Separation of Floating Oil Drops Based on Drop-Liquid Substrate

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Separation of floating oil drops based on drop-liquid substrate interfacial tension Thamarasseril Vijayan Vinay, and Subramanyan Namboodiri Varanakkottu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b01829 • Publication Date (Web): 18 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

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Separation of floating oil drops based on drop-liquid substrate interfacial tension Thamarasseril Vijayan Vinay† and Subramanyan Namboodiri Varanakkottu* †School

of Materials Science and Engineering,

National Institute of Technology Calicut, Kozhikode, 673601, India *Department of Physics, National Institute of Technology Calicut Kozhikode, 673601, India, E-mail: [email protected] KEYWORDS: Capillary interaction, droplet microfluidics, meniscus, self-assembly, surfactant.

ABSTRACT. Though various strategies exist for the transport of oil drops suspended on a liquid substrate, selective manipulation of different kind of drops based on their respective characteristics remains a challenge. In practical, it is possible to have multiple drops having different wetting states with the liquid substrate, whose separation is desired. In this work, we exploit curvature induced capillary forces for the selective manipulation (transport as well as separation) of oil droplets based on their interfacial tension (IFT) with the underlying liquid substrate. To demonstrate this, we have selected two oils having different IFT with the aqueous

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liquid substrate, and tuned their curvature induced capillary interaction (inward or outward from the source) by controlled addition of surfactant. We experimentally realize three droplet manipulation regimes: repulsion, attraction and separation regime. In the repulsion as well as attraction regimes, both the drops behave in the similar manner. Strikingly, in the separation regime, drops can be effectively separated based on their IFT; low IFT droplets are attracted towards the source while high IFT droplets do the reverse.

INTRODUCTION Droplet based microfluidic platforms have gained significant attention in many scientific and technological domains.1–5 Conventionally, solid substrates have been employed for realizing the same.6 Because of complex fabrication steps as well as high contact angle hysteresis7,8 associated with the solid substrates, slippery surfaces9,10 soft substrates11,12 including free liquid surface are gaining particular attention.13 Transport strategies of immiscible droplets on liquid substrates could be broadly categorized into two classes: individual and parallel manipulation. In the former, the target drops are to be addressed individually, where simultaneous handling of multiple drops is usually cumbersome. For example, oil droplet floating on an aqueous solution could be transported in forward and backward direction by changing the optical path of a laser beam.14 The laser beam locally heats the oil drop creating a convection inside the drop which in turn results in the motion of the drop. In another strategy called chromo-capillary effect, light controlled tuning of IFT could effectively transport oil drops over a photosensitive liquid surface.15 Photo-driven chemo-propulsion involves light irradiation to initiate a series of chemical processes in the drop causing a change in surface tension of the fluid surrounding the droplet, resulting in the movement of the droplet.16 Recently, a similar method demonstrated the


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capability of light based droplet manipulation techniques for 3D droplet motion i.e., through the bulk phase in addition to the surface movement.17 In a novel magnetic based method, efficient manipulation of aqueous droplets was achieved by using a ferrofluid film floating on a liquid surface which serves as magnetic actuator.18 Parallel manipulation of droplets, in which multiple droplets could be transported simultaneously, could be realized either by Marangoni effect13,19,20 or surface deformation based techniques.21–23 For instance, deformation induced on a magnetic liquid surface by a moving external magnet could be used to transport drops as well as liquid marbles.22 Similarly, by utilizing “Moses effect”, diamagnetic objects could be transported on the surface of even non-magnetic liquids such as water or organic liquids.23,24 Light induced surface flows over photosensitive liquid substrate have also been effectively utilized for the controlled transport of floating entities.25–27 Though these methods are effective for the directed transport, all the floating entities are driven simultaneously in the same direction, inward or outward, irrespective of their properties. Manipulation strategies based on capillary interaction of floating objects28,29 could be potential route towards selective manipulation of floating drops, where the mechanism depends on the curvature they create at the interface.30 When adjacent interface deformations from two objects overlap, lateral capillary forces gets triggered to minimize the net curvature of the interface, resulting in a relative motion of the objects.31 Though capillary interactions based transport and assembly of solid objects at a liquid surface are well explored,32–36 its applicability for the manipulation of floating drops remains largely unexplored. Recently, capillary induced motion of drops have realized on slippery surfaces.11, 37, 38

Directed transport of bubbles39 and drops40, 41 towards static menisci also have been briefly

explored recently. However, in practical situations, it is possible to have multiple floating drops


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with different wetting states on the liquid substrate, whose separation is desired. This is an important as well as practically relevant problem which needs to be investigated. PRINCIPLE The working principle is depicted in Figure 1. Lateral capillary force between the objects floating on a liquid surface will be attractive42, when sin 𝜓1sin 𝜓2 > 0

(1)

Where Ψ1 and Ψ2 are the contact angles created by the objects. This is possible when both the menisci created by the objects are positive or negative. The situation where both the menisci are positive is schematically shown in Figure 1 (A). If one of the menisci becomes negative, the force becomes repulsive (Figure 1 (B)), i.e. sin 𝜓1sin 𝜓2 < 0

(2)

In this work we utilize this principle to demonstrate a capillary interaction platform capable of selective manipulation of oil droplets (directional transport as well as separation), based on the IFT of the oil droplets with the underlying liquid substrate. This is achieved by tuning the nature of the meniscus created by floating oil drops by the controlled addition of the surfactant. Accordingly, we selected two oils of similar density with significant difference in their IFT with pure water: sunflower oil (TYPE A, IFT-19 mN/m) and oleic acid (TYPE B, IFT-11mN/m), both of which assume a ‘liquid lens’ shape at the water surface. We employ aqueous surfactant solution as the liquid substrate. We show that proper control the drop-substrate IFT by varying the concentration of the surfactant could be effectively utilized for segregating two kinds of oil drops.


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Figure 1. Principle of bubble mediated capillary interaction platform. (A) Two objects with similar menisci get attracted by lateral capillary forces. (B) Two objects with opposite menisci get repelled by lateral capillary forces. (C) Side view of bubble induced interface deformation. The scale bar corresponds to 1 mm. (D) Repulsion regime at a low surfactant concentration. (E) Separation regime at Intermediate concentration. (F) Attraction regime at high concentration. Upward deformation of interface is denoted as ‘+ve’ and downward deformation as ‘-ve’.

An air bubble growing beneath the surface of water, when touches the interface creates a convex deformation as shown in figure 1 (C) (positive).23, 35 Air bubble allows us to deform the liquid substrate in a non-contact and additive free manner. 35 Oil drops that assume a lens shape on the pure water surface are usually non-wetting and create a concave deformation (negative) at the interface. Such drops, if placed in the periphery of the bubble deformation span, moves away from the bubble because of the opposite nature of curvatures28, 42. This situation is schematically


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shown in Figure 1 (D), which we call as repulsion regime. The nature of capillary interaction can be reversed if the shape of the meniscus created by the drop is changed to convex (positive). This transformation is dependent on the oil-liquid substrate IFT. Oil droplets having a low IFT (TYPE B) undergoes meniscus switching at a lower surfactant concentration compared to that having a high IFT (TYPE A). In this condition, the interaction with a bubble induced deformation, results in an attractive motion of the TYPE B drop towards the bubble. However, TYPE A drop, which is still having a negative meniscus, moves away from the bubble as shown in Figure 1 (E). This scenario could be exploited for the separation of these oil droplets, which we call as separation regime. Further increase in the surfactant concentration, switches the second drop‘s meniscus also to ‘positive’, resulting in the attractive motion of both the drops towards the bubble, as shown in Figure 1(F) (attraction regime). EXPERIMENTAL SECTION In the experiments, we used aqueous Sodium dodecyl benzene sulphonate (SDBS) solution as liquid substrates. Droplet manipulation experiments were carried out in a square container made of transparent acrylic sheets (10 cm x 10 cm). At the center of the container, a vertical flat tip needle (0.72mm outer diameter) acts as an orifice for the creation of the air bubble. SDBS solution was filled in the container up to ~2.5mm above the needle tip. Air bubbles were generated using a motorized syringe pump (SPLF2D, Holmarc) connected to the needle. Bubble grows from the orifice and upon touching the interface creates a positive deformation (convex) field at the interface. The height of the water column is chosen in such a way that the bubble get trapped between the needle tip and the interface. The height of the bubble induced interface deformation was fixed at 1.5 - 2 mm with respect to the undisturbed liquid surface. For the better visualization of the drops, we added a small amount of SUDAN IV dye to the drops. The drops


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are placed on the liquid substrate prior to the creation of the bubble in such a way that the drops fall within the bubble deformation field, which spans about 10 mm from the bubble center. When the bubble deforms the interface, the drops respond to the newly formed interface curvature by translating relative to the bubble position. RESULTS AND DISCUSSIONS Initially, the bubble induced droplet interaction experiments for both the TYPE A and TYPE B drops were performed independently, at different SDBS concentrations of the liquid substrate (03.5 mM). The nature of interaction, whether repulsion or attraction is tabulated in the Figure 2 (See supplementary movies 1, 2, 3, 4 for the results of representative experiments).

Figure 2. Observations from bubble induced capillary interaction for both drops at various concentrations.

At low concentrations, both the drops repelled from the bubble induced deformation field. However, the direction of TYPE B drop got reversed after 1 mM while TYPE A drop required 2.5 mM for the reversal. To understand this behavior we have measured the IFT of both the oils for the above-mentioned SDBS concentrations, as shown in Figure 3. Each data point represents an average value of five data points with standard deviation as error bar.


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Figure 3. Interfacial tension variation of both the oils at different SDBS concentration in the liquid substrate.

Comparing the observations of Figure 2 and Figure 3, the reversal of capillary interaction occurs at a particular concentration where there is a reduction in IFT by about 60% with respect to the pure water. The mechanism behind this reversal can be explained in terms of switching of drop menisci from negative to positive. To confirm this, we have performed a series of experiments by introducing microbubbles (~300 µm) to the liquid and monitored their trajectory in the periphery of the drops. These microbubbles will essentially be guided by buoyancy alone and will follow any surface curvature to reach the topmost peak it can attain on the surface. This will enable us to map very small curvatures at the interface as schematically represented in Figure 4 (A) and 4(B).


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Figure 4. Microbubble experiment. (A) Schematic representation of motion of microbubble near a negative meniscus (low surfactant concentration). (B) Schematic representation of motion micro bubble near a positive meniscus (high surfactant concentration). (C) Snapshots of the repulsion of microbubble from a TYPE A drop on 1.0 mM SDBS solution (D) Snapshots of microbubble getting attracted towards a TYPE A drop on 3.0 mM SDBS solution. Microbubble is highlighted with blue dotted circles for clarity. Yellow arrows denote the direction of motion of bubble. Scale bars correspond to 2mm.

In the experiments, at low SDBS concentrations, the microbubbles moved away from the drop, which indicate that the deformation surrounding the drop is concave (negative meniscus). However, the situation reverses (positive meniscus), i.e., microbubble starts to move towards the drop, at a surfactant concentration of above 2.0 mM for TYPE A, while the it occurs above

1.0

mM for TYPE B drop. Figures 4 (C) and 4 (D) represents the snapshots of representative microbubble experiments (see supplementary movies 5, 6). Strikingly, the outcome of the microbubble experiment corroborates the observation presented in the Figure 2 and Figure 3.


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Based on these investigations we confirm that the menisci surrounding oil drops undergo a negative to positive switching at a particular SDBS concentration, depending on the IFT. The central objective of this work is to realize the three regimes of droplet manipulation by simply varying the surfactant concentration. According to the data in the Figure 2 and Figure 3, we select three different surfactant concentrations. At a concentration of 0.5 mM, both the drops move away from the bubble as shown in Figure 5 (A) (Repulsion regime) (See supplementary movie 7). We have used 5 µL oil droplets for these experiments.

Figure 5. Droplet manipulation regimes. (A) Top row corresponds to repulsion regime at low surfactant concentration (0.5 mM). (B) Middle row corresponds to separation regime at intermediate surfactant concentration (1.5 mM). (C) Bottom row corresponds to attraction regime at high surfactant concentration (2.5 mM). Scale bars correspond to 4 mm. Schematic representation of each regime is presented on the left of each row.

In the intermediate concentration range i.e., 1.0-2.0 mM, TYPE A and TYPE B drops move in opposite directions, which is defined as the separation regime. To test this scenario we have placed TYPE A and TYPE B drops around the periphery of bubble induced deformation, keeping the surfactant concentration at 1.5 mM. The time-lapse images of the motion of the drops around


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the bubble are shown in Figure 5 (B) (See supplementary movie 8). From the images, it is evident that TYPE A drop initially situated nearer to the bubble gets repelled, while the TYPE B drop moves inward from a distance. In other words, we have achieved separation of oil drops based on their IFT, which has not been reported so far. Upon increasing the surfactant concentration above 2.5 mM, both type of drops move towards the bubble center as depicted in Figure 5(C) (Attraction regime) (See supplementary text S1 and movie 9). Thus by simply altering the surfactant concentration, both type of drops could be made to reverse their transport direction with respect to bubble center. The attractive motion of oil drops towards a tunable interface deformation has the potential for their assembly as well as directed transport along complex pathways. Experiments revealed that the drops in the volume range of 1 to 20 µL could be accelerated to a maximum velocity of about 8 mm/s (for TYPE B). Drops could be pulled in from a maximum distance of about 7mm from the bubble. We believe, the presented experiments, in particular the selective droplet manipulation could strengthen the capabilities of open surface digital microfluidics platforms. CONCLUSION In summary, we have experimentally demonstrated a non-invasive capillary interaction platform for on-demand and interfacial tension based selective manipulation of oil droplets floating on liquid substrates. By proper control over the oil-liquid interfacial tension, the platform could perform in three modes: (1) the repulsion regime, where all the drops moves away from the deformation source which can be used for localized cleaning, (2) in the separation regime, different oil droplets can be segregated based on their interfacial tension; and (3) in the attraction regime, multiple droplets can be assembled near the source of deformation. We believe the


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presented results could significantly contribute to the growing interest in liquid surface based microfluidics.

ASSOCIATED CONTENT Supporting Information. Supporting information: Text S1: Experimental analysis of droplet dynamics in the attraction regime based on the surfactant concentration and the drop volume; legends of supplementary movies (PDF) The following files are available free of charge. Supplementary movie S1: Interaction of TYPE A drop with the bubble at a low surfactant concentration (AVI). Supplementary movie S2: Interaction of TYPE A drops with the bubble at a high surfactant concentration (AVI). Supplementary movie S3: Interaction of TYPE B drops with a bubble at a low surfactant concentration (AVI). Supplementary movie S4: Interaction of TYPE B drops with a bubble at a high surfactant concentration (AVI). Supplementary movie S5: Microbubble interaction with a drop at a low surfactant concentration (AVI).


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Supplementary movie S6: Microbubble interaction with a drop at high surfactant concentration (AVI). Supplementary movie S7: Repulsion regime (AVI). Supplementary movie S8: Attraction regime (AVI). Supplementary movie S9: Separation regime (AVI). AUTHOR INFORMATION Corresponding Author *Subramanyan Namboodiri Varanakkottu: [email protected] ORCID id: 0000-0001-7024-3882 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources SNV sincerely acknowledge the funding from Science and Engineering Research Board (SERB), Department

of

science

and

technology

through

Early

Career

Research

Award

(ECR/2017/000583). ACKNOWLEDGMENT SNV sincerely acknowledge the funding from Science and Engineering Research Board (SERB), Department

of

science

and

technology

through

Early

Career

Research

Award

(ECR/2017/000583).


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Separation of Floating Oil Drops Based on Drop-Liquid Substrate

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