The techniques and tools employed to manipulate and transfer thick, adherent fluids in laboratory settings. Unlike their less viscous counterparts, these liquids resist flow, behaving more like honey than water.
For lab scientists, this inherent resistance presents a unique set of challenges. The very properties that define these liquids—cohesion and adhesion—can lead to inaccuracies in measurements, inconsistencies in experimental results, and even physical strain during manual pipetting. As such, mastering the intricacies of handling viscous liquids is not just about precision; it’s about ensuring the reliability and reproducibility of scientific research.
Glycated liquids are composed of sugars and exhibit higher adhesion to the pipette tip wall and comparatively higher cohesive forces compared to other viscous liquid categories. The viscosity of these liquids varies depending on the dilution. As these liquids stick to the tip, slowing tip withdrawal rate after aspiration and dispensing from liquids aids in avoiding bubbles, droplet formation, and clean aspiration and dispensing.
Volatile viscous liquids are a blend of glycerol/PEG and a volatile solvent such as ethanol or isopropanol prototypical. The glycerol/PEG addition increases viscosity whereas the volatile solvent increases vapor pressure. Thus, to pipette such liquids, users can use the default aspiration flow rate used for water. However, the dispensing flow rate needs to be reduced. To counteract dripping problems due to higher vapor pressure, adding an air gap after aspiration is necessary to ensure accurate pipetting.
Viscous surfactant liquids are a blend of a surfactant and a viscous glycated liquid. The most popular viscous surfactants are Tween® 20 and Triton® X – 100 with viscosity as high as 400 millipascal-seconds (mPa). These liquids exhibit higher adhesion to the pipette tip but lower cohesive forces. The viscous surfactants must be aspirated with slower aspiration flow rates and withdrawn with minimal withdrawal speed. The dispensing flow rate needs to be reduced such that ample time is available for the liquid to slide off the wall of the pipette tip.
Oils exhibit similar properties to viscous surfactants and can be handled with the process described above. Oils require longer time to slide off the wall completely, meaning it takes longer for the pipette to cleanly dispense. Slower withdrawal helps reduce loss of oil sticking to the exterior wall of the pipette tip, but since the adhesive forces of the oils to the tip are higher than the cohesive forces of the oil, clean dispensing of oil requires longer liquid settlement time causing delays.
One of the most commonly employed techniques for viscous liquid handling is reverse pipetting. In this method, more liquid is drawn into the pipette than is needed. The desired volume is then dispensed, leaving the excess behind. This technique is particularly useful for reducing the formation of air bubbles and ensuring accurate volumes. By ensuring that the pipette tip is fully wetted and then dispensing only the desired volume, reverse pipetting can improve accuracy, especially for very viscous solutions.
The two-step dispensing method is designed to enhance precision when working with viscous liquids. Initially, a portion of the liquid is dispensed slowly to ensure that the desired volume is being transferred accurately. Following this, the remainder of the liquid is dispensed more rapidly. This two-phase approach ensures that the liquid is released in a controlled manner, minimizing the chances of air entrapment and ensuring a consistent and accurate dispense.
The touch dispense strategy involves making direct contact between the pipette tip and the receiving vessel’s wall or surface. By touching the tip to the side of the vessel, the viscous liquid can flow down the wall, reducing the chances of it sticking to the pipette tip or forming droplets. This method is especially beneficial for highly viscous or sticky liquids, ensuring that the entirety of the sample is transferred and minimizing waste.
Designed with a broader opening compared to standard pipette tips, wider bore tips are tailored to handle very thick solutions, gels, or slurries. The expansive opening reduces resistance during aspiration or dispensing of viscous liquids, facilitating smoother flow and minimizing the chances of air bubble formation. This design is particularly beneficial when working with extremely viscous samples where a standard tip might clog or deliver inconsistent volumes.
Crafted from a unique polypropylene material or coated with a hydrophobic substance, low retention tips are the go-to choice when maximum sample recovery is essential. These tips minimize the liquid’s adhesion to the surface, ensuring that almost all of the aspirated volume is dispensed. This feature is especially valuable when handling precious samples, ensuring minimal wastage and optimal accuracy.
Incorporating an integrated barrier or filter, these tips are designed to prevent the liquid, aerosols, or any contaminants from reaching the pipette. This design ensures sample purity and offers an added layer of protection for the pipette from potential contamination or damage. Filter tips are indispensable in workflows like PCR or when handling DNA/RNA, where even minute cross-contamination can compromise results.
Unique in their design, these disposable tips come equipped with an integrated piston. This piston directly interacts with the liquid, bypassing the air cushion typical in standard pipettes. The result is a precise and complete dispensing of even the most viscous or volatile liquids. For researchers dealing with highly heterogeneous samples or those that are prone to evaporation, these tips offer a reliable solution.
Longer than the standard pipette tips, extended length tips are designed for easy access to the bottoms of deep vessels or tubes. Their length ensures that the entirety of the sample is accessed and dispensed, minimizing dead volumes and ensuring consistent results. These tips are particularly useful when working with deep well plates or long tubes where a standard tip might not reach the sample effectively.
Beyond the standard polypropylene, some pipette tips are crafted from materials tailored for specific liquid properties. Depending on their composition, these tips can offer benefits like reduced static, enhanced chemical resistance, or other specialized properties. Their application is specific to the nature of the liquid or the experimental requirements, ensuring that the material of the tip complements the sample’s properties for optimal results.
The OT-2 is a bench-top liquid handler designed to be accessible and flexible enough to automate many common applications.
Automated liquid handling systems, like the OT-2 or other advanced robotic systems, can be programmed to handle viscous liquids. These systems can adjust pipetting speeds, use specialized tips, and employ algorithms to ensure accurate and reproducible results. The automation process typically involves: