Drying of the colloidal droplets has been a subject of great interest since the 1980s [1]. Studies of a drying droplet, including changes of drying rate and contact angle, the progression of its shape, and final deposition pattern, can help us under- stand many interesting phenomena, such as coffee ring effects [2], electro-wetting effects [3], and Marangoni effects that are driven by surface tension gradients [4].
Mechanisms behind these phenomena may involve a complex interplay of convec- tion and evaporation, surface tension and capillary force, particles’ interactions, rheology, substrate hydrophobicity, pinning of the contact line, and Marangoni forces [3, 5]. During drying, unlike pure water droplets, the constituents in colloidal droplets, including various concentrations of particles [6], polymers [7–9], surfac- tants [10], solvents [11], and salts [12] will alter the interactions among these mass transfer mechanisms, yielding different drying behaviors. Additionally, environ- mental conditions such as temperature and humidity and the substrate properties also affect the drying process and final deposition [13, 14]. Insights gained from these studies of evaporating droplets may ultimately translate to practical applica- tions in polymer science, biomedicine, and nanotechnology [14], such as inkjet printing [15], DNA chip [16], biosensor [17, 18] and disease diagnosis based on deposition pattern of biological fluid [19–22], food quality analysis [23], particle separation [24], and production of nanoparticles for drug delivery [25, 26]. All above-mentioned applications are related to mass transportation during drying of a liquid suspension and the resultant film property.
Using OCT to monitor the drying process of colloidal droplets has been con- ducted in a few studies. Table 1 summarizes the details of experimental designs for these studies. In conjunction, Figure 1 illustrates the schematics of the drying process of these colloidal droplet models.
Trantum et al. demonstrated the first study to utilize OCT to visualize the cross- section of drying water droplets with suspended particles (see Figure 1A) [27]. Low concentration of polystyrene, melamine formaldehyde, or silica particles with a diameter of 1 μm and a volume fraction of 0.005% was suspended in distilled water.
The water droplet was loaded on a hydrophilic glass slide. The drying experiment was conducted in the ambient condition with relative humidity (RH) controlled to 30 or 40%. A commercial spectral-domain OCT system with a central wavelength of 860 nm and a spectral bandwidth of 51 nm was used to image the droplet. Axial and lateral resolutions of the OCT system were 6.4 and 8 μm, respectively. The scan rate of the system was set to 10 kHz. Their results showed that both the sedimentation rate and evaporation rate would affect the drying dynamics of particle-containing water droplets. In fast evaporating mode, the descending rate of drop surface was faster than the rate of particle sedimentation, resulting in the particles being trapped at the air-water interface (top surface). Close to the water-substrate inter- face, the “coffee ring” flows (CF) played a major role to transport the particles to the pinned contact line, yielding a “coffee ring” final deposition pattern. Given the same particle size, increased particle densities, i.e., silica (2.00 g/cm3), as compared to polystyrene (1.04 g/cm3) lead to early sediment of particles and early transporta- tion of particles to the edge by “coffee ring” flows. Their results suggested a way to control the final deposition of coffee ring structure based on particles’ density.
Based on the previous observations of drying colloidal droplets, the same group proposed design of biosensor utilizing the final deposition pattern to track the existence and concentration of target biomarkers (Figure 1B) [17]. In the experi- ment, the tracer particles were coated with antibodies. These particles would aggregate in the presence of target biomarker (M13 bacteriophage in the study).
Polydimethylsiloxane (PDMS) with low thermal conductivity (0.15 W/mK) was
ReferencesTrantum et al. [27]Trantum et al. [17]Manukyan et al. [28]Davidson et al. [3]Edwards et al. [29]Huang et al. [30] Years201320142013201720182018 Suspended materialsPS, MF, Si*Monoclonal antibodies Glycerol with M13**
Model paintLCLC, SSF***Binary drops (ethanol and n-butanol)PS Latex Diameter (nm)10001000<1000N.A.N.A.L Latex 125 S Latex 53 Initial concentrationvol%: 0.005% (105/μL)vol%: 0.05% (106/μl) Glycerol: 8%
vol%: 10, 30, 50%wt%: 5–20%wt%: 2–20%wt%: L latex: 40.11%, S latex: 33.43% SubstrateGlass slidePDMSGlass coated by Tegotop® 210Glass slide and coverslipGlass coated by Flutech LE15Glass slide EnvironmentAmbientAmbientAmbientChamberedChamberedAmbient Droplet modelSessileSessileSessileSessileSessile or pendantSessile Transparency****TransparentTransparentOpaqueTransparentTransparentOpaque TracersNoneCopolymer microspheres diameter: 4.3 or 7.9 μmPolystyrene diameter: 1 μm0.01 wt% polystyrene diameter: 2 μmNone OCT typeSpectral domainSpectral domainSpectral domainN.A.Spectral domain λOCT (nm)86093080013001320 ΔλOCT (nm)51100220N.A.110 Axial and lateral resolutions (in air)6.4 μm 8 μm7 μm 9 μm1.9 μm 3.5 μm5.5 μm 13 μm6.8 μm 14 μm Scan speed (kHz)10N.A.20N.A.20.7 Imaging depth and scan rangeN.A.1.6 mm 4 mmN.A.N.A. 2.5 mm2.2 mm 5.0 mm
2. Drying process of colloidal droplets
Drying of the colloidal droplets has been a subject of great interest since the 1980s [1]. Studies of a drying droplet, including changes of drying rate and contact angle, the progression of its shape, and final deposition pattern, can help us under- stand many interesting phenomena, such as coffee ring effects [2], electro-wetting effects [3], and Marangoni effects that are driven by surface tension gradients [4].
Mechanisms behind these phenomena may involve a complex interplay of convec- tion and evaporation, surface tension and capillary force, particles’ interactions, rheology, substrate hydrophobicity, pinning of the contact line, and Marangoni forces [3, 5]. During drying, unlike pure water droplets, the constituents in colloidal droplets, including various concentrations of particles [6], polymers [7–9], surfac- tants [10], solvents [11], and salts [12] will alter the interactions among these mass transfer mechanisms, yielding different drying behaviors. Additionally, environ- mental conditions such as temperature and humidity and the substrate properties also affect the drying process and final deposition [13, 14]. Insights gained from these studies of evaporating droplets may ultimately translate to practical applica- tions in polymer science, biomedicine, and nanotechnology [14], such as inkjet printing [15], DNA chip [16], biosensor [17, 18] and disease diagnosis based on deposition pattern of biological fluid [19–22], food quality analysis [23], particle separation [24], and production of nanoparticles for drug delivery [25, 26]. All above-mentioned applications are related to mass transportation during drying of a liquid suspension and the resultant film property.
Using OCT to monitor the drying process of colloidal droplets has been con- ducted in a few studies. Table 1 summarizes the details of experimental designs for these studies. In conjunction, Figure 1 illustrates the schematics of the drying process of these colloidal droplet models.
Trantum et al. demonstrated the first study to utilize OCT to visualize the cross- section of drying water droplets with suspended particles (see Figure 1A) [27]. Low concentration of polystyrene, melamine formaldehyde, or silica particles with a diameter of 1 μm and a volume fraction of 0.005% was suspended in distilled water.
The water droplet was loaded on a hydrophilic glass slide. The drying experiment was conducted in the ambient condition with relative humidity (RH) controlled to 30 or 40%. A commercial spectral-domain OCT system with a central wavelength of 860 nm and a spectral bandwidth of 51 nm was used to image the droplet. Axial and lateral resolutions of the OCT system were 6.4 and 8 μm, respectively. The scan rate of the system was set to 10 kHz. Their results showed that both the sedimentation rate and evaporation rate would affect the drying dynamics of particle-containing water droplets. In fast evaporating mode, the descending rate of drop surface was faster than the rate of particle sedimentation, resulting in the particles being trapped at the air-water interface (top surface). Close to the water-substrate inter- face, the “coffee ring” flows (CF) played a major role to transport the particles to the pinned contact line, yielding a “coffee ring” final deposition pattern. Given the same particle size, increased particle densities, i.e., silica (2.00 g/cm3), as compared to polystyrene (1.04 g/cm3) lead to early sediment of particles and early transporta- tion of particles to the edge by “coffee ring” flows. Their results suggested a way to control the final deposition of coffee ring structure based on particles’ density.
Based on the previous observations of drying colloidal droplets, the same group proposed design of biosensor utilizing the final deposition pattern to track the existence and concentration of target biomarkers (Figure 1B) [17]. In the experi- ment, the tracer particles were coated with antibodies. These particles would aggregate in the presence of target biomarker (M13 bacteriophage in the study).
Polydimethylsiloxane (PDMS) with low thermal conductivity (0.15 W/mK) was
ReferencesTrantum et al. [27]Trantum et al. [17]Manukyan et al. [28]Davidson et al. [3]Edwards et al. [29]Huang et al. [30] Years201320142013201720182018 Suspended materialsPS, MF, Si*Monoclonal antibodies Glycerol with M13**
Model paintLCLC, SSF***Binary drops (ethanol and n-butanol)PS Latex Diameter (nm)10001000<1000N.A.N.A.L Latex 125 S Latex 53 Initial concentrationvol%: 0.005% (105/μL)vol%: 0.05% (106/μl) Glycerol: 8%
vol%: 10, 30, 50%wt%: 5–20%wt%: 2–20%wt%: L latex: 40.11%, S latex: 33.43% SubstrateGlass slidePDMSGlass coated by Tegotop® 210Glass slide and coverslipGlass coated by Flutech LE15Glass slide EnvironmentAmbientAmbientAmbientChamberedChamberedAmbient Droplet modelSessileSessileSessileSessileSessile or pendantSessile Transparency****TransparentTransparentOpaqueTransparentTransparentOpaque TracersNoneCopolymer microspheres diameter: 4.3 or 7.9 μmPolystyrene diameter: 1 μm0.01 wt% polystyrene diameter: 2 μmNone OCT typeSpectral domainSpectral domainSpectral domainN.A.Spectral domain λOCT (nm)86093080013001320 ΔλOCT (nm)51100220N.A.110 Axial and lateral resolutions (in air)6.4 μm 8 μm7 μm 9 μm1.9 μm 3.5 μm5.5 μm 13 μm6.8 μm 14 μm Scan speed (kHz)10N.A.20N.A.20.7 Imaging depth and scan rangeN.A.1.6 mm 4 mmN.A.N.A. 2.5 mm2.2 mm 5.0 mm
ReferencesTrantum et al. [27]Trantum et al. [17]Manukyan et al. [28]Davidson et al. [3]Edwards et al. [29]Huang et al. [30] Frame rate or time interval5 or 0.5 fps, 200 frames2 fps33 or 50 fps interval: ~3–4 s50 fpsInterval: ~10 s *PS: polystyrene, MF: melamine formaldehyde, Si: silica. **Particle surface was functionalized with anti-M13. ***LCLC: lyotropic chromonic liquid crystal. SSF: sunset yellow FCF. ****Optical transparency in near-IR wavelength range. Table 1. OCT studies on drying droplets.
used as the substrate. Thus, a temperature gradient was established along with the interface, inducing a surface tension gradient and promoting Marangoni flows.
During the drying process, the particle aggregates in the presence of biomarkers showed a thicker convective flow pattern in the cross-sectional OCT images, while the dispersed particles showed a uniformly distributed convective flow pattern in the absence of biomarkers [17]. Under the influence of Marangoni flows, increase of viscosity by glycerol, and effect of gravity, the particle aggregates were concen- trated and deposited at the droplet center, forming a concentric final deposition pattern. Note that, with the glass substrate, the particle aggregates will not concen- trate at the droplet center.
Manukyan et al. reported using OCT to characterize the internal flows in a dry- ing model paint droplet (Figure 1C) [28]. Commercial model paint droplets with various initial volume fractions were characterized. Copolymer microspheres were mixed in the model paint droplets as tracer particles. Model paint droplets with an initial volume of 5–8 μL were loaded on hydrophilic or hydrophobic substrates, and their drying behaviors were monitored separately. A spectral-domain OCT system was utilized to perform time-lapse imaging of model paint droplets. The central wavelength and the spectral bandwidth of the system were 930 and 100 nm. Axial and lateral resolutions of the OCT system were 7 and 9 μm, respectively. The OCT system can scan a maximum transverse range of 4 mm. In their results, the model paint droplets on a hydrophilic substrate exhibited an outward radial flow pat- tern and a donut-shaped final deposition pattern in cross-sectional OCT images, indicating that the drying processes of these droplets were driven mainly by the coffee ring effect. However, on the hydrophobic surface, a reversed Marangoni flow was established in the drying of model paint droplets, shown as the convective flow moving down along the surface and then moving inward along the droplet- substrate interface in the OCT image. As the model paint droplet continued to dry, the formation of a skin layer was observed inside the droplet. At this stage, no convective flows were observed. A cavity was observed beneath the outer skin layer in OCT cross-sectional images.
Davidson et al. investigated the drying process of water droplets containing lyotropic chromonic liquid crystals (LCLCs), using polarized optical microscopy (POM) and OCT (Figure 1D) [3]. LCLCs are composed of organic, charged, and plank-like molecules [31, 32]. During the drying process of LCLC containing
Figure 1.
Illustrations of drying process of six colloidal droplet models. (A) Water droplets with tracer particles, i.e., polystyrene, melamine formaldehyde, silica. (B) Water droplet with antibody-coated particles to detect biomarkers. (C) Model paint droplet. (D) Liquid crystal droplet. (E) Binary liquid droplet. (F) Waterborne latex droplet.
ReferencesTrantum et al. [27]Trantum et al. [17]Manukyan et al. [28]Davidson et al. [3]Edwards et al. [29]Huang et al. [30] Frame rate or time interval5 or 0.5 fps, 200 frames2 fps33 or 50 fps interval: ~3–4 s50 fpsInterval: ~10 s *PS: polystyrene, MF: melamine formaldehyde, Si: silica. **Particle surface was functionalized with anti-M13. ***LCLC: lyotropic chromonic liquid crystal. SSF: sunset yellow FCF. ****Optical transparency in near-IR wavelength range. Table 1. OCT studies on drying droplets.
used as the substrate. Thus, a temperature gradient was established along with the interface, inducing a surface tension gradient and promoting Marangoni flows.
During the drying process, the particle aggregates in the presence of biomarkers showed a thicker convective flow pattern in the cross-sectional OCT images, while the dispersed particles showed a uniformly distributed convective flow pattern in the absence of biomarkers [17]. Under the influence of Marangoni flows, increase of viscosity by glycerol, and effect of gravity, the particle aggregates were concen- trated and deposited at the droplet center, forming a concentric final deposition pattern. Note that, with the glass substrate, the particle aggregates will not concen- trate at the droplet center.
Manukyan et al. reported using OCT to characterize the internal flows in a dry- ing model paint droplet (Figure 1C) [28]. Commercial model paint droplets with various initial volume fractions were characterized. Copolymer microspheres were mixed in the model paint droplets as tracer particles. Model paint droplets with an initial volume of 5–8 μL were loaded on hydrophilic or hydrophobic substrates, and their drying behaviors were monitored separately. A spectral-domain OCT system was utilized to perform time-lapse imaging of model paint droplets. The central wavelength and the spectral bandwidth of the system were 930 and 100 nm. Axial and lateral resolutions of the OCT system were 7 and 9 μm, respectively. The OCT system can scan a maximum transverse range of 4 mm. In their results, the model paint droplets on a hydrophilic substrate exhibited an outward radial flow pat- tern and a donut-shaped final deposition pattern in cross-sectional OCT images, indicating that the drying processes of these droplets were driven mainly by the coffee ring effect. However, on the hydrophobic surface, a reversed Marangoni flow was established in the drying of model paint droplets, shown as the convective flow moving down along the surface and then moving inward along the droplet- substrate interface in the OCT image. As the model paint droplet continued to dry, the formation of a skin layer was observed inside the droplet. At this stage, no convective flows were observed. A cavity was observed beneath the outer skin layer in OCT cross-sectional images.
Davidson et al. investigated the drying process of water droplets containing lyotropic chromonic liquid crystals (LCLCs), using polarized optical microscopy (POM) and OCT (Figure 1D) [3]. LCLCs are composed of organic, charged, and plank-like molecules [31, 32]. During the drying process of LCLC containing
Figure 1.
Illustrations of drying process of six colloidal droplet models. (A) Water droplets with tracer particles, i.e., polystyrene, melamine formaldehyde, silica. (B) Water droplet with antibody-coated particles to detect biomarkers. (C) Model paint droplet. (D) Liquid crystal droplet. (E) Binary liquid droplet. (F) Waterborne latex droplet.
droplets, changes of local temperature and concentration of LCLCs induce multiple phases of the LCLCs, including isotropic, nematic, columnar, and crystalline phases [3]. In the experiment, a droplet containing sunset yellow FCF (SSY), a dye that belonged to LCLC family, was loaded on a premium coverslip of the substrate.
Polystyrene particles were added in the droplets as tracers to track fluid flows.
A semi-enclosed PDMS chamber was used to slow down the drying rate, yield- ing a total drying time of ~10–15 min for the LCLC containing droplets with an initial volume of ~0.2–0.5 μL. To visualize the small tracer particles in the LCLC containing droplet, an ultrahigh-resolution (UHR) spectral-domain OCT system was employed, with a central wavelength of 800 nm and a spectral bandwidth of 220 nm. Axial and lateral resolutions of the UHR-OCT system were 1.9 μm and 3.5 μm, respectively. The camera’s scan rate was set to 20 kHz. Figure 2 shows the development of drying SSY solution droplet by UHR-OCT. High-speed time-lapse OCT imaging showed that the convective flows were initiated right after the SSY solution droplet was loaded on the substrate. Substantial Marangoni flows were visualized in time-lapse OCT images, which were established due to the increased concentration of SSY particles near the pinned contact line during the evaporation, leading to increase in local surface tension gradient along with the droplet interface.
In the next stage, the formation of the nematic phase (N) pushed the isotropic (I)-nematic phase boundary to the center. Finally, the isotropic phase diminished at the droplet center. A volcano-shaped final deposition pattern is observed for SSY solution droplets.
Recently, Edwards et al. utilized OCT to investigate the flow patterns of the drying binary liquid droplets (Figure 1E) [29]. Low concentration of solvents, i.e., ethanol or n-butanol, was diluted in the water solution to form binary liquid droplets. To monitor the drying binary liquid droplets, a 1300 nm OCT system was employed, with an axial resolution of 5.5 μm and a lateral resolution of 13 μm.
Figure 2.
Drying process of SSY solution droplet imaged by ultrahigh-resolution optical coherence
microscopy (UHR-OCM). A schematic illustration of phase behaviors of SSY solution droplet at different drying stages was shown in (A). (B–G) UHR-OCM images of the SSY solution droplet at different drying time points. White spots: polystyrene particles as tracers. Cr: crystalline. C: columnar. N: nematic. I: isotropic. White dashed line in (D): columnar-nematic boundary. White arrows in (C): nematic-isotropic boundary. Yellow arrows: columnar line structures. White arrow in (G). Crystallized state. Image reproduced from Ref. [3].
A scan range of 2.5 mm was set, which was sufficient to cover the whole droplet.
Different from the previous three studies, they investigated the flow pattern under different tilting conditions, including the sessile droplet mode with 0° tilting and pendant droplet mode with 180° tilting. In the tilted configuration, axisymmetric toroidal flow patterns were observed by OCT in the middle of drying processes. In the sessile droplet, a convective flow pattern in the same direction as expected by Marangoni driven flow was observed. However, in the pendant droplet, the flow direction was opposite to the Marangoni flow direction, suggesting that Marangoni flows might not dominate in the evaporating binary liquid droplets. They suggested that the convective flows were driven by the density of the liquid and gravity. In the case of the water-ethanol droplet with the preferential evaporation of ethanol, the binary liquid close to the air-water interface was denser than the bulk. Thus, surficial binary liquids would fall along the air-water interface from top to edge in sessile droplets and from edge to bottom in the pendant droplets, driving convec- tive flows in different patterns. Also, they monitored the full drying process of the binary liquid droplets with OCT, showing a three-stage drying process, which are chaotic flows, convective flows, and outward “coffee ring” flows.