Drop deformation in miniature channels under electric field

Original paper: AC electric field induced droplet deformation in a microfluidic T-junction 

Microfluidics is the science and technology of manipulating small volumes of fluid in channels with dimensions as small as the size of human hair. You can think of a microfluidic system as a plumbing network composed of miniature pipes. Microfluidics has the potential to advance biology, chemistry, and medical diagnostics by allowing many operations such as mixing of fluids, and synthesis of materials, as well as lab analysis to be miniaturized and integrated into a single device. Such a device is typically only a few cm² in size and is called a lab-on-chip platform. Many analyses that are done using lab-on-chip devices use droplets. For example, instead of growing cells on a flat surface, such as Petri dishes, it is possible to grow cells inside a droplet. The advantage is that one can better control the microenvironment, allowing high throughput single cell manipulations. 

Electric fields are often used in lab-on-chip systems to control droplet generation, sorting, merging, and mixing. These active droplet manipulation methods often involve deformation of droplets. Although there are other techniques for droplet manipulation such as thermal, magnetic, and acoustic, electric fields are often preferred as they provide a faster response time. However, the interaction between electric fields and droplets in lab-on-chip systems is poorly understood. Thus, it is of vital importance to have a better understanding of an electric field induced droplet deformation.

In general, when a uniform electric field is applied to a conductive drop, for example, a water drop containing salt, suspended in an insulating liquid such as mineral oil, charges will accumulate at the drop interface due to a mismatch between the electrical properties of the water and oil. The accumulation of charges at the drop interface (shown in Figure 1) will induce electric stresses that will deform the drop. Today’s paper focuses on the on the effect of electric fields due to alternating current (AC), which is a current that periodically reverses its direction. AC current has many potential biological applications, and its effect on droplets has not been studied extensively.1

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Figure 1. Schematic of electric field induced polarization (movement of charge carriers to the interface) of a conductive drop suspended in a non-conductive medium. Electric stresses deform the drop and result in its elongation in the direction of the electric field.

The researchers used a microfluidic device to make water-in-oil emulsion droplets. The droplets are generated using a T-junction geometry as shown in Figure 2. Water is injected into a flowing stream of oil where it is sheared off into individual droplets. An electric field is applied using two electrodes (shown in black and red in Figure 2) that are positioned on both sides of the droplet channel. The electrodes are not in contact with fluids to prevent electrolysis of water (a process where electricity is used to break apart water into hydrogen and oxygen).

The deformation of the droplet is imaged as it passes through the electrodes using a microscope and a high-speed camera at 5000 frames per second. Before entering the electric field, the droplet takes the shape of a horizontal ellipse due to deformation by the flow. When the droplet enters the electric field, the shape of the droplet changes from the horizontal ellipse to an ellipse with a flattened back side, illustrated in Figure 2,  as electric stresses act mainly in the direction of the electric field.

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Figure 2. Schematic sketch of the microfluidic device and the electric field induced deformation. The droplet deforms as it passes through the electrified region. The blue and gray streams are water and oil phase, respectively. Adapted from Tan et al.

The effect of field strength and AC frequency on droplet deformation is shown in Figure 3 and is captured by a dimensionless parameter D which represents the change in the droplet aspect ratio (the ratio of the droplet length along the electric field direction to the length in the flow direction) after deformation.  At low electric field strength2, D Is directly proportional to the electric field strength and is independent of AC frequency. One possible explanation for this lack of relationship between D and AC frequency is that at low electric fields, viscosity and interfacial tension (a measure of the tendency of liquids to resist deformation by an external force) are the dominant factors determining the droplet deformation.

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Figure 3. Deformation of droplets expressed as D (change in droplet aspect ratio after being subjected to an electric field)  as a function of electric field amplitude at different AC frequencies. Images of the droplet at different field strengths are also presented to show the shape change of the droplet (AC frequency remains at 40 kHz). Adapted from Tan et al.

A promising feature of this setup is the ability to induce controlled oscillation in droplets. The authors used amplitude modulation to induce oscillation in droplets. Amplitude modulation is a technique commonly used in communication applications, such as radio, to transmit information.

In this technique, we start with a wave of high frequency – a carrier wave and add to it a wave of low frequency – a signal wave, as shown in Figure 4. The combination of both will give a wave with the same frequency as the carrier wave and an amplitude which is higher than the original carrier wave.

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Figure 4. Amplitude modulation: the combination of a carrier and signal wave to result in a wave with characteristics of both carrier and signal wave. Image adapted from  https://www.tutorialspoint.com/communication/amplitude_modulation.asp

Tan and his coworkers used amplitude modulation as a periodic on-off signal. The droplet deformed when the signal is on (100% amplitude) and goes back to its original shape (0% amplitude) when it is off, as shown in Figure 5. Here, higher frequency of the AC field corresponds to faster switching of the electric field amplitude.

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Figure 5. Periodic droplet deformation under different AC frequencies.  Adapted from Tan et al.

In summary, the research group led by Tan have provided a unique platform to deform and oscillate the deformation of microdroplets in microfluidic channels. This result has tremendous potential in many future applications including drug screening, cell study, chemical reaction and any other applications for which enhanced mixing conditions are preferred.

1 Both AC and direct current (DC) field can induce drop deformation in a similar physical mechanism as described in the article. However, the use of DC fields often involves heating of the sample due to the use of large direct current. Such heating might be undesirable in biological samples that are heat sensitive. In the case of AC fields, the current is smaller compared to DC, thus there is less heating.

2 At higher field strengths, the droplet aspect ratio depends non-linearly on the field. In fact, high enough fields can even break the droplet apart completely.


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