We demonstrate an all-optical active mode of dielectric microdroplet splitting in a sandwich structure consisting of two anti-symmetrical y-cut LN:Fe substrates. The dynamic process of the microdroplet splitting and the simulation of the electrostatic interaction inside the sandwich gap show that the combination of two anti-symmetrical substrates are capable to provide a sufficient dielectrophoretic force and to reduce the unbalance of the drag forces for a stable and efficient splitting of the microdroplet. The dependences of the splitting time on the illumination intensity and the initial microdroplet size are also studied, and the results show that the microdroplet splitting process is fully governed by the establishment of the superposed photovoltaic field inside the sandwich gap. A key ratio Er/E0, representing the microdroplet splitting difficulty for a given sandwich structure, is found linearly dependent on the initial microdroplet size. These points are quite important to the integration of splitting functionality on the LN-based microfluidic chip.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Lithium niobate (LN)-based microfluidic chip is regarded as a potential platform for biomedical applications such as biological analysis, clinical diagnostics and drug discovery [1,2]. As a key functionality of this chip, photo-assisted microdroplet splitting on LN substrates has attracted broad interests in the past few years [3,4]. The first demonstration of photo-assisted microdroplet splitting was made by Esseling et al . By optically creating attractive virtual rails on LN:Fe substrates they realized a passive mode of microdroplet splitting with a pumping stream being as an actuator. Chen et al. then reported the active photo-assisted splitting mode in 2016, and the dielectric microdroplet was split in a controllable way without the help of the external pumping system . However, this splitting process utilizes both the pyroelectric and photovoltaic effects of LN crystals, and the substrate has to undergo a pre-polarizing procedure, i.e. a temperature variation of the substrate before the microdroplet splitting. Apparently, the required pre-polarizing procedure makes this splitting way quite complicated, which is inconvenient for the integration of splitting functionality on a LN-based microfluidic platform. Thus, it is very urgent to develop an all-optical active mode for the dielectric microdroplet splitting on the LN substrate.
The force required for the photo-assisted microdroplet splitting essentially stems from the space charges generated on the LN substrate [5,6]. The inhomogeneous electrostatic field induced by these charges allows manipulating the dielectric liquid through dielectrophoretic (DEP) interaction [7,8]. By this way, researchers also realized various optical manipulations on other kinds of neutral objects including nanoparticles [9,10], microcrystals , and biological cells [12,13]. The optical anisotropy of LN crystals makes them behave differently depending on the direction on their c-axis (c-orientation), and different features have been reported in the previous works regarding LN-based optical manipulation [14-16]. However, in the most of them only a single LN:Fe substrate was utilized and no attempt was made for combining LN:Fe substrates with different c-orientations.
In this letter, a new type of sandwich structure consisting of two y-cut LN:Fe substrates is proposed, and their c-orientations are set to be anti-symmetrical to each other. Inside this sandwich structure, an all-optical active mode of the dielectric microdroplet splitting is demonstrated. It will be shown that the combination of two anti-symmetrical substrates is quite crucial to the stable and efficient microdroplet splitting. Moreover, the mechanism of this all-optical microdroplet splitting is compared to the previous photo-assisted microdroplet splitting , and the dependences of the splitting time on the initial microdroplet size is studied. The outcome are quite important to the integration of splitting functionality on the LN-based microfluidic chip.
2. General principle
Nominally pure as-grown LN crystals exhibit weak photovoltaic effect in the visible range. The dopant Fe introduces electron traps (FeLi2+/3+) in the LN lattice and they can induce a strong photovoltaic effect in the visible range. Under the illumination, the electrons can be easily photo-excited from Fe2+/3+ traps and transport along the + direction of the optical axis (c-axis) due to the photovoltaic effect. Thus, the photo-excited electrons are easily accumulated at the + c end of the LN:Fe crystal, leaving a large amount of positive charges at the -c end. Due to the accumulation of the photo-excited electrons, strong inhomogeneous electrostatic field (i.e. photovoltaic field) comes into being in different configurations depending on the orientation of the sample. If the sample is illuminated by a Gaussian laser beam (Fig. 1(a)), the photovoltaic field is mainly perpendicular (Fig. 1(b)) and parallel (Fig. 1(c)) to the cutting surface in the c-cut and y-cut cases, respectively. In this work, a sandwich structure consisting of two y-cut samples are used and their c-orientations are set to be anti-symmetrical to each other, i.e. their c-axes are parallel but point to opposite directions. Under the superposed electrostatic field generated inside the sandwich structure, the dielectric liquid is polarized, and moved, governed by the DEP force, toward the place where the maximum of electrostatic field is located. By using this DEP force, different kinds of manipulation including microdroplet splitting can be developed for various biological applications.
The samples used in our experiments were two y-cut congruent LN crystals doped with 0.03 wt% Fe2O3. The crystals were in as-grown state and no post-growth heat-treatment was performed. The thickness of both samples was about 0.5 mm and their absorption at 405 nm was about 2.9 cm-1 (Fig. 2(c)). The doping rate of 0.03 wt% was selected because it was proved to be a proper value for various photovoltaic manipulations of microdroplets [4,16]. It should be noted that changing the doping rate or reducing the sample may dramatically change the photovoltaic property of the crystal and probably results in better manipulation effects. However, the optimization of the crystal property is not included in this work and it will be published elsewhere.
The experimental setup for realizing microdroplet splitting is shown in Fig. 2(a). The 405 nm-laser source (CNI Laser) was selected because it was proved to be high-efficiency for producing photovoltaic current as compared to other longer-wavelength laser. For example, the photovoltaic coefficient at 405 nm for moderately doped LN is about twice of that at 473 nm . Besides, according to the spectrum shown in Fig. 2(c), the sample absorption is just a little higher at 405 nm than at longer wavelength, and therefore the temperature increase due to the sample absorption is expected to be slight. As the matter of fact, our preliminary simulation showed that the temperature increase at the focus is below 1 oC for the moderate illumination. The laser beam was focused by a microscope objective on a LN-based sandwich structure. The aperture was used to screen the low-angle scattering lights from the laser source. A camera was responsible for capturing the dynamic process with the white light used as a background. The laser reflector has a high reflectivity to the light with the wavelength around 405nm but it has good transparency in other visible range. Therefore, the white light from the light source can pass the laser reflector and get to the camera. Since the 405 nm-intensity of laser spot at the focus is very high, only the laser reflector is not enough to attenuate the spot brightness to below the saturation value of the camera. To solve this problem, another 405 nm-filter was placed before the camera to further attenuate the spot brightness. The two sandwich substrates were fixed through substrate holders onto two linear stages with the sandwich gap thickness being adjusted. Two substrates were mounted with their c-orientations being anti-symmetrical to each other (Case2 in Fig. 2(b)). Before experiments, we ejected dielectric microdroplets (transformer oil) onto the substrates by using a nozzle and then we carefully tuned the distance between the two substrates. By applying laser irradiation on the microdroplet, the splitting can be performed in an all-optical way. For comparison, we also tried the microdroplet splitting in a sandwich structure consisting of one y-cut LN:Fe sample and one quartz plate (Case 1 in Fig. 2(b)). Note that the gravity effect on the microdroplet splitting can be ignored because the gravity is weak as compared to the photo-induced electrostatic force acting on the microdroplet.
4. Results and discussions
In , the active splitting of dielectric microdroplets inside a c-cut-LN-based sandwich structure needs the help of pyroelectric charges. This is because the photovoltaic field generated on the c-cut LN substrate is usually unipolar  and without the external help it cannot induce opposite drag forces required by the microdroplet splitting. But in this work, y-cut-LN-based sandwich structure are used and the dipolar photovoltaic field  generated on the substrate is beneficial to the formation of the opposite drag forces. Under an anti-symmetrical substrate configuration, the active splitting of dielectric microdroplets can be performed in an all-optical way.
As shown in Fig. 3(b), upon the focused laser illumination the microdroplet starts to deform gradually, changes from the initial round shape to a dumbbell-like one, and finally splits into two sub-droplets with similar sizes. This splitting process is totally automatic and usually completed in several seconds. The necessity for using anti-symmetrical substrate configuration can be seen from Fig. 3(a), where only one y-cut LN substrate is used in the sandwich structure. In this case, the microdroplet splitting cannot be completed even after laser illumination for tens of seconds and only microdroplet deformation is induced by the laser illumination. Note that the laser power and focused beam size used in Fig. 3(a) and 3(b) are the same. Two problems cause the failure of microdroplet splitting in the structure using a single LN substrate. Firstly, the DEP force provided by the single LN substrate is not sufficient for the splitting. Secondly, the unbalanced drag forces generated by the dipolar photovoltaic field easily makes a large part of microdroplet deviate from the splitting position (see the frame @ 37 s in Fig. 3(a)) and finally leads to the failure of the microdroplet splitting.
As mentioned above, the DEP interaction is the main mechanism responsible for the optical manipulation of dielectric liquids . Since the DEP force is dependent on the electrical field gradient (determined by the space charge distribution) rather than the electric field direction (determined by the charge sign) [20,21], the unbalance of the drag forces originates from the difference between the space charge distributions at the + c and –c ends. So far no particular study has been reported on this issue, but the possible reasons for the difference of space charge distribution may be some kind of anisotropic charge transport feature or photorefractive effect under the focused laser illumination. In Fig. 9 of  and Fig. 4 of , uncharged nanoparticles seem to deposit in a relatively narrower region at the –c side than at the + c side of a illuminated region, which could be due to the difference between the space charge distributions at the + c and –c ends. If there is the similar situation in our case, the denser charge distribution at the –c end may result in a stronger drag force toward the –c end, i.e. the unbalance of the drag forces. Thus, the microdroplet deviates from the splitting position to the –c end, as shown in Fig. 3(a).
The above problems leading to the failure of the microdroplet splitting can be solved by using two y-cut LN substrates with anti-symmetrical c-orientations. Figure 4 shows the simulated distribution of DEP forces inside the gap for both the single and anti-symmetrical LN sandwich structures. The basic space charge profile on the y-cut substrate is given by solving the Kukhtarev equations numerically . The space charge is assumed to distribute differently at the + c and –c ends, and the maximum of surface charge density is set to be 15% different between the + c and –c end. For an explicit charge density σ, the electric potential V, the electric field E and the DEP force FDEP are calculated by:
Figure 3(c) shows the dynamic process of a special microdroplet splitting process in which the laser illumination is switch on and off alternately. It can be seen that the microdroplet response to the laser illumination is almost instantaneous. Moreover, only upon the laser illumination the microdroplet deforms while it keeps totally frozen without any relaxation effect as the illumination is off. In other words, the progress of the microdroplet splitting is savable by controlling the On/Off status of the illumination. The above result indicates that the microdroplet splitting reported here does not associate with any pyroelectric charges or thermal effect [4,23], and it is indeed an all-optical process connected with the establishment of the photovoltaic field inside the sandwich gap.
To further confirm the mechanism of this microdroplet splitting, the time (splitting time) required by the whole splitting process is recorded at different illumination intensities. The dependence of the splitting time on the illumination intensity is plotted in Fig. 6. The splitting process is found to be accelerated with the increase of the illumination intensity, which is consistent with the basic law for the establishment of the photovoltaic field. It is well known that the photorefractive field E develops with the time (t) in an exponential way :
The intensity dependence of the splitting time can be well fitted by using Eq. (6) (see Fig. 6), indicating that the microdroplet splitting process is fully governed by the establishment of the photovoltaic field inside the sandwich gap. The fitting also yields the value (5.287*105 Ws/m2) of the parameter K, and the ratio Er/E0 is calculated to be about 67.5%, meaning that there is still 32.5% of capability left for the splitting of a much larger microdroplet.
The dependences of the splitting time on the microdroplet size (d and D) at the fixed illumination intensity are plotted in Fig. 7(a). It is also shown here some typical frames of the moments when the microdroplets are just splitting. Note that the different height of each microdroplet can be distinguished directly from the boundary linewidth of the microdroplet in these frames. It is found that the splitting time goes up with the increase of the microdroplet height d and diameter D, because longer time is needed for the establishment of a high critical photovoltaic field required by the splitting of a large microdroplet. It is apparent that the splitting time does not depend linearly with the parameter d and D. By using the Eq. (6) and fixing the illumination intensity, the ratio Er/E0 can be calculated from the splitting time t for different parameter d and D. As shown in Fig. 7(b), the dependences of the ratio Er/E0 on the parameter d and D are roughly linear, and the linear fitting yields the slope of 0.075 and 0.0071 µm-1 for d and D, respectively.
Theoretically, the critical photovoltaic field Er cannot exceed the saturated photovoltaic field E0, i.e the ratio Er/E0 should be lower than 1. Experimentally, we indeed found that the microdroplet is very hard to split when the sandwich gap thickness is beyond 20 µm (i.e. Er/E0>1), and only the permanent microdroplet deformation is observed in this case. We also made several splitting attempts for the microdroplets with much larger diameters (i.e. Er/E0>>1), but the results are complicated because the boundaries of large microdroplets are usually not the regular circular shape.
As the microdroplet splitting technique reported here aims at biological applications in the future, the cell viability during microdroplet manipulation has to be discussed. It is well known that UV illumination, especially the illumination below 365 nm, can decrease the cell viability significantly. The 405 nm-light used here is located in the visible range and its damage to the cell can be ignored theoretically. Moreover, as mentioned in the experimental section, selecting 405 nm as the operating wavelength is for a high-efficiency manipulation. If necessary, the wavelength of the manipulation light can be replaced by 473 nm or even by 532 nm. The light at these wavelength should be much safer for cell, and moreover they are valid for the microdroplet splitting because the photovoltaic effect of LN:Fe can be provoked principally by the light in the whole visible range. Very recently, we studied the effect of photovoltaic manipulation on the viability of Escherichia coli inside LB microdroplets diluted by normal saline. Till now we found no evidence about the damage induced by the 405 nm-illumination to these bacteria.
Another issue that has to be discussed is about the applicability of splitting technique on water droplets. It is of key interest for biology because an aqueous environment is necessary for cell culture. The difficulty that lies in water-microdroplet splitting is associated with the high permittivity and conductivity of water: polarization charges and free charges produced in water microdroplets can reduce significantly the static DEP interaction from the LN:Fe surface. Although we recently demonstrated a successful photovoltaic actuation of water microdroplets on the LN:Fe surface through the electrowetting mechanism , the splitting of water microdroplets is much more complicated and difficult as compared to the actuation and it cannot be fulfilled simply by the previous way. Lately, we found a kind of super-hydrophobic porous film permeated with insulating oil, and we realized the splitting of water microdroplets on the y-cut LN:Fe crystal coated with this film. The details about the progress will be reported in another paper soon.
In summary, we demonstrate an all-optical active mode of dielectric microdroplet splitting in a sandwich structure consisting of two anti-symmetrical y-cut LN:Fe substrates. As compared with the sandwich structure using only a single y-cut LN:Fe substrate, the combination of two anti-symmetrical substrates is capable to provide much stronger and more balanced drag forces, which is beneficial to a stable and efficient microdroplet splitting. No thermal effect is found involved in the dynamic process of the microdroplet splitting and the splitting is fully governed by the establishment of the photovoltaic field inside the sandwich gap. The dependence of the splitting time on the initial microdroplet size are also studied, the key ratio Er/E0, representing the microdroplet splitting difficulty for a given sandwich structure, is found linearly dependent on the initial microdroplet size.
Natural Science Foundation of Hebei Province (No. 17JCYBJC16500); National Natural Science Foundation of China (No. 11874014); Natural Science Foundation of Tianjin City (No. F2017202238).
We thank Prof. Yongfa Kong for his help on sample preparation. The authors are indebted to the referee for the valuable comments.
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