Hydrogel (beacon). pNIPAm and PEG-DA gels were used as SI structures and fabricated using the microscope projection lithography technique (54). An ultraviolet (UV) lamp was set to 30 mW/cm2 (measured at an empty objective slot). A 1000-μm diameter circular photomask was inserted into the field stop of an inverted microscope (Nikon TE-2000U) and aligned as described previously (55). pNIPAm/PEG-DA precursor solution was injected until the channel was filled. The syringe was disconnected, and 2 min was allowed for flow to relax. Then, 20-s (for pNIPAm gels) and 500-ms (for PEG-DA gels) UV exposures were used with a 10× objective to photopolymerize the gel. This step results in hydrogel posts with diameters 375 to 425 μm. The precursor solution was flushed from the device by flowing DI water for 2 hours for the PEG-DA gels. An NOA81 post was used as the inert structure in Fig. 3C and fabricated using the same procedure as with the PEG-DA beacons.

The pNIPAm gel was flushed with DI water for 24 hours for the control experiment, and only 10 min when some amount of residual photoinitiator was desired inside the gel. The microfluidic device was mounted on a thermal stage (Instec TSA12Gi) to control the temperature of the pNIPAm gel. The device was maintained at 50°C at the beginning of the experiment. Negative PS suspension at the same temperature was introduced into the device, flow was stopped using technique described in (35), and video recording (Andor iXon 885 fluorescence camera) was started with a 4× objective at one frame per second with 0.1 s exposure time. The temperature of the stage was then lowered to 25°C to allow the gel to cool down and cross the LCST. Video recording was stopped 30 min after the gel undergoes the transition.

Source-sink dipole. Two streams, one with 5 mM SDS in water and the other with clean DI water, were flowed simultaneously into the channel using the two inlets of the device. In this way, one of the beacons (the source) was loaded with SDS, while the other (sink) remained unloaded. After loading the source for 20 min, the two streams were flushed out by displacing with the suspension of negative PS particles. The flow was stopped, and video recording was started.

Multiple sources. Two streams, one with 5 mM SDS in water and the other with 5 mM [C6mim][I] in water, were flowed simultaneously into the channel. Both the sources were loaded for 20 min before flushing with a suspension of negative PS particles/decane drops. Flow was stopped, and image series were recorded.

Multiple sources with reaction. Two streams, one with CaCl2.2H2O (0.15 g/ml) in ethanol and the other with benzoic acid (0.4 g/ml) in ethanol, were flowed simultaneously into the channel. The two beacons were loaded for 10 min and subsequently flushed with NaOH (0.08 g/ml) in water and DI water, respectively. The NaOH reacts with CaCl2 to precipitate out Ca(OH)2 in the first beacon, while water reduces solvent quality for benzoic acid, resulting in benzoic acid crystal formation in the second beacon (Fig. 5C, inset). The two streams were then flushed by displacing with the PS suspensions. Flow was stopped, and image series were recorded.

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