Figure 1 shows the changes in GFs and morphology of a ∼ 80 µm droplet (at ∼ 90 % RH) consisting of 1, 2, 6-hexanetriol and AS with OIR = 1:1 during an RH cycle. For the dehumidification process, the droplet is first exposed to a high RH of ∼ 91.1 % with a GF of ∼ 1.50 at the beginning (time t=0). The particle remains as one single liquid phase, as shown in Fig. 1. As RH decreases, LLPS occurs at ∼ 79.3 % RH, detected by the sudden appearance of schlieren (small separated regions), which will be discussed in detail below. After that, two liquid phases are gradually formed, i.e., an inner AS solution phase and an outer organic-rich phase. The water release continues with decreasing RH, showing a continuous reduction in GFs. The core–shell particle undergoes a crystallization transition from ∼ 47.7 % RH to ∼ 47.2 % RH. However, the particle size continues to decrease due to the continuous water release by the outer organic-rich phase consisting mainly of aqueous 1, 2, 6-hexanetriol, which can absorb and release water continuously without any phase transitions during the whole RH cycle, as shown in Fig. S1 in the Supplement. Thus, we can conclude that the nucleation of all the mixed droplets is owed to the crystallization of AS. At very low RH, the smooth surface of the particle becomes irregular. Upon hydration, the outer phase begins to take up water even at very low RH, showing that the GFs increase gradually, similarly to the hygroscopic behavior of pure 1, 2, 6-hexanetriol. At ∼ 80.2 % RH, the GFs begin to increase abruptly, indicating the dissolution of the inner AS crystal, which can also be identified by the images. Indeed, determination of deliquescence relative humidity (DRH) is always prone to uncertainties, not like the SRH and efflorescence relative humidity (ERH), because the occurrence of AS crystal dissolution is not easy to be clearly judged from the images and the turning point of humidification curve, in view of the continuous water uptake by organic coating. At ∼ 83.4 % RH, the particle has been completely deliquesced, showing only one liquid phase. Above this RH, the GFs are slightly lower than those in the dehumidification process, which should agree with the weak volatility of 1, 2, 6-hexanetriol (Lv et al., 2019).

In addition, we determine the SRH, ERH and DRH of OIR = 1:1 particles with different particle diameters of 25–87 µm (∼ 90 % RH), as shown in Fig. S2 in the Supplement. The results show that the SRH does not depend on particle sizes. The DRH shows no dependency due to the large particle sizes (Gao et al., 2007; Ebert et al., 2002), considering that the DRH of particles smaller than 60 nm increases with decreasing particle sizes (Hämeri et al., 2001; Russell and Ming, 2002), while the ERH decreases slightly with the particle sizes, consistent with classical nucleation theory and earlier studies (Pant et al., 2004; Gao et al., 2006).

The optical images and corresponding illustrations for the same OIR = 1:1 particle during LLPS, secondary LLPS, efflorescence and deliquescence are depicted in Fig. 2. During LLPS, the particle first exists in homogenous mixed phase at ∼ 79.4 % RH, as shown in the first frame of Fig. 2. Note that both the bright globe in the center and the dark ring at the edge are owed to the optical effect of light scattering (Bertram et al., 2011). When the RH arrives at ∼ 79.3 %, the schlieren over the whole droplet appears suddenly, indicating the onset of LLPS by spinodal decomposition. It is noteworthy that the LLPS mechanism can also be judged from the temporal changes of the number of AS inclusions (Ciobanu et al., 2009; Song et al., 2012b), which will be discussed in detail in Sect. 3.4. Then, the dispersed clusters grow and coalesce, leading to the separated inclusions consisting mainly of AS solution, followed by the coalescence of these inclusions to form an inner AS solution phase at t=275.0 s. The large aggregation coexists with amounts of small AS inclusions, meaning the phase evolution continues until an equilibrium (Ciobanu et al., 2009). From t=275.0 to 307.0 s, the AS inclusions become bigger and merge into uniform AS solution phase. At ∼ 75.2 % RH, the equilibrium partitioning is reached and the morphology containing an inner AS solution phase linked to several inclusions and an outer organic-rich shell is presented. Importantly, a secondary LLPS occurs at ∼ 68.6 % RH, showing a brighter aqueous phase present in the center of inner phase. The new phase is attributed to more concentrated AS inclusions, as confirmed by our Raman spectra in next section, and becomes more visible with decreasing RH. This is because the inner AS-rich phase first contains small amounts of organics; then, the continuous water release causes a gradual increase in sulfate concentration in the inner phase, which ultimately results in the occurrence of secondary LLPS. Based on the phase rule, the degrees of freedom of the mixed system is zero in the case of coexistence of the three liquid phases during secondary LLPS. In the phase diagram of three pairs of partially miscible systems, the concentration of the three phases cannot be changed when the three phases coexist, but the relative content of the three phases can be changed according to the position of the system points in the phase diagram. For clarity, the concentrated AS inclusions are marked with different shades of aqua in the illustrations to indicate the degree of secondary LLPS. At 61.9 % RH, the central AS inclusions can be clearly distinguished. At t=735.0 s, an AS crystal appears at the edge of the droplet, indicating the onset of efflorescence at 47.7 % RH. The following crystallization of the AS phase and inclusions proceed until 47.2 % RH. Upon hydration, the solid AS crystals begin to dissolve at the DRH of ∼ 80.2 % and are deliquesced completely at ∼ 83.4 % RH. Note that the effloresced particle transfers into the homogenous mixed phase without LLPS after deliquescence because the DRH of AS crystals is above the SRH of the mixed particle.

To clearly illustrate the LLPS and secondary LLPS of mixed particles, Raman spectra acquired on the surface and at the center of the OIR = 1:1 particle are collected at constant RH. The dehumidification process is shown in Fig. 3a and humidification process for the effloresced particle is shown in Fig. 3b. Next to the spectra are the high-quality images corresponding to the same RH conditions from video microscopy, not the low-resolution images by the Leica DMLM microscope (Fig. S3 in the Supplement). As seen in Fig. 3, the O–H stretching vibration, ν(O–H), of liquid water is identified at 3170–3715 cm-1. The bands at 980 and 975 cm-1 are assigned to the symmetric stretching vibration of SO42-, νs(SO42-), in the solution and crystalline states, respectively. The band of C–H stretching vibration, ν(C–H), of 1, 2, 6-hexanetriol is observed at 2798–2995 cm-1. Based on these, the intensity ratios of the νs(SO42-) band to ν(C–H) band are determined and depicted in Fig. 3c to identify the component distribution of the mixed particle. For the dehumidification process, the particle first exists as only one liquid phase at ∼ 85.3 % RH, confirmed by almost identical intensity ratios of a1, a2 and a3. Then, two separated phases are presented at ∼ 77.0 % RH. It is clear that the intensity ratio at the center increases significantly, and the intensity ratio of a5 is much higher than that of a4 and a6, indicating the morphology of an AS solution phase surrounded by an organic-rich shell. Note that the signatures of both sulfate and organics can be observed in spectra a4–a6, suggesting there is a small amount of AS and 1, 2, 6-hexanetriol present in the organic-rich and sulfate-rich phases, respectively. As the RH decreases to 68.1 % and 58.2 %, the intensity ratio at the center increases to 6.07 and 8.17, respectively, about 2–4 times higher than that of a5, demonstrating the formation of more concentrated AS inclusions in the inner phase. When the RH increases to 76.3 % after the particle is fully effloresced, there is an AS crystalline phase in the center and an organic-rich phase in the shell, as identified by the spectra b1–b3. Notably, the band intensity of C–H in b2 is much higher than that in a5, a7 and a8, implying the presence of 1, 2, 6-hexanetriol in the veins of the AS crystal as discussed below. At 84.5 % RH, the intensity ratios of b4, b5 and b6 are almost the same, indicating the full deliquescence of the particle.

Figure 4a and b display the GF changes and morphological changes in an RH cycle for mixed 1, 2, 6-hexanetriol / AS particles with OIR = 1:4 and 1:2, respectively. The hygroscopic growth of the OIR = 1:4 particle follows an entirely different route from the OIR = 1:1 particle. First, the GF decreases in an approximately linear manner upon dehydration until ∼ 46.4 % RH. Then, the onset and end of efflorescence is in a very narrow RH range around 46.4 %, meaning a faster crystal growth due to the weaker transfer limitation of water molecules, in view of the thinner viscous organic-rich shell for the OIR = 1:4 particle. Finally, the particle size remains constant until 84.0 % RH and then increases steeply to the initial size. However, the DRH of the mixed particle is ∼ 80.2 %, as identified by the occurrence of AS crystal dissolution with almost unchanged particle size under ∼ 80.2 % RH shown in the optical images. Coupled with the morphological changes upon crystallization (Fig. S4 in the Supplement), we can conclude that aqueous 1, 2, 6-hexanetriol will enter into the veins of the AS crystal and then be enclosed by a crystalline AS crust; namely, the organics may be trapped within the AS crystalline phase after crystallization(Sjogren et al., 2007; Ciobanu et al., 2009). Likewise, Sjogren et al. (2007) reported that effloresced AS crystals could exist in the veins or could even be enclosed with organics. Ciobanu et al. (2009) introduced that liquid PEG-400 could be trapped by the solid AS for the PEG-400/AS particle with OIR = 1:2. Song et al. (2012b) found that the outer organic-rich phase was sucked into the cavities of the inner AS crystal due to capillary forces. Upon hydration, the GFs after full deliquescence overlap with those in the dehumidification process, suggesting almost no volatilization of 1, 2, 6-hexanetriol due to capture by the AS crystal. For the particle with OIR = 1:2, LLPS occurs at ∼ 73.4 % RH and ends at ∼ 73.2 % RH. After that, the GFs decrease gradually until 41.9 % RH, at which point a rapid reduction in GFs appears. Hence, the ERH is ∼ 41.9 %. Upon hydration, a gradually faster increase in GFs is apparent first, together with the transition of particle morphology from irregular to spherical-like. The dissolution of the inner AS crystal begins at ∼ 77.3 % RH, judged mainly from the optical images.

The dynamic processes of LLPS, secondary LLPS and efflorescence for the same OIR = 1:4 and 1:2 particles are shown in Fig. 5. In Fig. 5b, it is clear that LLPS for the OIR = 1:4 particle occurs by growth of a second phase from the particle surface over a wider RH range of 78.3 %–76.3 %. Importantly, the secondary LLPS occurs at ∼ 77.9 % RH, almost the same RH as the appearance of LLPS. Then, the degree of secondary LLPS increases gradually along with the water release from the inner AS solution phase. The size of the concentrated AS-rich phase in the center of the AS solution phase almost attains the size of inner phase at ∼ 50.7 % RH, owing to the very high sulfate fraction. The crystallization occurs at 46.4 % RH. After ∼ 2.0 s, the particle morphology becomes rough, meaning rapid crystal growth and trapping of organics into the cavity of the AS crystal. Then, the crystal growth continues, resulting in the formation of an AS crust (Fig. S4 in the Supplement). For the OIR = 1:2 particle, LLPS occurs at ∼ 73.4 % RH (t=211.0 s), showing that the schlieren appears suddenly. The growth and coalescence of these regions lead to the generation of AS inclusions. The inclusions further merge together and fade away, followed by the growth of a second phase from the rim of the particle starting at ∼ 219.0 s. Hence, LLPS is observed to occur by the first spinodal decomposition and then growth of a second phase from the particle surface. Here, the AS solution phases in different illustrations during LLPS are marked with different shades of blue to indicate the degree of LLPS. Indeed, the growth of a second phase is not always clearly distinguished from the images due to the optical effect, but it is clearly visible in movie S8 in the Supplement (i.e., the LLPS process of the OIR = 1:2 particle). A secondary LLPS occurs at ∼ 69.0 % RH. The number of concentrated AS inclusions increases with decreasing RH, along with the coalescence and growth of inclusions. At ∼ 41.9 % RH, the inner phase turns into crystalline AS phase.

The hygroscopic cycles for mixed particles with OIR = 2:1 and 4:1 are shown in Fig. 6. LLPS occurs at ∼ 73.6 % RH and ∼ 76.6 % RH, respectively. The efflorescence begins at ∼ 43.2 % RH and ∼ 50.6 % RH, respectively. For the humidification process, the OIR = 2:1 particle first absorbs water very slowly at low RH and then the size of particle increases abruptly above ∼ 50 % RH, whereas for the OIR = 4:1 particle, an appreciable size increase occurs even at very low RH (∼ 20 % RH), followed by a considerable and continuous size growth as RH increases, indicating a more similar hygroscopic property to pure 1, 2, 6-hexanetriol. Then, the DRH for the two particles are ∼ 74.9 % and ∼ 75.5 %, respectively, as identified by the occurrence of crystal dissolution from the images.

Figure 7 shows the LLPS, secondary LLPS and efflorescence processes of the OIR = 2:1 and 4:1 particles. For the OIR = 2:1 particle, LLPS occurs first by spinodal decomposition at ∼ 73.6 % RH. After that, another LLPS mechanism, nucleation and growth, is exhibited at about 72.9 % RH–71.7 % RH. The ultimate particle morphology from nucleation and growth commonly consists of spherical droplets of the minor phase, i.e., AS inclusions in this case, dispersed in the major phase, i.e., the organic-rich phase (Ciobanu et al., 2009). Crystallization of dispersed AS inclusions starts at ∼ 43.2 % RH and ends at ∼ 41.4 % RH. For the OIR = 4:1 particle, it is clear that the critical AS solution nuclei appear at ∼ 76.6 % RH, which further grow and coalesce into bigger spherical inclusions dispersed in the organic-rich phase until ∼ 72.8 % RH. Crystallization transition of this particle occurs over an RH range of 50.6 %–50.2 %. Furthermore, LLPS dynamic processes are also observed under two faster RH changing conditions, i.e., 0.14 and 2.40 RH s-1 (Fig. S5 in the Supplement). Apparently, the number of formed AS inclusions increases significantly with higher RH changing rates. This is owed to the greater kinetics limitation in the viscous outer organic-rich phase derived from the faster water release, which inhibits the coalescence of AS inclusions, as discussed by Fard et al. (2017).