simulated and experimental particle fluorescence for different particle configuration (P1-P7). Each data point corresponds to a unique droplet–particle configuration; see the Supplementary Material Supplementary_Material for $D_{\text{d}}$, $r_{\text{p}}$ and $\theta_{\text{p}}$ values. A calibration constant ${k_{\text{PMT}} = 3.01 \times 10^{-6}}$ was applied using the relation ${S_{\text{fl},{\text{sim}}}} = k_{\text{PMT}} {I_{{\text{fl}},{\text{sim}}}}$, (d) Simulated fluorescence intensity from 135$\degree$ MMF for varied particle-to-droplet size ratio with particle positioned at $\theta_{\text{p}} = 90\degree$ with fixed $D_{\text{d}}$ of 30 µ m.