Non-Contact Liquid Handling: Basics and Technologies
Authored by: P. Koltay and C. Ernst
Reprinted with permission by Biofluidix GMBH
Small liquid droplets with volumes of only a few nanoliters or picoliters are used in many applications today. Still much research work is dedicated to understand and to describe the numerous existing droplet generation mechanisms, as well as to propose new technologies and devices particularly designed for specific applications. The major areas of interest that can be identified today are printing and coating, liquid handling in life science and pharmaceutical research, industrial fabrication, and some other smaller applications. Many of these application areas nowadays rely on non-contact technologies to create minute droplets. Non-contact in this context means that a liquid droplet is formed and detached from a nozzle prior to impinging on a substrate. This is in contrast to contact based technologies like pin printing, stencil printing, screen printing or the like which are not in the scope of this article.
Printing and coating in general is certainly the biggest industrial and consumer market. The well known home and office printers based on inkjet technology  make up the largest market share of all microfluidic devices sold today. The same technology is also applied by many industrial large format printers and also in industrial production, for example for optical devices like flat screen displays. High quality color image, low machine cost and low printing noise are basically the main advantages of such inkjet printers producing droplets in the 1 – 100 picoliter range.
Life sciences research as another important application area exhibits completely different requirements compared to printing and coating. For the fabrication of microarrays, lab-on-a-chip systems and liquid handling for drug discovery research for example, a large number of very complex liquids with ever varying properties has to be dealt with . Furthermore, typically hundreds and thousands of different liquid solutions are handled simultaneously whilst avoiding contamination by all means. In more and more cases even particle laden liquids containing cells or beads have to be dispensed, which presents additional technical challenges due to orifice clogging and related issues. In most cases the well established inkjet technology does not fit these requirements and other approaches have to be followed.
Droplet dispensing in the field of industrial fabrication like for the assembly and packaging of semiconductor chips or the fabrication of printed circuit boards (PCB) has again requirements of its own. In this important application area many hot and aggressive as well as highly viscous and particle filled liquids have to be dispensed. Some of these requirements can be met by inkjet techniques, like for example the deposition of molten solder on chips and circuit boards  or adhesives for packaging. However, in many cases the liquids are too viscous for inkjets and often pressure driven fast switching valves are applied to generate nanoliter sized droplets.
Besides the mentioned modern application areas there are also very classical applications especially for spray generation like fuel injection systems for combustion engines or spray technologies for painting, coating and production of powders and micro particles. These applications have developed their own set of specific technologies not covered by this article. The focus of this article will remain on devices being able to deposit individual droplets one by one in a controlled way. Amongst them there are the classical drop-on-demand inkjet devices but there are also other technologies capable to produce individual droplets as the following sections will show.
Droplet Breakup conditions
Before considering different technologies for non-contact droplet generation the conditions for droplet breakup will be derived in the framework of a simple physical model. This model can also help to better understand the influence of liquid and nozzle properties on the dispensing result.
Criteria for droplet generation
In a very simple model a droplet dispenser can be considered to consist of a circular orifice with diameter D filled with a liquid having density ρ, surface tension η and viscosity σ like sketched in figure 1.
| Figure 1: Sketch of the energy required to create a droplet. Es = surface energy, |
Ed = viscous dissipation, Ek = kinetic energy
In order to create a small droplet emerging from such a nozzle, the energy driving the droplet ejection has to be delivered quickly. If not, the droplet is wetting the rim of the orifice and a pending droplet is created. Sufficient energy needs to be provided to the nozzle to account for following effects which consume energy:
- The generation of the surface energy of a droplet Es is requiring a part of the supplied energy. Similar to a balloon that needs energy to be inflated; also the surface tension of the liquid stores some energy (indicated by arrows in figure 1) which has to be delivered during droplet generation. The amount of this energy scales with the surface tension and is inversely proportional to the droplet size. This means that more energy is required to generate the surface of a small droplet than for a large one.
- The flow of liquid through the nozzle during droplet ejection causes viscous dissipation Ed through the inner friction of the liquid molecules represented by the viscosity of the liquids. The longer the flow path, the smaller the nozzle diameter and the higher the viscosity the more energy is required to flow the liquid through the nozzle.
- Finally, the fraction of energy remaining, after the liquid has passed the nozzle orifice and has established the surface of the droplet is provided as kinetic energy Ek to the droplet. This kinetic energy determines the velocity of the droplet and is also responsible for secondary processes like satellite formation if it is too large.
Regardless by which actuation mechanism the energy is supplied to the orifice to eject a droplet or jet, a certain minimum power is required to release a droplet of a specific liquid from a nozzle of given size. In order to determine suitable conditions for the ejection process, droplet generators are studied throughout the literature in terms of the so called Weber number and the Ohnesorge number . Such dimensionless numbers are commonly used in fluid dynamics to characterize the flow situation qualitatively. The Weber number, being defined as the ratio of surface tension energy to kinetic energy,
provides a good estimate to determine whether a droplet has sufficient kinetic energy to overcome the surface tension at the orifice and to create a free flying droplet. In other words the Weber number helps to distinguish a situation where a droplet drips out of an orifice (low Weber number) from a free flying droplet shot from a nozzle at sufficiently high velocity (high Weber number). In particular it turns out that for inviscid liquids a free droplet of approximately the size of the orifice can be produced only for Weber numbers larger than 12, which is the so called critical Weber number .
In cases where the liquid’s viscosity is not negligible the critical Weber number can be much larger than 12 and a second quantity – the Ohnesorge number - is required to determine the conditions for free droplet ejection. The Ohnesorge numbers helps to discriminate “water like” flow situations (i.e. liquid jets disintegrating due to surface instabilities into many droplets) form “honey like” situations (i.e. liquid jets which form long tails). The Ohnesorge number considers all liquid properties and is defined as follows:
In terms of physical properties the Ohnesorge number can be interpreted as the ratio of kinetic energy compared to the energy dissipated by the viscous flow. The higher the Ohnesorge number the higher becomes the critical Weber number and the more difficult it is to create a free flying droplet at all. The steep increase of the critical Weber number with increasing Ohnesorge numbers is the reason why creating small droplets from viscous liquids is so difficult.
Depending on the actual values of the Weber number – assuming the Ohnesorge number to be close to zero for the remainder of this article – in general three different regimes of droplet or jet breakup can be distinguished  as depicted in figure 2:
|Fig. 2: Different breakup regimes separated by lines representing Weber numbers We=8, We=12 and We=40 as function of the velocity v versus the droplet diameter D.|
This breakup regime is characterized by an ejection of a single droplet or jet with a diameter equal or slightly bigger than the nozzle diameter. The ejected droplet or jet can be followed by smaller satellite droplets or a tail which could also disperse into single satellite droplets after a while. To ensure droplet breakup a fast actuation is required to create a sufficiently high Weber number in the range from We = 12 to We < 40. All of the devices discussed in the following sections are operating in this regime.
Rayleigh breakup regime
The Rayleigh breakup regime is characterized by a continuous operation. A liquid jet is ejected out of a nozzle continuously that disperses into single droplets due to the so called Rayleigh instability. Typical Weber numbers are in the range of about We = 8 to We < 12. Below a value of We = 8 there are no free flying jets or droplets, but dripping is observed.
The atomization breakup is mainly characterized by a high speed liquid jet which disperses into a fine spray of many single droplets outside the nozzle. The actuation is continuous and very strong, which leads to very high velocities at the orifice producing Weber numbers larger than We > 40.
Based on this very basic consideration, relying on the Weber number only, important design rules can be derived readily. In figure 2 for example the Weber number is plotted as function of liquid velocity at the orifice and droplet diameter. It can be deduced easily which flow velocity has to be achieved by an actuator mechanism to be able to eject a droplet of given size. How this velocity is achieved by a certain dispensing technology is a different matter. In particular this turns out to be the most difficult part in practice; or in the words of E.R. Lee: "The process of drop ejection is not as simple as taking a fluid chamber with a small hole and pressurizing it enough for fluid to start emerging from the ejection nozzle hole" .
Classification of droplet dispensers
As pointed out before, the actuation method which drives the droplet ejection is a key element of any dispensing device and therefore it is natural and common practice to classify dispensers according to the adopted actuator (e.g. piezo-electric, thermo-electric, pressure driven etc.). However, ultimately the effect of the actuator on the liquid determines the droplet ejection and not the actuator itself. Therefore it is more precise to consider the fluidic boundary condition (BC) applied to eject a droplet from the orifice of a dispenser for classification.
Grounded on the concept applied in computational fluid dynamic (CFD) simulations to apply pressure boundary conditions respectively flow boundary condition to model the effect of the actuation, dispensing devices can be classified in two categories: Either a predetermined pressure is provided by the actuator and the flow is free to evolve or the flow is set by the actuator and the pressure is free to take on a certain value depending on the liquid properties, geometry, etc. In both cases the actuator is assumed to be able to provide an infinite pressure, respectively flow, which is obviously an idealization.
In fact, there exist real droplet generators where a combination of pressure and flow represents the correct boundary condition. In this case neither an ideal pressure source nor an ideal flow source is the correct assumption. The pressure provided by the actuator is influenced by the flow and vice versa. Due to this, also a third group of droplet generators with a combined pressure and flow boundary condition has to be considered.
Finally, a fourth group is required to complete the classification that accounts for acoustic actuation. Devices driven by acoustic actuation are characterized by rapid pressure oscillations which propagate through the liquid without inducing a substantial net flow. The basic model of a dispenser sketched in figure 3 illustrates the difference: A pressure or flow BC produces a net liquid flow through the dispensing device where pressure and flow are closely coupled. In contrast an acoustic actuation generates a periodic pressure distribution inside the device (represented by the periodic color shades) and pinches the droplet off at the nozzle due to a high local pressure gradient. The net liquid flow through the device is negligible in the latter case.
The acoustic actuation shows an excellent reproducibility, but is sensitive to the liquids properties (viscosity, surface tension etc.) In contrast the displacement principle relying on a flow boundary condition is the more robust actuation principle, but it is more difficult to miniaturize.
Non-contact dispensing technologies in the nanoliter to picoliter range
Valve dispensing technologies
Pressure driven valves
One of the most common methods to generate droplets in the nanoliter range is to apply fast switching solenoid or piezoelectric valves. Such valves are typically fed by a fluidic line from a pressurized reservoir or by syringe pumps like sketched in figure 4 a). Upon fast opening of the valve droplets are ejected.
|Fig. 4 a) Pressure driven dispensing valve b) Syringe Solenoid dispenser (graphics from Innovadyne)|
The pressurized valve technology has the advantage that in principle arbitrarily high pressures can be applied to realize the required Weber numbers. Therefore this technology is very prominent for jetting adhesives in industrial applications. Main drawbacks of this technology are the costs of the high performance valves, the low degree of miniaturization, clogging issues with particle laden liquids and high maintenance efforts for cleaning the valves. The smallest volumes achievable with this method are in the range of 50 nL.
Syringe Solenoid Technology
In terms of the presented classification the pressurized valve technology clearly falls into the category of devices driven by a pressure BC. The pressure in the reservoir – which is created typically by some large external compressor – can be considered to be not influenced by the droplet that is issued from the orifice. The control of the pressure inside the system is complete and defined by the settings of the device (pressure, valve opening time, resistance of tubing, etc.).
However, if the pressure is generated by a controlled displacement through a syringe like displayed in figure 4 b, such syringe solenoid dispensers have to be considered to be driven by a flow BC rather than by a pressure BC. The reason for this is that the total amount of liquid that can flow through the nozzle is pre-defined by the syringe displacement. The liquid volume displaced by the syringe creates an overpressure in the elastic tubing, which is suddenly released upon opening the valve. Due to the pre-defined liquid volume this technology is less sensitive to temperature variations and variation in valve switching times. The accuracy is improved through the previous metering of the liquid by the syringe. A comprehensive comparison of valve based dispensing technologies and others is provided by Innovadyne (http://www.innovadyne.com/tech_comparison.html.
- Pneumatic micro dispensing valves
- Liquid handling systems based on valve technology
Inkjet printheads are the most popular and well know droplet dispensing devices, the most prominent technologies being the thermal inkjet or bubble-jet closely followed by piezoelectric technologies. An excellent overview on all the inkjet technologies is given by . Though, all inkjet devices are used commonly for a similar purpose, mainly printing, they can rely on quite different actuation technologies. In terms of the proposed classification they can fit in various categories depending on the design of the device and the power of the actuator. Typically the actuators are small and do not have more power than the required minimum. Therefore often a coupling between fluid flow and actuator movement is given through fluid structure interaction. In these cases the classification as device being driven by a combined BC is appropriate. In other cases the actuation is guided or amplified acoustically. Such devices are best characterized by an acoustic BC.
|Fig.5: functional principle of a thermal bubble jet|
As an example, the very prominent bubble-jet technology will be discussed briefly. The functional principle of a thermal bubble jet printhead is displayed in figure 5. An electrical current applied to a micro heater leads to a very short heating pulse at the solid-liquid interface. Consequently, a small vapor bubble is generated which expands explosively. The increasing vapor bubble leads to a volume displacement of the ink towards the nozzle and finally to a droplet ejection. After switching off the heater the bubble cools down and collapses. The suction of the collapsing bubble and the capillary forces inside the printhead lead to refilling of the nozzle chamber that is finished before the next shot within approximately 10 µs.
Other dispensing technologies
Glass Capillary Dispensers
One prominent example for a dispenser driven by an acoustic BC is realized by the working principle of glass capillary dispensers. The glass capillary is enclosed by a piezoelectric crystal which generates a pressure wave, in this case an acoustic wave, caused by the electrical actuation. The pressure wave travels to the nozzle, where a high local acceleration up to 100,000 g occurs which ejects the droplet. The ejected droplet size mainly depends on the size of the nozzle. The glass capillary dispensers show a reproducibility of < 0.1 %. Drawback of this technology is its sensitivity to liquid and reservoir pressure changes. The origin of this technology dates back to early patents of Steven Zoltan in the 1970’s. A comprehensive review of the applications and physics of this technology is given in the excellent book of E.P. Lee .
|Fig. 6: Glass capillary dispensing system from Microdrop|
Ultrasonic droplet generation
The ultrasonic or acoustic droplet generation technology was originally invented by Xerox for printing applications at the Palo Alto Research Center (PARC). This technology basically relies on the principle to focus ultrasonic acoustic energy on the surface of a fluid sample to eject small droplets. Sound waves generated by a piezo transducer are focused by an acoustic lens through the bottom of the fluid reservoir (e.g. wells of a microplate) and through the fluid. The pressure of the focused acoustic waves generates a high local gradient, and a droplet is ejected from the surface of the liquid.
The volume of the ejected droplet ranges from several picoliters up to hundreds of nanoliters of a single droplet. So, the droplet volume is adjustable in a wide range. However, the ejected volume depends on the liquid properties. The acoustic technology has applications in specialized particle manufacturing for production of pharmaceuticals, life science reagents and cosmetics. It is as well applied for liquid handling in pharmaceutical research. Drawbacks of the technology are the relatively large equipment featuring a complex control systems and the single channel serial dispensing process. If no acoustic focusing lens is applied, the piezo actuation of a free liquid surface creates an undefined mist of droplets with a random size distribution of microdroplets. Fluid nebulizers based on this effect are often used for humidification devices in various applications http://www.americanpiezo.com.
- Acoustic Droplet Ejection
The key element of the PipeJetTM technology is an elastic plastic tube which is squeezed by a piston. By the deformation of the tube a volume displacement and subsequently a liquid flow is induced. If the flow is fast enough to overcome the critical Weber number a droplet is ejected. The volume of the droplet is adjustable by the tube size and the piston displacement in a range from one to several hundred nanoliters .
Another nice feature of the PipeJetTM technology is that all fluid contaminated parts can be exchanged very easily. Due to the low costs of the plastic tube they can even be considered disposable. Thus, many drawbacks associated with cleaning and cross contamination in other dispensing systems can be eliminated. Due to the straight geometry of the tube (no corners, edges or bends hinder the liquid flow) clogging is hardly observed for this method compared to valve based or inkjet systems where this is a frequent problem.
- BioFluidix/ PipeJet Technology
- BioFluidix/PipeJet Products
In dispensing applications it is often required that a multitude of different liquids can be handled simultaneously. For fabrication of microarrays or biochips often even more, like hundreds to thousands of different solutions have to be printed to form a regular array of DNA, antibody or protein spots. Such printing can be achieved by inkjet technologies in a serial way (spot by spot) by inkjet or glass capillary dispensers. As an alternative the TopSpot® method has been proposed  for printing microarrays in a highly parallel way (see figure 11 and video).
Fig. 11: a) Working principle of the TopSpot® method b) TopSpot® 24-channel printhead c) parallel droplet ejection from the printhead d) video TopSpot® printing principle
The TopSpot® technology relies on a micro fabricated printhead which can be filled with different liquids. The printhead consists of three layers, which are manufactured with lithographic techniques. The top layer contains either 24, 96 or 384 reservoirs for the sample fluids. The liquids are transported from the reservoirs to the nozzles by capillary forces. The nozzles are pressurized from the back by a piezostack actuator driving the piston. The piston movement compresses the air in a closed cavity in the back of the nozzles and generates the required pneumatic pressure. The pressure pulse acts equally upon all the nozzles, causing them to simultaneously eject a single droplet. The TopSpot® technology is a perfect example for a dispenser driven by a pressure BC.
The volume of the droplets ejected by TopSpot® devices is typically in the order of 1 nL. The exact amount of the dosage volume is determined – like in inkjet devices – by a complex interplay between liquid properties and actuation parameters. Different volumes can be achieved by using different nozzle diameters, liquids and piston movement.
EMB (Electro-Magnetic Bellows)-technology
The key element of the EMB-technology is a metallic below driven by an electromagnetic actuator. The Electro-Magnetic actuator acts like a motor or a linear actuator of the system by creating force to move the bellows. There is no friction in the system because the whole dispensing system is based only on internal spiral spring forces: the bellows spring force and the electro-magnetic actuator spring force. The bellows act like a piston in the system and both aspiration and dispense are done with the same module like in any syringe system. In some sense the EMB combines the performance of a dispensing valve with the function of a pump in one single part. The total dispense / aspirate range offered by EMB-technology can vary from 50 nanoliters to 1450 microliters. EMB dispensers are claimed to have a very long maintenance free lifetime.
Non-Contact quality control methods
Since all non-contact dispensing technologies produce tiny and fast droplets, measurement and control of the individual droplets is a difficult task. For calibration purposes often fluorescent or gravimetric methods are used which are applied to number of individual droplets which constitute a larger volume. This approach has only a limited accuracy and does not provide information on the single droplet level. Furthermore such a method leads to the loss of the investigated droplet. For most inline quality control applications the measurement technology however needs to be contact free, otherwise the droplet could not be used for the desired purpose. On the one hand there are few measurement technologies available for this task. On the other hand online volume control of droplets is becoming increasingly important, especially for high-throughput experimental technologies and for quality management in production.
Stroboscopic & high speed camera systems
The most prominent method for observing and controlling dispensing processes is provided by stroboscopic or high speed camera systems. With such systems the shape, size and velocity of the droplets can be monitored and documented contact free during the dispensing process . From the generated droplet images a lot of information can be deduced, like for example droplet shape, presence of satellites, droplet velocity etc. However, since the volume scales with the third power of the droplet diameter, it turns out that this method is not very accurate for determining droplet volumes. Furthermore, expensive camera and illumination equipment is required and the method is – due to spatial restrictions – not applicable, if the dispenser is in close proximity to the substrate like in many printing or liquid handling applications.
Fig 12.: Stroboscopic video of a droplet ejection process by the PipeJetTM Technology
Optical droplet sensor
A simple but efficient non-contact process control, recently released by BioFluidix, is the innovative optical droplet sensor DropSense. The sensor enables the detection of micro droplets in flight by measuring the absorption and refraction, caused by a free flying droplet that passes the sensor's active area. The sensor is well suited for monitoring dispensing processes, since it delivers qualitative information about the volume, shape and velocity of the droplet in terms of a so called "finger print" signal.
Droplet volumes in the range between 1 nL to 100 nL can be detected with the optical droplet sensor. Due to its small size, the optical sensor can be adapted to different non-contact dispensing systems and droplet generators and can be operated inline. By this each individual droplet can be monitored, even the dispenser is in close proximity to the substrate.
Flow sensors basically measure the flow through a fluidic line. With the advent of micro technology, ultra miniaturized flow sensors have become available, which are even able to detect the flow generated single nanoliter droplet ejected out of an orifice. Therefore, a straight forward method to control the dispensing process is to monitor the flow from the reservoir towards the nozzle. Different approaches based on thermal flow sensors or differential pressure flow sensors have been suggested for this .
The big benefit of this technology is that no sensing device has to be placed outside of the nozzle. Therefore, pipetting and dispensing systems can be highly integrated and fit easily into existing automation equipment. Drawbacks of the technology are that the sensors have to be calibrated for a specific fluid type (e.g. system fluid) and can be sensitive to environmental temperature and pressure changes. Nevertheless, the company Seyonic for example is successfully marketing flow controlled dispensing systems for non-contact dispensing applications as well as for conventional air displacement pipetting with disposable tips since several years (Seyonics).
As highlighted by the selected examples, there are many ways to overcome the critical Weber number and to produce droplets on demand. Each method can have its specific advantages as well as its shortcomings, and there is still a lot of room for new technologies to be invented. In principle, dispensing of droplets is more or less only about driving “a fluid chamber with a small orifice”. But this orifice has to be driven “the right way” to achieve the desired droplet properties in terms of size, shape and velocity. A further challenge becoming increasingly important is to take the many requirements of the various applications into account, like for example the need for disposable dispensing tips, multi nozzle systems or inline droplet quality control.
- ↑ 1.0 1.1 H. P. Le, Journal of Imaging Science and Technology, vol. 42, no. 1, pp. 49-62, Jan.1998.
- ↑ J. Comley, Drug Discovery World, vol. summer 2004, pp. 1-8, July 2004.
- ↑ D. Schuhmacher, et al., Proc. IEEE-MEMS 2007, Kobe, Japan, pp. 357-360, 2007.
- ↑ C. Weber, Zeitschrift für angewandte Mathematik und Mechanik, vol. 11, no. 2, pp. 136-154, 1931.
- ↑ W. von Ohnesorge, Zeitschrift für angewandte Mathematik und Mechanik, vol. 16, no. 6, pp. 355-358, 1936
- ↑ G.O.Thomas, „The Aerodynamic Breakup of Ligaments“, Atomization and Sprays, vol.13,pp. 117-129, 2003; PDF
- ↑ S. P. Lin and R. D. Reitz, Annual Review of Fluid Mechanics, vol. 30, pp. 85-105, 1998.
- ↑ 8.0 8.1 E. R. Lee, Microdrop Generation, 1 ed. Boca Raton: CRC Press, 2002.
- ↑ W. Streule, et al. J. of the Assoc. for Lab. Automation, vol. 9, no. 5, pp. 300-306, Sept.2004
- ↑ J. Ducrée, et al., in Proc. IEEE-MEMS 2007, Mizyazaki, Japan, pp. 317-322, 2000.
- ↑ K. Thurow, et al., Journal of Automated Methods and Management in Chemistry, Vol. 2009 (2009), Article ID 198732
- ↑ W. Streule, et al., Proc. Mikrosystemtechnik-Kongress, 2005.
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