On-demand light-driven release of droplets stabilized via a photoresponsive fluorosurfactant

The study addresses the challenge of selectively releasing targeted droplets from passive microfluidic traps after long-term monitoring in high-throughput droplet microfluidic assays. Existing trapping is mature, but selective retrieval typically requires complex chip designs (e.g., pneumatic valves, embedded photothermal materials) or sophisticated flow control that scale poorly with trap number. The authors propose using a photoresponsive fluorosurfactant composed of fluorinated plasmonic nanoparticles that generate on-demand vapor bubbles under 532 nm laser illumination. The bubbles impart momentum to trapped droplets to achieve selective release in both hydrodynamic and floating traps. The goal is to enable rapid, simple, and scalable droplet release while minimizing fabrication complexity and control overhead, thereby improving downstream workflows such as sequencing and iterative evolution.

Droplet traps are commonly categorized as passive (hydrodynamic traps based on hydraulic resistance; floating traps exploiting density differences) and active (e.g., electric fields). Selective release strategies include: (1) breaking pressure balance by adjusting hydraulic or Laplace pressures to move droplets forward or backward; (2) actuating pneumatic valves; and (3) using light-induced bubbles via photothermal effects. Pressure-based and valve- regulated releases demand complex fabrication and individualized control channels, which scale poorly. Light-induced bubble methods reduce controller complexity because a movable laser can address traps individually; however, they often require integration of photothermal materials (e.g., aluminum patches or layers) at each trap, complicating fabrication, or they rely on UV pulsed lasers to heat pure water, which is inefficient and slow (~15 min per release). Common fluorosurfactants (PFO, PFPE-PEG, PFPE-Tris) stabilize droplets but offer no photothermal functionality. Prior work from the authors introduced a fluorinated plasmonic nanoparticle (f-PNP) fluorosurfactant enabling rapid oil vaporization and droplet motion under 532 nm illumination. This study extends that capability to selective release from passive traps with faster response and simpler chip requirements than earlier light-induced approaches.

Photoresponsive fluorosurfactant synthesis: Fluorinated Au–SiO2 core–shell nanoparticles (f-Au@SiO2) were prepared via three steps. (1) Gold nanoparticle cores (~13 nm) were synthesized by reducing HAuCl4 with trisodium citrate in water. (2) A ~5 nm silica shell was formed by condensing Na2SiO3 using APTMS as a primer. (3) The silica-coated particles were functionalized with PFOTES to impart fluorophilicity. The extinction peak was measured at 524 nm. Nanoparticles were weighed and redispersed in HFE-7500 at 1% w/w (also compatible with FC-40 and FC-70).
Fluorescence-activated droplet release (FADR) system: A custom dual-laser microscope platform was built. A 532 nm laser (21.6 mW after the objective, ~6 µm beam waist) provided release actuation; an AOM served as a high-speed shutter (rise time ~25 µs). A 488 nm laser (2.4 mW, ~10 µm beam waist) excited laser-induced fluorescence (LIF). Emission was split to a PMT for quantification and a high-speed camera for visualization. A motorized stage positioned the microfluidic device to target individual droplets. Fluorescence signals were sampled at 1 MS/s; an intensity threshold (computed from average intensities of all droplets) was used to binarize and trigger release. For a 5×5 array, processing time was <1 ms.
Trap design: Hydrodynamic traps comprised a main flow channel and a parallel trapping channel with a central trap region flanked by two narrow channels. Hydraulic resistance followed Hagen–Poiseuille scaling with geometric correction C(α). Geometry was engineered so Rm/Rt > 1 to direct droplets into the trap region; two narrow channels reduced overall resistance and prevented premature bubble escape. Traps of 60 µm diameter were optimized for 50 µm droplets in a single-layer device of 55 µm height. Floating traps used a two-layer design: a 65 µm bottom flow layer and a 55 µm upper trap layer with a 60×60 µm trap region for 50 µm droplets.
Device fabrication: Standard PDMS soft lithography was used. SU8-3050 was spin-coated on 4-inch silicon wafers to define heights; UV exposure through film masks patterned single-layer hydrodynamic traps, while double exposure with alignment marks defined floating traps. PDMS (10:1 base:curing agent) was mixed, degassed, cast, and cured at 70 °C for 2 h, then peeled, inlet/outlet holes punched, plasma-bonded to glass, and post-baked at 80 °C overnight.
Operation: 50 µm W/O droplets stabilized by f-Au@SiO2 were generated via flow-focusing (per prior work) and injected into traps at 0.5 µL/min. The continuous phase was then exchanged to pure HFE-7500 via a side channel at 5 µL/min to ensure photothermal activity localized at droplet interfaces. Hydrodynamic trapping was robust over 0.5–10 µL/min; floating traps required lower flow (<1 µL/min) to allow buoyant migration to the trap layer. Excess droplets were flushed at 20 µL/min. During release tests, HFE-7500 flowed at 1 µL/min. For hydrodynamic traps, the 532 nm beam was focused on the inner interface of the trapped droplet to push it back into the main flow. For floating traps, the 532 nm beam was focused near the upper droplet interface (laser entering from below) so the expanding vapor bubble would push droplets downward into the flow layer.

• f-Au@SiO2 at droplet interfaces produced rapid photothermal heating under 532 nm illumination, vaporizing the fluorocarbon oil and nucleating a vapor bubble at the interface within ~0.15 ms.
• In hydrodynamic traps, 21.6 mW laser power with 50 ms illumination generated bubbles large enough to push 50 µm droplets out of the trap and into the main channel. Bubble growth scaled approximately linearly with illumination time (bubble volume vs. time).
• In floating traps, focusing on the upper droplet interface produced a bubble that drove droplets downward into the bottom flow layer; an illumination time of ~5 ms yielded >95% release efficiency. Proper axial focus was critical; mispositioning could generate two bubbles (above and below) and reduce reliability.
• Flow-rate dependence: Hydrodynamic-trap release efficiency decreased at higher oil flow rates due to increased opposing hydraulic pressure. Floating-trap release efficiency was minimally affected by flow rate.
• Through FADR, fluorescence thresholds (from 488 nm LIF) were computed and binarized with <1 ms processing for a 5×5 array, enabling automated selective release.
• Overall release speeds were faster than previously reported light-induced bubble approaches: single-event release times of ~50 ms (hydrodynamic) and ~20 ms (floating) were demonstrated, at relatively low laser power (21.6 mW), without integrating photothermal layers into the chip.

The work demonstrates that a photoresponsive fluorosurfactant can both stabilize W/O droplets and serve as an on-demand local heater to generate interfacial vapor bubbles that impart momentum for selective release. This decouples selective release from complex chip architectures (e.g., pneumatic valve networks or embedded photothermal patches) and from global flow manipulations, thereby simplifying scaling to large trap arrays. The approach is compatible with both hydrodynamic and floating passive traps, broadening applicability. The observed linear bubble growth with illumination time enables predictable control of momentum transfer via laser pulse duration, while the low laser power requirement reduces thermal load compared with UV-based water heating. Automated FADR closes the loop between fluorescence-based selection and release, which is key for high-throughput screening workflows such as isolating droplets with desired reaction outcomes. Sensitivities to flow conditions (in hydrodynamic traps) and beam positioning (in floating traps) define practical operating windows but do not negate the core advantages in speed, selectivity, and ease of fabrication.

A plasmonic, photoresponsive fluorosurfactant (f-Au@SiO2) enables on-demand, light-driven selective release of trapped aqueous droplets in fluorocarbon oils. By generating interfacial vapor bubbles under 532 nm illumination, droplets are rapidly released from both hydrodynamic (≈50 ms) and floating (≈20 ms event; ~5 ms illumination, >95% efficiency) traps. The method eliminates the need for on-chip photothermal layers or complex valve networks and integrates seamlessly with a fluorescence-activated droplet release (FADR) system capable of automated selection with sub-millisecond processing for arrayed traps. This platform promises simpler scaling for large droplet screening campaigns. Future work could optimize laser-beam positioning algorithms, expand to different oils and droplet sizes, quantify thermal impacts on sensitive bioassays, and integrate with downstream collection and analysis modules for closed-loop workflows.

• The approach requires the use of a specific photoresponsive fluorosurfactant (f-Au@SiO2) and an oil-phase exchange to pure HFE-7500 to localize photothermal effects, which may not be compatible with all assays or oils.
• Release efficiency in hydrodynamic traps decreases at higher continuous-phase flow rates due to counteracting hydraulic pressure, limiting operating throughput in that mode.
• In floating traps, precise axial focusing is required to generate bubbles at the desired interface; misalignment can produce multiple bubbles and reduce reliability.
• Potential localized heating at the droplet interface is intrinsic to the mechanism; while low-power and fast, thermal effects on sensitive biomolecules were not quantified here.
• Only selected trap geometries and droplet sizes (≈50 µm) were tested; generalizability across broader designs and sizes remains to be validated.