Color Tuning Using Scanning Optical Tweezers

Color tuning plays an increasingly vital role in daily life, including painting, printing, decoration, and animation display. Among many nonemissive color tuning technologies with low‐cost and low‐energy consumption, direct color mixing is simpler without fine and complex display structural designs or specific synthetic materials. Nevertheless, it yet faces an enormous challenge in precise color tuning due to high requirement for integrating dyes in a designable way. Herein, a scanning optical tweezers‐based color mixing method is proposed for precise color tuning. Water/oil‐soluble dye microdroplets with different sizes and colors can be stably captured and then rapidly fused by setting and exerting different trajectories of optical traps under optical tweezers. Thus, the color hue, saturation, and value of the mixed microdroplet can be accurately tuned. Besides, this is further applied to the optical assembly of colorful patterns in a programmable manner. The proposed method is more effective in harvesting pure color and high color quality without introducing exogenous materials contaminating the sample during the manipulation process, which is believed to have potential applications in liquid animation, human–robot interactive painting, and color sensing.


Introduction
Due to the ability of carrying rich information, colors are important for perception and identification of human. [1][4][5][6] As an effective method with low-cost and low-energy consumption, nonemissive color tuning technologies have been widely studied, which are based on the interaction with transmitted or reflected ambient light and can produce a visual stimulus in a subtractive fashion. [7]5][16][17] Structural color technology mainly relies on various periodic micro-/nanostructures (such as gratings, thin films, and photonic crystals) to reflect specific wavelength of ambient white light. [4]The induced color can be tuned by changing the performance parameters of the structures and the corresponding substrate.The required structures are generally fabricated by the state-of-the-art top-down lithography and the bottom-up assembly techniques, which undergo a long design and complex processing procedure to obtain the production and integration of different high-resolution pixels. [8,9]ull-color electrophoretic technology widely used in achromatic eReader produces and tunes colors by moving and combining four dye particles (yellow, cyan, magenta, and white colors) in an electrophoretic fluid. [10]The dye particles with different colors are moved to the display surface by applying different voltages to form different color combinations, which needs a certain response time and requires fine and complex voltage control to ensure the correct color to be displayed. [11,12]For field-induced chromic technology, it relies on specific synthetic materials to enable reversibly changing their colors under an external stimulus including electric, [13,14] thermal, [15] or optical [16,17] field.These materials can achieve fast color switching, but they have monotonous color changes and wide color gamut modulation cannot be generally achieved.In the above methods, due to the intricacies of micro-/ nanostructure or synthetic material and color relationships, fine color tuning is difficult.By contrast, direct color mixing of dyes is a simpler approach, which is also a well-established practice in the ink-jet printing fields. [10]Nevertheless, it yet faces an enormous challenge in fine color control due to high requirement for integrating dyes in a designable way. [10]Therefore, a simple and efficient way for precisely tuning color is highly required.
In recent years, some literatures have reported that different microdroplets could be fused and mixed by applying an external field (such as acoustic, [18][19][20] magnetic, [21][22][23][24] or electric [25] field).Inspired by the above works, fine color tuning is expected to be realized through fusing dye microdroplets of different colors.Among the manipulation technologies of microdroplets, the magnetic-actuated method required the addition of magnetically responsive materials into the microdroplets, and the electric actuation generally needed the physical contact of electrodes with the microdroplets and it could introduce some surfactants into the microdroplets, all of which resulted in the decline in color quality.For the acoustics-based method, a complex microfabrication was required to provide acoustic transducers or arrays to ensure the flexible manipulation of microdroplets, [20] which increased the complexity of color mixing obviously.Compared with the above methods, scanning optical tweezers-based micromanipulation technology has great advantages such as noncontact, no label, and ease of operation, while it can also precisely manipulate large number of samples simultaneously and does not contaminate the sample. [26]It has been successfully used to manipulate and fuse oil/water microdroplets. [27,28]So, it is possible to achieve a fine color tuning by fusing multiple dye microdroplets at the same time.
Therefore, in this work, we propose a scanning optical tweezers-based color mixing method for fine color tuning.Scanning optical tweezers that can provide multiple optical traps simultaneously is used to capture dye microdroplets with different sizes and colors in solution and then fuse them.Thus, the color hue, saturation, and value of the mixed microdroplet can be accurately tuned.Moreover, the obtained color is predictable.It can be further applied to the optical assembly of colorful patterns in a programmable manner.With the advantages of easy operation and widely available inks, this technology is expected to be combined with photothermic programmable liquid display technology to enable colorful liquid animation. [29]It can also be combined with robotics for human-robot interactive color mixing and painting. [30,31]In addition, this method also has great potential for reflective displays and color sensing.

Experiment Setup
The experimental setup was constructed around a scanning optical tweezers system (Tweez250si, Aresis, Europe), as shown in Figure 1.The wavelength of the trapping laser was 1064 nm (excited by a semiconductor-pumped Nd:YAG laser).An incident laser beam could be deflected at a tunable angle and distance through an integrated acousto-optic deflector (AOD), which can modulate the spatial-temporal distribution of the laser beam in real time.This method allowed us to create several optical traps simultaneously and each optical trap could be further set with a predefined trajectory by programing in MATLAB.After the modulation, the laser beam was expanded through a beam expander and then reflected upward by the dichroic mirror.Finally, the laser beam was focused in the sample chamber by a 60Â water immersion objective lens (numerical aperture: 1.0).Another illumination light was focused through a condenser for irradiating the sample, and real-time image acquisition and video recording were performed by a high-speed charge-coupled device (CCD) camera (Nikon, white balance).

Sample Preparation
Slides (26 Â 76 mm) and coverslips (24 Â 50 mm) were ultrasonically cleaned, washed with alcohol, and then dried naturally.A sample chamber was prepared by gluing two long sides of one slide and one coverslip together using a double-sided tape with a thickness of about 100 μm, as shown in the upper left illustration of Figure 1.The experiment was performed using two different types of samples and liquid environments (sample 1: colored dye water microdroplets in oil; sample 2: colored dye oil microdroplets in water), which also indicates the ability of the proposed method to be applied in multiple scenarios.For sample 1, three water-soluble dyes (eosin (red), citrine travertine (yellow), and phycocyanin (blue) as examples) were first dissolved into deionized water to form three basic colored aqueous solutions (concentration: 1 mol L À1 ) and then colored dye water microdroplets in an oil environment were further prepared by mixing the basic colored aqueous solution and oil (silicone oil as the oil environment in this work) under an action of ultrasound.For sample 2, solvent blue (blue), Sudan I (yellow), and Sudan III (red) were selected as three oil-soluble dyes, which were dissolved in dichloromethane to form three basic colored oil solutions (concentration: 1 mol L À1 ), and then the basic colored oil solutions were further dispersed into aqueous sodium chloride solution under the action of ultrasound; thus, colored dye oil microdroplets in water were also ready.Here, it should be pointed out that choosing the aqueous sodium chloride solution as the aqueous environment is mainly because the surfaces of oil microdroplets have a certain charge, resulting in an electrostatic repulsion between oil microdroplets and then hindering the fusing of oil microdroplets; Introducing an ionic solution can reduce the electrostatic repulsive force between oil microdroplets and makes it easier to fuse the oil microdroplets.When the prepared dye microdroplets suspension was sandwiched between two glass slides, a colored display window was also created synchronously, which have a potential application in reflective color display, colorimetric sensor, and data encryption.
In addition, the sizes of the fabricated dye microdroplets above could be controlled by adjusting ultrasonic time and ultrasonic frequency.In general, the longer the ultrasonic time or the higher the ultrasonic frequency, the smaller is the microdroplet size.As an example, Figure 2a shows optical micrographs of the fabricated dye oil microdroplets (blue) in water with increasing ultrasonic time at a constant ultrasonic frequency (40 kHz).The diameters of the microdroplets were measured as about 20.0, 12.0, 7.0, 4.5, and 3.0 μm with 30, 60, 120, 180, and 240 s of ultrasonic time, respectively.In addition, the fabricated microdroplets also presented a relatively uniform size at a given ultrasonic frequency and time.As an example, Figure 2b(I,II) shows an optical micrograph of the fabricated dye oil microdroplets (blue) in water with 30 s of ultrasonic time and 40 kHz of ultrasonic frequency and the corresponding diameter distribution, respectively.The equivalent diameter of the fabricated microdroplets was evaluated as 19.3 AE 1.1 μm.The above results indicate the size controllability of microdroplet.In the following experiments, various sizes of microdroplets need to be fabricated and used to achieve color tuning with a wide color gamut range.

Working Principle
Figure 3 shows the working principle.As water and oil microdroplets are subjected to different optical forces, different trajectories of optical traps are set to capture the microdroplets.For dye water microdroplets dispersed in oil, the optical force exerted by a single optical trap on each of them acts as a repulsive or pushing force due to its smaller refractive index than that of the oil environment.To achieve a stable trapping of the water microdroplet, a quasicontinuous optical trap with a circular trajectory is constructed and then placed on the equator periphery of the microwater droplet.In this case, the surrounding repulsive force can confine the water microdroplet within the circular trajectory, and thus the water microdroplet can be trapped stably.By moving the quasicontinuous optical trap, the water microdroplets can be further transported accordingly.The stable trapping and controlled transportation of microdroplets make it possible to achieve the fusion of water microdroplets, so, by moving the laser trap to each other, two trapped dye water microdroplets can be translated, contacted, and then fused (Figure 3a).Here, due to unobvious directional arrangements of molecules on surfaces of the water microdroplets, the electrostatic repulsive force between the water microdroplets can be directly overcome during the fusing process by the optical force of optical tweezers.In contrast to the dye water microdroplets dispersed in oil, the refractive index of oil microdroplet is larger than that of water environment (aqueous sodium chloride solution in this experiment), so only a single optical trap can directly trap the oil microdroplet dispersed in water stably and then move it in three dimensions (Figure 3b).Due to introducing ionic solution to reduce the electrostatic repulsive force on the surfaces of oil microdroplets, two dye oil microdroplets can be easily fused with each other under the actions of optical forces.
A new color is created by fusing two dye microdroplets with different colors.After two dye microdroplets were mixed, one of them could absorb the reflective light from the other dye microdroplet again.It involves the interactions of three factors including a white light source, dye microdroplets, and the viewer (CCD) in the work, which was based on the subtractive color mixing theory producing a new mixing color through removal of certain wavelengths from a visual stimulus. [32]More new colors are created by fusing more dye microdroplets with different colors and then fine color tuning can be achieved.The quality of color can be determined by the three parameters, [33] including hue (H), saturation (S), and value (V).Among the three parameters, hue is the appearance of a color, such as red, yellow, green, and blue, which can be tuned by fusing different proportions of colored microdroplets.Taking Figure 3c as an example, an orange microdroplet can be obtained by fusing a red microdroplet with a yellow microdroplet.Similarly, fusing a blue microdroplet with a yellow microdroplet can get a green microdroplet.Fusing a red microdroplet with a blue microdroplet can get a purple microdroplet.For the other two parameters, saturation refers to the vividness of color and value indicates the brightness degree of the color.The saturation of the primary color is the highest and as the saturation decreases, the color gradually becomes colorless.That is, the color loses its hue.For object color, the value relates to the transmission ratio and reflection ratio of the object.Thus, the color saturation and value can be tuned by fusing different ratios of colored microdroplets with colorless microdroplets under the actions of optical forces.As shown in Figure 3d, fusing a red microdroplet with a colorless microdroplet can get a light red microdroplet with a lower saturation and a greater value.Similarly, the same results can be obtained in blue and yellow microdroplets.By fusing more colorless microdroplets, the saturation of the colored microdroplets will continue to reduce, while the value will increase regularly.Therefore, fine color tuning can be achieved by fusing two or more different dye microdroplets (colored or colorless).
To further verify the effectiveness of this color tuning method based on optical tweezers, CIEL* a* b* model was used to characterize the quality parameters (especially the hue parameter) of a new color mixed from three basic colors (red, yellow, and blue), which is based on subtractive color mixing theory and has a wider color gamut range than that of the HSV (hue, saturation, value) model.Here, L* indicates white and black balance, a* indicates green and red balance (þa* = red and Àa* = green hues), and b* indicates blue and yellow balance (þb* = yellow and Àb* = blue hues). [32]Therefore, the a* and b* chromaticity coordinates of the CIEL* a*b* model could be used to quantitatively represent the hue parameter in the above HSV model.Moreover, higher a* and b* values correspond to higher color saturation.According to the subtractive color mixing theory, [7] if the chromaticity coordinates (a* and b* in the CIEL*a*b* system) of the mixed color stimulus lie on the line connecting the two stimuli being mixed, then the mixed chromaticity coordinates should follow the trend expressed by the following equations: [16] ( Here, a 1 * and a 2 * are the a* coordinates of the two microdroplets being mixed; a 3 * is the a* coordinate of the mixed microdroplet; similar to b 1 *, b 2 *, and b 3 *, x and y are fractional contributions weighted by the extinction coefficients of each component.Therefore, we can predict the hues of the mixing colors based on the above Equation (1).By comparing the predicted chromaticity coordinates with the measured chromaticity coordinates, the closer they are, the better is the hue consistency, otherwise not.Thus, if the predicted chromaticity coordinates of the mixing color are close to the actual measured chromaticity coordinates, it means this color tuning method based on optical tweezers is effectiveness.
The experiments were performed in an experimental light environment with a standard white light source.The color parameters of the microdroplets were measured by ImageJ software.The final coordinate value of the color was the average value after ten measurements.

Tuning of Color Hue
From the above, by fusing different proportions of colored microdroplets, the color hue can be tuned.The colored microdroplet samples (see Section 2.2) used in our experiments had the same initial concentration (1 mol L À1 ), which is helpful for quantitative analysis.Taking colored dye oil microdroplets in water as examples, Figure 4a shows the fusion process of two different colored oil microdroplets (I: yellow-blue; II: red-yellow; III: red-blue) based on optical tweezers.First, two static optical traps were constructed to stably capture two microdroplets, respectively; then one of the microdroplets was translated toward the other microdroplet by moving its optical trap; finally, two contacted microdroplets were fused under actions of optical forces and a new mixing color was produced.Then, we measured the colorimetric values (a*b* chromaticity coordinates) of the mixing color as discussed in the above working principle.In Figure 4a(IÀIII), a green, orange, and purple microdroplet with its a*b* values of (À20, À6), (22, 53), and (30, À11) was obtained, respectively, which indicates the different hue.The hue can be further tuned by fusing more colored oil microdroplets.As shown in Figure 4b, one yellow oil microdroplet d 1 (À6, 70) and one green oil microdroplet d 2 (À24, 3) were manipulated and fused as a new colored microdroplet d 3 (À18, 37).Then, the microdroplet d 3 was moved toward another yellow microdroplet d 4 (À6, 70) and both of them were fused as a new colored microdroplet d 5 (À15, 53).Thus, fusing different proportions of colored microdroplets based on optical tweezers, the color hue could be tuned.Furthermore, due to the convection and diffusion of the liquid during the fusion process, the time required for color mixing is very short.For example, Figure 4b(IIÀIV) shows the fusion process of two colored oil microdroplets d 1 and d 2 in Figure 4b(I) and it only took 0.10 s from surface contact of the microdroplets to realization of uniform fusion, which is basically the same as the time required for the fusion of two water microdroplets in oil.Therefore, it is possible to achieve a fast response of color tuning.
To further demonstrate the effectiveness of color tuning based on optical tweezers, more experiments for fusing different proportions of colored microdroplets have been performed, and the corresponding hue parameter measurements based on image-j software and the theoretical predictions based on the Equation ( 1) have been shown in the a*b* chromaticity coordinates in Figure 4c,d.In both figures, the used samples were colored water microdroplets in oil and colored oil microdroplets in water, respectively.For the former, circular dynamic optical traps were set to manipulate its movement and achieve its fusion (see Section 2.3), which is obviously different with manipulation method of the latter.Three red, yellow, and blue squares on the vertices of the big triangle represented the three basic colors (red, yellow, and blue), respectively.Taking 1:3, 1:1, and 3:1 as the mixing ratios of arbitrary two basic colored microdroplets (i.e., (x, y) in Equation ( 1) were (75%, 25%), (50%, 50%), and (25%, 75%), respectively), the stars on the black straight lines are the predicted color values for mixtures.The outer circles are the chromaticity coordinates of the actual measured values.Obviously, for colors in each group of mixtures, the predicted and measured hue values are very close to each other, indicating the good prediction and accuracy of hue tuning based on optical tweezers.Here, it should be reminded that the dye solutes of microdroplets in this work can be also substituted by fluorescent particles.By mixing various ratios of fluorophores, a broader range of emitted colors on the CIE diagram including white light can be accomplished. [34]

Tuning of Color Saturation and Value
By fusing different proportions of colored microdroplets, the color hue can be tuned in a controlled manner.Correspondingly, the tuning of color saturation and value can be achieved by fusing different proportions of colored and colorless microdroplets based on optical tweezers.As an example, Figure 5a shows the fusion process of one colored and one colorless oil microdroplet (I: yellow-colorless; II: red-colorless; III: blue-colorless) in water.Obviously, the hue remains basically the same, while the saturation decreases and the value increases.This is mainly because the dye distribution in mixed or diluted microdroplets was more dispersed and more light was transmitted through the microdroplets.To quantitatively analyze the tuning of color saturation and value, three basic colored (red, yellow, and blue) oil/water microdroplets were fused with colorless microdroplets in the mixing ratio of 1:1, 1:2, 1:3, 1:4, and 1:5 (colored:colorless), respectively, under optical tweezers, as shown in Figure 5bÀe (Figure 5b,d: oil microdroplets in water; Figure 5c,e: water microdroplets in oil).Figure 5b,c shows the corresponding optical micrographs of mixed oil microdroplets in water and mixed water microdroplets in oil, respectively.Figure 5d,e shows the measured relationship between color saturation/value and mixing ratio in Figure 5b,c, respectively.With the decreasing concentration, the color saturation of the microdroplets decreases, while the color value increases.Specifically, the color saturation of the oil microdroplets decreases by an average of 12 and the corresponding color value increases by an average of 7 in Figure 5d.It shows good linear fittings with the saturation fitting degrees R 2 of 0.998, 0.999, and 0.998, respectively, and the value fitting degrees R 2 of 0.983, 0.990, and 0.954, respectively.For the mixed water microdroplets in Figure 5c, the color saturation decreases by an average of 14 and the color value increases by an average of 7 (Figure 5e).By linear fitting, the fitting degrees R 2 of color saturation are 0.994, 0.986, and 0.995, respectively, and the R 2 of color value are 0.995, 0.974, and 0.995, respectively.The results indicate the color saturation and value of the microdroplets can be well tuned by fusing different proportions of colored and colorless microdroplets based on optical tweezers.
In addition, further experiments show that color saturation and value can be also controlled linearly by the photothermic effect.When a colored microdroplet was irradiated with a large laser power, the solvent in the microdroplet could be evaporated and the concentration of the solute was increased correspondingly.Thus, the saturation of the microdroplet will increase and the value will decrease.This method achieves a purpose for controlling color saturation and value using photothermic tweezers.As an example, Figure 5f shows the changes of size, color saturation, and value of a blue water microdroplet with laser irradiation time.With a continuous irradiation of 40 mW laser within 12 min, the microdroplet size decreased from 47.9 to 38.0 μm (inset of Figure 5f ), which shows a good linear fitting with the fitting degree R 2 of 0.9971.The saturation increased from 42 to 57, while the value decreased from 76 to 68.By linear fitting, the fitting degrees of color saturation and value were 0.9978 and 0.9704, respectively.The good linear fitting indicates that the tuning of color saturation and value by the photothermic effect is feasible and linearly controllable.Further experiments show that with an increasing laser power, the tuning time of color saturation and value would be gradually decrease.
In addition, it is important to point out that the laser powers used in the other experiments were very low (≤10 mW) and did not cause evaporation of the microdroplets.All in all, by fusing different proportions of colored and colorless microdroplets based on optical tweezers, the color saturation and value can be accurately tuned.Moreover, they could also be tuned by the photothermic effect.

Optical Assembly of Colorful Pattern
From the above, microdroplets with different colors could be obtained by the fusion manipulation technology of optical tweezers, which is beneficial to achieve optical assembly of colorful pattern.By programming optical traps through MATLAB, optical traps could be assembled into complex patterns.Each trapped microdroplet in the optical trap could be regarded as a pixel, and each pixel could be composed of different colors.As an example, Figure 6 shows an optical assembly of colorful pattern "SYSU" (an acronym for "Sun Yat-sen University").Here, orange, green, and purple oil microdroplets in water were formed by fusing red/yellow, blue/yellow, and red/blue oil microdroplets in water, respectively, as shown in the schematic diagram (Figure 6a).Then, the optical traps are activated following a preprogrammed sequence, resulting in the formation of the letters "SYSU".The corresponding experimental result (Figure 6b) verified the feasibility and effectiveness of this method.It took about 80 s to build the pattern.In several repeated experiments, as an example, Movie S1, Supporting Information, shows the assembly process of colorful pattern "SYSU".The assembly time is a little long, which is mainly because of the slower transportation speed of the trapped microdroplets induced by the lower optical power (10 mW) acting on them.It can reduce the thermal effect, prevent the reagent evaporation in dye microdroplets, and ensure a display with high color quality.
Additionally, it is worth pointing out that, the results of Figure 6b can be achieved in two working modes.For colorful pattern assembly of dye water microdroplets in oil, it only needs to replace every single optical trap (for trapping every single oil microdroplet in water in Figure 6b) into a quasicontinuous optical trap with a circular trajectory (for manipulating every water microdroplet in oil).Looking forward, this method has also shown a critical application in liquid painting and animation.

Conclusion
In this work, we proposed a scanning optical tweezers-based color tuning method, which could be implemented in both water and oil environments.By fusing basic color microdroplets in various ratios with each other under optical tweezers, different hues of colors could be obtained, which agrees with the theoretical prediction quite well; it also verified a good accuracy in predicting the hue coordinates of a given mixed microdroplet.fusing different proportions of colorless microdroplets and colored microdroplets under optical tweezers, the color saturation and value could be accurately tuned; the good linear fittings indicated the saturation and value coordinates of the mixing color could also be accurately predicted.On this basis, an optical assembly of colorful pattern "SYSU" was achieved by programming optical traps through MATLAB.
Compared to other existed methods, the proposed method in this work has the following advantages.First, it retains the noncontact, no label, ease of operation, and flexibility of optical tweezers.Second, the used color mixing and display structure sandwiching the suspension of dye microdroplets between two glass slides is simple enough, which does not require complex microfabrication and specific synthetic materials.Third, the method does not need to introduce exogenous materials and does not contaminate the sample during the manipulation process, which results in pure color and high color quality.Fourth, due to introducing the scanning optical tweezers, the method can be applied in multienvironments including oil microdroplets in water environment and water microdroplets in oil environment.Fifth, the method needs only a low laser power (≤10 mW) to capture and fuse the target microdroplets and so it would not cause evaporation of the microdroplets; with an increasing laser power (tens of mW), the system can be also used to control color saturation and value linearly based on the photothermic effect.Sixth, due to the liquid diffusion and convection during the fusion process of microdroplets, the time required for color mixing is very short, which provides a fast response and high efficiency of color tuning.This method can be further combined with programmable liquids display technology to realize the application of colorful liquid painting and liquid animation.In addition to the above advantages, the proposed method still has some unique characteristics compared to other external field-based methods.First, scanning optical tweezers can set multiple optical traps and independently control the optical power and position of each optical trap, so it can control each of microdroplet precisely while manipulating multiple microdroplets, which is highly favorable to achieve a more colorful color world.Second, optical tweezers can trap and manipulate smaller microdroplets (hundreds of nanometers to tens of microns), which results in a higher pixel density, so it is helpful to obtain a colorful pattern with a higher resolution.Third, optical tweezers could be also utilized to trap and manipulate microdroplets in a gas-phase environment, [35] so the color tuning is expected to be realized in gas environment.

Figure 1 .
Figure 1.Schematic diagram of the experimental setup.

Figure 2 .
Figure 2. Size controllability of the fabricated dye microdroplets.a) Optical micrographs of the blue oil microdroplets in water with increasing ultrasonic time at a constant ultrasonic frequency (40 kHz).b) Optical micrograph of the blue oil microdroplets in water with 30 s of ultrasonic time and 40 kHz of ultrasonic frequency (I) and the corresponding diameter distribution (II).

Figure 3 .
Figure 3. Experimental design.a) Fusion process of water microdroplets in oil.Two circle optical traps were programmed to capture two water microdroplets, respectively.They can be fused under actions of optical forces.b) Fusion process of oil microdroplets in water.Every oil microdroplet can be captured by a single optical trap.Ionic solution was added to create an environment where oil microdroplets can be easily fused.c) Tuning of color hue by fusing different proportions of colored microdroplets under actions of optical forces.d) Tuning of color saturation and value by fusing different ratios of colored microdroplets with colorless microdroplets under actions of optical forces.

Figure 4 .
Figure 4. Fusion of colored microdroplets based on optical tweezers.a) Fusion of two different colored oil microdroplets (I: yellow-blue; II: red-yellow; III: red-blue) in water based on optical tweezers.b) Tuning of color hue by fusing more colored oil microdroplets.The fusion time is within 0.10 s (IIÀIV).c) Predicted and measured a*b* chromaticity coordinates of mixed dye water microdroplets in oil.d) Predicted and measured a*b* chromaticity coordinates of mixed dye oil microdroplets in water.

Figure 5 .
Figure 5. Fusion of colored and colorless microdroplets based on optical tweezers.a) Fusion of one colored and one colorless oil microdroplet (I: yellowcolorless; II: red-colorless; III: blue-colorless) in water.bÀe) Tuning of color saturation and value with mixing ratio of 1:1, 1:2, 1:3, 1:4, and 1:5 (colored microdroplets:colorless microdroplets).(b) Optical micrographs of mixed oil microdroplets in water; (c) optical micrographs of mixed water microdroplets in oil; (d) measured color saturation/value of mixed oil microdroplets with different mixing ratio; (e) measured color saturation/value of mixed water microdroplets with different mixing ratio.f ) Tuning of color saturation and value based on thermal effect of optical tweezers.Inset shows the size change of a blue water microdroplet with laser irradiation time and the corresponding optical micrographs.

Figure 6 .
Figure 6.Optical assembly of colorful pattern.a) Schematic diagram.b) Optical micrographs of experimental results.Here, orange, green, and purple oil microdroplets in water were formed by fusing red/yellow, blue/yellow, and red/blue oil microdroplets in water, respectively.Optical traps were activated following a preprogrammed sequence, resulting in optical assembly of the colorful pattern "SYSU".