Considerable efforts have been spent over the last decade developing hydrophobic surfaces exhibiting very large contact angles with water. Many of these methods require complex and expensive fabrication techniques. We demonstrate that sanding Teflon can produce superhydrophobic surfaces with advancing contact angles of up to 151 • and contact angle hysteresis of less than 4 •. Furthermore, we show that a wide range of both advancing contact angles and contact angle hysteresis can be achieved by varying the grit size of the sandpaper, allowing for future hysteresis and contact angle studies. Scanning electron microscopy images of the roughened surfaces depict the range and amplitude of length scales imparted on the surface by the sandpaper, which leads to deeper understanding of the state of wetting on the surface.
In order to make an effective droplet-based microfluidic device, one must be able to precisely control a number of key processes including droplet positioning, motion, coalescence, mixing, and sorting. In a typical three-dimensional device, these processes are well understood. However, for planar or open microfluidic devices, many of these processes have yet to be demonstrated. In this paper, a series of superhydrophobic surfaces created by sanding Teflon are used as the microfluidics platform. The superhydrophobic surfaces used in this study all have advancing contact angles of 150° but have contact angle hysteresis that were varied smoothly from 3° to 30° as the grit size of the sandpaper is changed. Drop motion was initiated by placing the surface on an inclined plane. To deflect and move droplets along the surface, single and multiple transition lines in receding contact angle were created by spatially varying the surface roughness of the Teflon. The degree of droplet deflection was studied as a function of droplet size, droplet speed, and the angle that the transition line in contact angle hysteresis made with the principle direction of droplet motion. Droplet deflections across a single transition as large as 140% the droplet diameter were observed. The droplet deflection was found to increase with increasing difference in contact angle hysteresis across the transition and increasing transition angles up to about 40°. The largest deflections were observed over a very narrow range of droplet velocities corresponding to a range in Weber numbers between 0.1 and 0.2. This narrow range in Weber number suggests that transitions in receding contact angle can be used to sort drops based on velocity, size or wetting properties with a strong degree of selectivity. The direction of deflection was observed to change depending on whether the drops transitioned from a region of low to high or high to low contact angle hysteresis. In a transition from low to high hysteresis, a large portion of the drop's kinetic energy is converted into interfacial energy as the receding contact line of the drop is deformed. Alternatively, a transition from high to low hysteresis results in some of the drop's interfacial energy converted into kinetic energy as the deformation of the droplet is reduced. The result is either a reduction or increase in the droplet's velocity normal to the line of transition depending on the sign of the transition in contact angle hysteresis. Finally, single and multiple stripes of different contact angle hysteresis are also shown to be effective at deflecting droplets.
Large‐scale genetic screening of neonatal dried blood spots for episomal DNA has a great potential to lower patient mortality and morbidity through early diagnosis of primary immunodeficiencies. However, DNA extraction from the surface of dried blood spots remains one of the most time consuming, costly, and labor‐intensive parts of DNA analysis. In the present study, we developed and optimized a rapid methodology using only 50 V and heat to extract episomal DNA from dried blood spots prepared from diagnostic cord blood samples. This electric field DNA extraction is the first methodology to use an electric field to extract episomal DNA from a dried blood spot. This 25‐minute procedure has one of the lowest times for the extraction of episomal DNA found within the literature and this novel procedure not only negates the need for costly treatment and wash steps, but reduces the time of manual procedures by more than 30 min while retaining the 75–80% of the yield. Combined with real‐time PCR, this novel method of electric field extraction not only provides an effective tool for the large scale genetic analysis of neonates, but a key step forward in the simplification and standardization of diagnostic testing.
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