Figure 1. Chemical Structure of F-127 [1].

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1 Effect of Surface Properties on Pluronic Deposition Vivian Cheng 1, Christopher Lafergola 1, Melanie Noye 1, Clement Marmorat 1, Miriam Rafailovich 1 1 Chemical and Molecular Engineering Department, Stony Brook, NY, 11794, USA Abstract This paper investigates ways in which Pluronic F-127, a biocompatible triblock copolymer, interacts with substrates based on their surface properties including surface tension, which can be calculated from the resulting contact angle when F-127 is deposited on a given substrate. This testing will aim to provide insight on the physics involved when F-127 dries on a given substrate, since this is commonly seen in topical medications, drug delivery systems, and skin products. By creating F-127 solutions of different weight percentages, and applying them to spin casted polystyrene-silicon wafer substrates, the resulting contact angle can be measured in order to determine surface tension. F-127 solutions were made in 1 weight percent, 5 weight percent, and 10 weight percent batches, and a 15 mg/ml solution of polystyrene was spin casted onto cut silicon wafer squares. Each F-127 solution was dried onto this substrate, as well as DI water as a control, and the contact angle was measured as each droplet dried. This is important because one can determine how F-127 will deposit on a given surface depending on different surface tensions, as well as with different concentrations of F-127 in solution. Optical microscopy and atomic force microscopy (AFM) will also be utilized in order to obtain images of the F-127 that is deposited on the substrate, so the pattern of the deposition can be observed. The coffee ring effect is observed in the images because as a droplet dries, the height decreases as a function of time, with the radius remaining the same, causing deposition to occur on the outer rim of the drying droplet. We hypothesize that we can control the evaporation rate of water by using varying concentrations of F-127, and that surface tension will affect how F-127 deposits on a given surface. This research will ultimately serve the purpose of providing an in-depth analysis of how F-127 deposits on given substrates, so that observed trends can then be applied to how topical medications, cosmetics, and drug delivery systems will work when being deposited on human skin. Keywords Pluronic, F-127, Droplet, Evaporation, Surface Pattern, Coffee Ring Effect. 1. Introduction Pluronic F-127 is a nonionic triblock copolymer composed of a central hydrophobic block of polypropylene oxide surrounded by two hydrophilic blocks of polyethylene oxide. Figure 1. Chemical Structure of F-127 [1]. F-127 can be used as an antifoaming agent, wetting agent, dispersant, thickener, or emulsifier. By understanding how the conditions of a given substrate affect F-127 deposition, it is easier for dermatologists as well as the cosmetic industry to better understand how people s skin will be affected. Currently, there are many different products that use F-127, namely in drug delivery, but in order to understand how well it will be received by a given individual, the physics involved with F-127 deposition needs to be researched. F

2 solutions were created for varying weight percentages and spun cast onto PS-silicon wafer substrates. Then contact angle measurements were taken for the different solutions. DI water was used as a control, so that one could determine how F-127 will deposit on the surface due to varying surface tensions. Atomic force microscopy and optical microscopy was conducted in order to observe surface patterns of the F-127 that was deposited on substrate. Temperature was kept constant as well since this can also affect the drying process, and adding this variable into the experiment would only serve to complicate results. This research will help to explain how F-127 deposits on given substrates, so that observed trends can then be applied to a variety of application in both the cosmetic and pharmaceutical industry. A common example of an evaporating droplet is a coffee droplet on a table. After evaporation, the droplet leaves a ring-shaped stain often referred to as the coffee ring effect. Droplets placed on solid surfaces can adopt different shapes ranging from spherical to flat shaped based on surface tension, which is excess surface energy per unit area of liquid surfaces that are in contact with another material. The contact angle of the droplet on a solid can be found by the equilibrium forces due to the surface tensions at the liquid-gas, solid-gas and solid-liquid interface. Figure 2. Droplet Evaporating [2]. In figure 2, an evaporating droplet which has a contact line pinned on the surface is shown. The evaporation flux that diverges at the contact line is represented by the arrows above the droplet. It is important to note that while the radius of the droplet remains constant, the height, h(t), slowly decrease as the water evaporate over time. The arrows inside the droplet represent the flow of the liquid in order to compensate for the liquid that is evaporating from the edge. The flow then brings dispersed particle to the outer rim of the droplet until the droplet is fully evaporated, leaving a ring shape around the outer line of the droplet. Interestingly, when the contact line is pinned the evaporating is faster and when the contact angle approaches zero it creates a rush of particles that accumulates around the outer rim which can help explain the coffee ring effect. The top of the droplet is the coldest due to longer conduction paths from the substrate, so evaporation occurs faster along the contact line [2] Figure 3. Electron Micrograph of a sample of 10% F-127 solution [3]. Individual micelles can be visualized on the micrograph of the F-127 sample in figure 3, which shows that it is spherical in shape and demonstrates that a core-shell structure is present. This correlates to F-127 consisting of a hydrophobic poly (propylene oxide) core and a shell comprising of poly(ethylene oxide) and water. The core is lighter due to the lower electron density compared to the shell. 21

3 A previous study by Yeganehdoust et al. looked at the kinetic energy and average pressure of a droplet during the evolution of the droplet. The study found that the curvature can be calculated more accurately when there are a larger number of particles, which results in a higher surface tension in that region. The higher surface tension also correlates to a higher initial kinetic energy and larger average pressures [4]. This removes the coffee ring formation since the substrate slows edge evaporation more than in the center. Developing a better understanding of how droplets dry and the coffee ring effect can help us also better understand how F- 127 will deposit on different substrates as a result of surface tension. We are specifically interested in how F-127 will deposit since F- 127 can be used by many different industries. 2. Materials and Methods 2.1 Creating Solutions for Pluronic Droplets The solutions for our droplets were created using Pluronic F-127 purchased from Sigma-Aldrich and DI water. We measured 1, 5, and 10 weight percent s of F- 127 in relation to DI water and stored in test tubes. A separate test tube was also prepared for 100% DI water for the testing of a control droplet. Figure 4. Cross section and 3D projection of single droplets [5]. The shape of the ink droplets at different temperatures were observed in a previous study by Soltman et al and can be visualized in figure 4. The flux of a fluid to the edge of the drop can lead to buildup of solute as the drop evaporates. The geometric nature of pinning and the curvature at the drop s edge, leads to increased evaporation which can enhance the coffee ring effect. The study explains how heat is easily transferred from the substrate to the edge of the drop which results in improved evaporation near the edge of the droplet compared to the center of the drop. In addition, the study found that increasing the temperature of the substrate also increases the amount of solute transferred near the contact line, while decreasing the substrate temperature decreases rim evaporation. [5] 2.2 Spin Caster The Spin Caster was used to deposit an even film over a silicon wafer purchased from Sigma- Aldrich. The silicon wafers were cut into 1x1 centimeter squares using an X-Acto knife. Polystyrene (PS) was purchased from Sigma-Aldrich. PS solutions were dropped onto the surface of a wafer immediately before spin casting. Tweezers were used to handle wafers because it is important to avoid creating defects on the wafer. In total, thirty PS wafers were spin casted. 2.3 Contact Angle Goniometer The contact angle goniometer was used to measure the contact angle and volume of the droplets of F-127 on PS wafers. A total of fifteen frames were taken with 60 seconds in between frames, in order to track and observe the evaporating droplet. After the completion of all frames for the droplet, each frame was individually 22

4 analyzed by using its contact angle and volume in relation to the time the frame was taken. The contact angle goniometer acquired the core of our data by measuring the contact angle of a droplet in relation to decreasing time. 2.4 Atomic Force Microscopy The surface patterns of each wafer will be observed using AFM. AFM will detect the patterns of the particles deposited from F-127 solution onto the PS wafer. Using data from AFM, we will be able to compare the coffee ring effect amongst different concentrations of F-127. The coffee ring effect is an indication of particle movement and deposition, thus advancing our research on the varying effects of surface tension on F Optical Microscopy The surface patterns of each wafer were also observed using optical microscopy. Images will be taken using different lenses. The microscopy images allow for visualization of a dried droplet on different surfaces. The surface patterns detected using optical microscopy can help to explain the coffee ring effect. 3. Results and Discussion 3.1 Contact Angle Goniometer We started our data collection using the PS wafers. We conducted trials to calculate contact angles for 1 wt% F-127, 5 wt% F-127, 10 wt% F-127 and DI water. For each concentration, we used droplet sizes of 5 µl and 10 µl. Figure 5 a. Volume vs. Time for 5 µl samples. The rate of evaporation can be calculated by finding the slope of each sample. Figure 5 b. Volume vs. Time for 10 µl samples. From this graph it is clear that the rate of evaporation is greatest in the 1 wt% F-127, and slowest in 10 wt% F

5 Concentration of F µl DI Water % wt Rate of Evaporation ( µl/s) reflection from the light onto the droplet could have skewed the contact angle goniometer s ability to calculate the volume of the droplet. In addition, all 5 µl of the solution may not have ejected from the pipet when the droplet was released, thereby altering from the desired results. The white overcast in conjunction with an error with the pipet could have caused the anomaly of the 5 µl, 5 wt% data. 5% wt %wt Time (s) Average of Right and Left Contact Angles ( ) 10 µl DI Water 1% Wt 5% Wt 10% Wt DI Water % wt % wt % wt Table 1. The rate of evaporation is compared between samples. For 5 µl samples, 5% wt concentration evaporated the fastest and 10% wt concentration evaporated the slowest. For 10 µl samples, 1 wt% concentration evaporated fastest and 10 wt% evaporated the slowest. Table 1. indicates that for both droplet sizes of 5 µl and 10 µl, the droplet containing the highest concentration by weight of F-127 took the longest to evaporate. Compared to our control sample of DI water, droplets with F-127 showed a trend of evaporating slower. Therefore, a higher concentration of F-127 hinders the evaporation of water, thereby indicating that F-127 delays the reduction of the height of the droplet. The 5 µl, 5% wt% F-127 data deviated from the norm in regards to evaporation rate due to some possible errors. While gathering contact angle data, the Table 2 a. Contact Angles for 5 µl samples for the first 3 minutes. The contact angle was found using the contact angle goniometer, and by averaging the right and the left contact angles of each individual droplet. The errors found in tables 2 a. and b. was the standard deviation of a single sample s left and right contact angle. The right and left contact angles should similar, if not exact since the liquid from the droplet disperses outward from the center symmetrically. The reflection from the light blurred the edges of our droplet in certain 24

6 Time (s) Average of Right and Left Contact Angles ( ) DI Water 1% Wt 5% Wt 10% Wt Figure 5 c. The volume of four samples of a 5 µl droplet of 5 wt% F-127 compared to its height Table 2 b. Contact Angles for 10 µl samples for the first 3 minutes. frames, making it difficult for the goniometer to exact the contact angles. A discrepancy between the right and left contact angle measurements could also have resulted from existing scratches and particles on the substrate. The standard deviation was found by averaging the right and left angles, subtracting the average from each number, squaring the results, calculating the mean of those squared differences, and lastly square rooting the mean. A high contact angle indicates that the liquid is hydrophobic and displays a low wettability. A lower contact angle has higher wetting which means the liquid spreads over the surface compared to a higher contact angle. The surface energy is the amount of intermolecular force created at the surface of a substrate. [6] For a low wetting substrate, the surface tension of the liquid is stronger than the surface energy. Thus, the higher the contact angle, the lower the surface tension. Figure 5 d. The volume of four samples of a 10 µl droplet of 5 wt% F-127 compared to its height. In Figure 5 c. for the 5 µl sample of 5 wt% F-127, it is apparent that the height remains relatively constant as the volume decreases. The drying of a droplet begins when its height begins to decrease and its liquid begins to flow out in opposite directions. Since the height remains drops slower than the control, the droplet can be categorized as frustrated because the surface tensions of the substrate hold on to the liquid at the edges of the droplet. The other samples of a 10 µl droplet and 5 wt% F-127 shown in figure 5 d. showcase normal drying conditions. 25

7 Our contact angle results show that the droplets with F-127 concentrations have a smaller contact angle compared to DI water. Therefore, according to our results, droplets with F-127 have a higher surface tension with the polystyrene substrate than DI water alone does with the same substrate. 3.2 Atomic Force Microscopy (AFM) and Optical Microscopy Atomic Force Microscopy was used to observe how the F-127 deposited on the PS substrate for each concentration. This reveals a lot about how the F-127 deposits on a nanoscale. Optical microscopy is also an important tool since surface patterns can be observed, which provides insight as to how different concentrations of F-127 will deposit. From the images we obtained it was clear that concentrations of F-127 that are 5 wt% or higher do not produce clear ridges, rather, the surface is much more bulky and rough. This is most likely due to the fact that concentration was simply too high, hence why the focus was on concentrations of 1% and 0.1%, since these concentrations produced images that were much more telling about how F-127 will deposit on a given surface. Figure 6 b. 1 wt% AFM image, stripes region. stripes in figure 6 a show that there is some indication of a surface pattern from F-127 deposition, but there is too much F-127 to be able to analyze the surface structure in more detail. The AFM image in figure 6 b shows one of the stripes in detail, but it is hard to make out any details because of this bulk amount of F-127. Figure 7 a. 5 wt% microscopy image at 10X Figure 6 a. 1 wt% microscopy image at 10X. Figures 6 a,b are images found using 1WT% 5 µl F127 on PS substrate. The Figure 7 b. 5 wt% AFM image, center region. 26

8 Figures 7 a,b correspond to images of the 5WT% 5 µl F127 on PS substrate. These images were even more difficult to interpret due to the higher concentration of F-127. For this reason, we chose to create samples for 0.1 wt%. Since this concentration produced samples that were much easier to analyze, since they had clearer surface patterns. products. A perfectly smooth silicon wafer substrate is great in a lab setting, however, in real-life applications the surface will rarely be smooth, especially when considering how unique each individual s skin is. Figure 8 a. 0.1 wt% microscopy image at 5X. Figure 9 a. 0.1 wt% microscopy image at 5X. Figure 9 b. 0.1 wt% microscopy image at 50X (left) and an AFM image of the area (right). Figure 8 b. 0.1 wt% microscopy image at 50X. Figures 8 a,b show optical microscopy images for a 5 ul drop of 0.1 wt% F-127 on a PS substrate. From these images, it is clear that the lower concentration of F-127 results in a smoother surface with more distinct surface patterns. Interestingly, defects present on the surface cause the F-127 to wrap around the defects as it deposits, as a result of increased surface tension in the area of the defect. This sheds light on the how surface defects will affect F-127 deposition which is important in medical applications that utilize F-127 in topical medications, or in the cosmetic industry that use F-127 in topical cosmetic Figures 9 a.b show microscopy and AFM images of a 5 µl drop of 0.1 wt% F- 127 on a PS substrate. The dark spots seen on the surface are most likely the substrate showing through a very thin layer of F-127 deposited in the area. This is most likely due to how F-127 wets a surface selectively, since it will not wet a surface uniformly. When water wets a surface, there is a uniform coating that occurs, however, with F-127 it will wet selectively because of the nature of its amphiphilic structure. This duality results in this unique surface pattern when F-127 wets a surface. 4. Conclusions 27

9 In conclusion, the surface properties of Pluronic F-127 were investigated in order to better understand how it deposits on a given substrate. The contact angles were determined for different solutions of varying F-127 concentration in order to observe the effect of F-127 on the evaporation rate of the droplet. A larger contact angle represents a more hydrophobic substance and demonstrates low wettability. The results indicate that droplets of the F-127 have a higher surface tension with respect to the polystyrene substrate compared to the control, which is DI water. The surface properties of F-127 were observed using AFM and optical microscopy at varying F- 127 concentrations, which showed many stripes originating from the center of the droplet for the 1WT% 5 µl F-127 on PS substrate indicating a change in structure to a hexagonal shape. The coffee ring effect can be observed in the microscopy images since the height of the droplet decreases with respect to time while the radius remains constant, which causes the solution to deposit on the outer rim of droplet. From figure 9 b, we also learn some interesting things about how F-127 will wet a surface since it is not uniformly distributed like water is. Rather, because of F-127 s amphiphilic nature small gaps form where F-127 does not deposit as heavily, most likely due to its selectivity when it comes to the surface it is depositing on. It is difficult to observe these trends in concentrations of F-127 higher than 1 wt%, which is made clear by figure 7 b, which shows bulk amounts of F-127, without any clear surface patterns. Interestingly, the 0.1 wt% AFM images provide insight on how surface defects affect the deposition of F-127. The F-127 deposited in a ring around the defects, in figure 8 b, shows that increased surface tension from the defect results in F-127 deposition that is not uniform. Thus, areas of greater surface tension will have a greater buildup of F-127 compared to the surface of the rest of the sample which is shown to be very smooth and uniform. We had hypothesized that surface tension would affect F-127 deposition, and this has been proven from our research. We also have proved that we can control the evaporation rate of water by increasing the concentration of F-127, since increased concentrations of F-127 results in a slower evaporation rate of water. When it comes to applications to the cosmetic and medical industries, this plays a vital role in how topical products should be designed. Everyone s skin is unique with a variable number of defects, which is why this research can lead to further study of how products can be designed to take advantage of how surface tension affects Pluronic F-127 deposition. Acknowledgements We are grateful for the financial support from the Department of Materials Science & Engineering and the Program in Chemical and Molecular Engineering at Stony Brook University through research funding. References [1] "Applications of Thermo-Reversible Pluronic F-127 Gels in Pharmaceutical Formulations." J Pharmaceut Science. Canadian, 27 Nov Web. 09 Apr [2] Mampallil, Dileep. "Some Physics Inside Drying Droplets." Resonance: Journal Of Science Education 19.2 (2014): Education Source. Web. 12 Oct [3] Lam, Yeng-Ming, Nikolaus Grigorie, and Gerhard Goldbeck-Wooda. "Direct Visualisation of Micelles of Pluronic Block Copolymers in Aqueous Solution by Cryo- TEM." The Royal Society of Chemistry, 28 May Web. 22 Feb

10 [4] F. Yeganehdoust, M. Yaghoubi. H. Emdad, M. Ordoubadi. Numerical Study of Multiphase Droplet Dynamics and Contact Angles By Smoothed Particle Hydrodynamics. Journal of Applied Mathematical Modelling Vol. 40 (2016): Academic Search Complete. Web. 26 Oct [5] Soltman, Dan, Subramanian, Vivek. "Inkjet-printed Line Morphologies and Temperature Control of the Coffee Ring Effect." Engineering Village. Elsevier, 4 Mar Web. 16 Nov [6] Yuan, Yuehua, and Randall Lee. "Contact Angle and Wetting Properties." Springer-Verlag Berlin Heidelberg, Web. 7 Dec