Four reversible and reconfigurable structures for three-phase emulsions: extended morphologies and applications

Here in this article, we classify and conclude the four morphologies of three-phase emulsions. Remarkably, we achieve the reversible transformations between every shape. Through theoretical analysis, we choose four liquid systems to form these four morphologies. Then monodispersed droplets with these four morphologies are formed through a microfluidic device and captured in a petri-dish. By replacing their ambient solution of the captured emulsions, in-situ morphology transformations between each shape are achieved. The process is well recorded through photographs and videos and they are systematical and reversible. Finally, we use the droplets structure to form an on-off switch to start and shut off the evaporation of one volatile phase to achieve the process monitoring. This could be used to initiate and quench a reaction, which offers a novel idea to achieve the switchable and reversible reaction control in multiple-phase reactions.


Equation deviation
In 1967,Torza [1] discovered the relationship between spreading coefficients and morphologies.With time proceeding, the understanding of the morphologies becomes more specific because of the intense needs of the particle applications. The morphologies of the three phase system include core-shell structure,Janus structure and separating structure.
When applied in the researches and industries, the separating structure is regarded as two separating droplets and not regarded as multiple emulsions.For core-shell structures,the core and the shell materials are the main considerations for applications. When using this structure, which is the core and which is the shell is initial in the choosing of ambient atmosphere and the actives' hydrophobicity or hydrophilicity.Thus we separate the core-shell structure into core-shell structure and reverse core-shell structure to distinguish the components of the core/shell.
As for the Janus structures,the integral structure(sphere or non-sphere) and the connecting condition of two phases are most considered.In my previous work,we have achieved the methodology of designing Janus structure with simulation in the interfacial tensions between three phases and the flow ratio.But in the most researches and industry applications, they care first about the integral structure of the Janus.Jan Guzowski [2] first describe the requirement for forming the perfect-state Janus by analyzing the interfacial tension relationship between three liquid phases. After that,researchers have named the spherical Janus morphology as perfect Janus. On the contrary, dumbbell Janus/snow Janus which describes one non-sphere structure composed of two lobes. Here in this paper,we unify these names as perfect Janus and dumbbell Janus and conclude them through more fundamental methods:the interfacial tension relationship.Then we used this relationship to choose and adjust liquid phases to form these two morphology droplets and proceed their transformation.
When &~'~&' ,we could calculate the angles roughly to be about 60° which would be a typical dumbell Janus. [2] When & , ' ≫ &' , would be close to zero which is a typical perfect Janus.It has a strict restriction on forming perfect Janus.Only those which meet the restriction of Janus structure and & , ' ≫ &' it would form perfect Janus.When & , ' are higher than &' ,the Janus is more perfect. [2] The three categories according to Torza and the specific classification on the Janus as well as the contacting angles relationship is shown in Figure S1. As the core-shell and reverse core-shell structure, the distinction is the value of γ B and γ A . When γ B is larger, B phase is inside and whenγ A is larger, A phase is larger.
According to the core-shell interfacial tension relationship, the equations are shown as: Equation (4) could be adjusted to: Thus, for simplification, we can conclude that: when γ B − γ A > γ AB , it means γ B is larger and B phase is inside to form a core-shell structure; When γ A − γ B > γ AB , it means γ A is larger and A phase is inside to form a reverse core-shell structure;

2.Selection of the liquid systems
To achieve the transformation of these morphologies, we have to apply for our working systems and operation orders strictly according to the theory and the logic analysis. Here we take the transformation from the perfect Janus to dumbbell Janus as an example. From the previous analysis between the morphology and the interfacial tension, we know that other than meeting the needs to form Janus droplets, to form a Perfect Janus, the working liquids should meet & , ' >> &' while forming a dumbbell Janus, the working liquids should meet &~'~&' . Thus, to change from one to another, we have four possible ways, which is shown in Figure S2(a), that is: ' which approach to small &' to achieve from Perfect Janus to Dumbbell Janus; 2) Decrease the big &' which has been relatively the same to big & , ' to approach to small &' to achieve from Dumbbell Janus to perfect Janus; 3) Increase the & , ' which has been relatively the same to the small &' to big & , ' to achieve from Dumbbell Janus to Perfect Janus; 4) Increase the small &' to approach to big &' which is relatively to big & , ' to achieve from Perfect Janus to Dumbbell Janus.
When applying into real working systems, we usually use two immiscible oil phases and one water phase to work as system liquids with surfactants to change the interfacial tension between water and oil.
For these systems, four combinations can be listed which shown in Figure 2   and changing bigγ 12 to small γ 12 ; 3) Keeping small γ 12 and changing small γ 1 , γ 2 to big γ 1 , γ 2 ; 4) Keeping big γ 1 , γ 2 and changing small γ 12 to big γ 12 . For these four possible ways, 1) and 3) are the transformation between big γ 1 , γ 2 to small γ 1 , γ 2 and keeping the small γ 12 while 1) and 4) are the transformation between big γ 12 to small γ 12 and keeping the big γ 1 , γ 2 . (b) The analysis tree of the working system transformed from Perfect Janus and Dumbbell Janus. 1) A and B are oil phases while the continuous phase is water phase with or without surfactant to achieve the transformation between big γ 1 , γ 2 to small γ 1 , γ 2 . The continuous phase is relatively simpler to replace, thus this method is relatively easy.2) A and the continuous phase are oil while B is water with or without surfactant to adjust the value of γ 12 .The problem might be that it is difficult to add or remove surfactants in the dispersed oil phase after forming droplets to achieve in-situ droplets reconfiguration. 3) A and the continuous phase are oil while B is water with or without surfactant to adjust the value of γ 12 or γ 2 .The problem is that γ 1 might keep the same that fail to achieve the transformation. 4) A and B are oil phases while the continuous phase is water. Here the surfactant is added into one oil phase(A or B) to adjust γ 1 or γ 2 .The problem might also be difficult to operate and fail to achieve the transformation. Thus through the enumeration method, we could conclude that the optimum system combination is two dispersed oil phase and the continuous water phase with or without surfactant.(c) 1) The transformation of the interfacial tensions and the possible liquid systems.
2) The interfacial tension between ETPTA and liquid paraffin with the increasing concentration of While in real operation, it is relatively difficult to add a surfactant into dispersed phases which have been sheared to be one part of droplets. Thus we could ignore the methods above of adding surfactants into dispersed phases. As for the condition of adding surfactants into the continuous oil phase, it has the problem that it just change the interfacial tension between water and oil rather than that between two oil phases. That is, & might keep unchanged while ' is decreasing largely.
To make all the logic analysis a conclusion, we found that the system composed of two oil phases working as A and B and water working as the continuous phase fits the reality best. When operating, we only need to replace the pure water with water solution adding surfactants or do the reverse.
With the analysis in the system combination, it has been listed to choose the oil/oil/water with(without) surfactants to achieve the transformation between Perfect Janus and Dumbbell Janus structures. Here we choose ETPTA and Liquid paraffin as oil phases and water as continuous phase. ABIL 90 is an oil-soluble surfactant and PF-127 is water-soluble surfactant to adjust the interfacial tensions. In this system, the &' needs to be small (the smaller, the better), and the & and ' need to be changed from big to small as shown in Figure 2 Janus. Thus we could try this system as one of our transformation systems.
To make a conclusion, the systems we used in the manuscript is shown in Table S1.

3.The experiment process diagram
We use the double-core capillary microfluidic device to form the droplets and then collect them into a cap which is used to capture the emulsions in the petri-dish. Then we replace the ambient solution of the droplets in the petri-dish through Polytetrafluoroethylene (PTFE) pipe by pumping new ambient solution and extracting original ambient solution. After replacing the ambient solution, the in-situ morphology transform to another shape. To observe the process, microscope connecting to the computer is used to record videos. The process is shown in Figure S3.