Patent Application: US-201013265720-A

Abstract:
production and the distribution of pico and nano - drops , which are extracted by the effect of a strong electric field generated by pyroelectric effect , in particular , but not exclusively , from a sessile drop or by a liquid film , and distributed on a dielectric substrate . the electric field is advantageously generated applying a heat source on the dielectric substrate or utilizing a laser source emitting in the infrared region . in this new approach , it is not necessary to use fixed electrodes , circuits , high tension generators or to design intentionally , and therefore to realize , pico - and nano - nozzles .

Description:
the apparatus according to the invention is shown in fig1 ( a ) and 1 ( b ). in both the shown embodiments 100 , 100 ′, a liquid drop 150 or a liquid film is deposed on a slide 110 , while an upper plate 120 is at a distance d from the slide by electrically isolated spacers 140 . the upper plate 120 is a z - cut ln wafer ( lithium niobate ) ( see section of methods for details concerning the preparation ), for example of 500 μm of thickness . the spacers can be more or less thermally conductors , allowing a greater or lesser activation of the drops ( see later on ). in a first preferred embodiment ( fig1 ( a )), some thermal stimuli are applied in a contactless modus by an infrared source ( laser co2 ) 130 that emits at a wavelength of λ = 10 . 6 μm . in a second form of favourite realization , the heated tip source 130 ′ is in contact with a ln crystal 120 ( fig1 ( b )). the laser and the tip can be moved so as to induce local punctiform point - like thermal stimuli . the distance between the slide 110 and the plate 120 is , for example , of 1 mm . in both the illustrative cases , the z - cut ln crystal reacts to the thermal stimuli by forming an electric potential through its two surfaces ( z +, z −) due to the piezoelectric effect . the pyroelectric effect consists in the change of spontaneous polarisation δps consequent to a temperature variation δt [ 29 ]. at the equilibrium , the crystal ps is completely screened by the charge of external shield and no electric field exists . when the point - like source or the laser beam heats locally the crystal , suddenly a superficial charge density σ appears given by : neglecting the losses , where pc is the specific pyroelectric coefficient of the material ( pc =− 8 . 3 × 10 − 5 c /° c ./ m 2 per ln a 25 ° c .). the electric field has an attractive force on a liquid as shown in fig1 ( c ). the electric potential is due to the electric effect induced on the streamlined substrate by the thermal stimuli operated by the heated tip . in the case of a sufficiently strong electric field , thin jets of liquid can be released by a conical tip structure ( similar to the taylor cone usually utilized in the electro - spray ; nevertheless , in the present case , the liquid is not conductive , consequently there is not exactly the taylor cone regime ) [ 15 - 16 - 17 ]. when the liquid ( either sessile drops or a film ) starts to get deformed under the action of the electric field , two evolutions are possible . in the first case ( i ): if the liquid volume and the separation distance d between the two plates are appropriate , then a stable liquid bridge can be formed ( see fig1 ( c )). for a specific volume , the critical distance under which a liquid bridge can be formed is expressed by [ 30 ]: wherein θ is the contact angle and v the volume of the liquid bridge . for contact angle , here , the starting angle of the cone formed by the liquid is intended , angle that may be different at the ends of the bridge , as shown in the following . this angle is univocally defined both by the liquid cone generated by a drop and by the liquid cone generated by a liquid film , the starting angle being always measured with respect to the substrate or the surface of the remaining liquid parallel to the substrate ( see i . e . fig1 ( e ) and 2 ( b )). a typical liquid bridge is shown in fig1 ( d ). the different contact angles at the upper and lower solid - liquid interfaces are to be noted , what is indicative that the liquid bridge is formed between the two materials with different wettability properties . nevertheless , for the present invention the second case ( ii ) is more important : if the separation d is over the critical value , a stable liquid bridge cannot be stabilised between the plates . the present invention has been reached by searching a way for using this instability with the aim of dosing and distributing the liquid drops . in fig1 ( e ) and 1 ( f ) the typical situations of liquid discharge for two different liquids are shown , almond oil and pdms ( poly - dimethyl - siloxane ) respectively . due to the high viscosity , the emission liquid cone is continuous . the results shown in fig2 ( a ) and 2 ( b ) demonstrate the distribution of almond oil nano - liter drops from a liquid film and from a sessile drops reservoir , respectively ( see methods section ). in the performed experimentation with the liquid film ( see fig2 ( a )), a heated tip has been utilized , whereas , for the sessile drop , the heating has been stimulated by the laser in the infrared zone . the dynamic evolution shows the liquid deformation in both the situations and it is possible to observe from the sequences ( not shown ) how the film and the drop heights grow under the ehd force action . these phenomena have been shot with a cmos camera with a rate of recording photogram of 125 photogram / s . in fig2 ( c ) and 2 ( d ), traces of liquid volumes transferred from the substrate as a function of time ( square ) and the height change ( principal plot ) during the discharge are shown . in the case of fig2 ( a ), the rate is approximately equal to 30 nl / ms . after five discharges , the total volume transferred to the pending drop ( the one formed by the destination substrate ) is 160 nl . the drop acts as a dispenser gun with a rate of repetition that depends on the liquid response to the ehd force . it can be observed that , after the formation of the taylor cone , in order to permit the first discharge , the dispenser gun shoots periodically liquid drops if the electric field is still active . the period of discharge has been of 50 ms . the cycle is repeated few times during the cooling . as shown in fig2 ( b ), which has been obtained utilising a co 2 laser ( but a similar result can be obtained other laser or other electromagnetic radiation sources ) it is possible to proceed to the complete extraction of the liquid from the drop reservoir . this fact could be due to the improved mechanism of heating efficiency . after the irradiation of five impulses of 10 w , each one of 100 ms duration , it has been observed that the drops reservoir dispense until 55 discharges . the initial volume was of 180 nl while the total final volume transferred to the photogram in fig2 ( b ) was 164 nl and the remaining drop was 16 nl . the discharge period has been of 200 ms while the volume transferred in each discharge has been estimated to be 3 nl . a physical global image of the mechanism of cone formation , jet emission and interruption , that happen during the continuous emission from the ehd tip from a liquid film of finite conductivity , has been provided only recently by the basaran group [ 17 ] ( obtained through the use of high electrodes and tensions as previously described ), even if the process is well known and utilized since decennia . the simulations and the experiments there described are referring to an axial symmetric case . obviously , in the case of the present invention , the situation is more complicated since experimentations with more complicated configurations are described . nevertheless , from the conducted trials , the inventors have understood that by modifying some parameters such as the distance , the fluid , the volume and the heating , it will be possible to regulate the system performances . the flexibility of the approach according to the invention is demonstrated by the following experiments in which different functionalities have been tested . the movement of the heated tip or of the laser permits , for example , to change the emission direction of a drop in a large solid angle as shown in fig3 ( a ). in fact , by changing the heated tip position , it is possible to rapidly change ( about 2 s ) the electric field distribution . the region with the highest electric field follows the tip movement . angles of discharge out of axis reach in this case values until about 20 °. this may give the possibility to dispense liquids in an area of 23 mm 2 even if the reservoir drop maintains a fixed position . more large angles imply movement of the reservoir drop , as described in the following . in an additional different embodiment , the “ dispenser gun ” is moved at the same time as it is discharged in a continuous manner allowing the liquid patterning . in this case it is important to select conditions in which dragging the drop in different positions is possible , as described in the following . in both cases the “ dispenser gun ” can be easily moved , by simply moving the heated tip . nevertheless , in the first case the sessile drop starts to move only at a critical angle . indeed , the asymmetric deformation experimented by the drop , under the non - axial electric force , generates an unbalance of the solid - liquid interfaces tensions with a net resulting force ( see fig3 ( b )) that pushes forward the drop . this unbalance causes the drop displacement similarly to the thermocapillarity where the thermal gradient causes the loss of balance of the solid - liquid tensions . in fig3 ( b ), how the drop movement permits the pattering is shown , by the drops distribution from the reservoir drop in three different position along a single scanning line . in case of liquid film , the “ dispenser gun ” can be simply removed since no solid - liquid interfaces tension is in opposition with the emission cone movement . a sequence of images ( fig3 ( c )) shows the lateral movements ( x - axis ) until 1 . 6 mm without distribution axis interruption , and 1 mm along the y axis ( the shifting along the y axis is observable since the liquid cone is better brought in focus in the final frame ( base ) with respect to the out - of - focus third position that is observed in the first ( upper ) frame ). a more enchanting function of the present invention is the harmonic combination of the distribution function synchronised with the drops transport , at the same time as they are continuously formed , function that is shown in the sequence of fig4 ( a ). the sequence of images in fig4 ( a ) clearly shows the formation and the synchronised transport , on the destination substrate , of three different drops in straight line on the right side . these drops can be easily collected and managed in a microfluidic system . this function may be successfully improved by selecting in an appropriate manner the position of the thermal stimuli ( heated tip , in this case ). the beauty of the physical phenomenon is that the process seems to be auto - organised as a function of two different physical effects : ehd and thermocapillarity , but activated by an only single external stimulus . the lateral shift is activated by thermocapillarity , that pushes the drops towards colder regions ( in our case left and right sides ) ( see fig4 ( a )). the reason why in the shown experiment the x direction is preferred depends on the geometric design of the cell . the upper substrate ln is in contact with the glass at the + x and − x ends ( therefore the heating exchange with the glass is larger than in air ). the contact with the glass permits a heat exchange that is higher along this direction than in the y direction . nevertheless , in principle + x and − x sides are equivalent but if the heated tip source is not symmetrically positioned , in axis , with respect to the bottom of the reservoir drop , the liquid discharges have a moment with horizontal components that dynamically pushes the liquid preferably along the axis of the device ( right side ). the angle , in this case , is clearly visible from the image and it is of about 11 ° with respect to the normal at the substrate . it is important to note that by using a laser it is easy to direct the thermal stimulus in different points , in other words in correspondence of different sessile reservoir drops . in addition , with the laser it is possible to better proportion the heating energy by modulating the beam power . besides , through a simply lenticular focalisation , it is possible to narrow better the area on which the thermal stimulus is applied . as shown in fig4 ( b ), the laser beam has been directed at three different reservoir drops ( see fig4 ( c )) stored on a glassy substrate . the drops have been sequentially activated . an elevated distribution of volume is possible by separating the laser beam so as to obtain a parallel emission from many “ dispenser guns ” ( thus accelerating the distribution process ). finally , in order to demonstrate the picoliter drops distribution in specific points , an additional experiment has been carried out wherein a sample of functionalized ln has been utilized in which periodically polarised structures have been micro - engineered . the structure has got hexagons displaced in a square matrix ( see method section ). this regular structure permits to form some cones starting from the liquid that will spread itself exactly over and in correspondence of the hexagons . a liquid pdms layer has been spread over the glassy substrate . by using thermal stimuli ( in a flexible manner through the use of a laser or large heating ) the formation of three tank drops has been demonstrated in correspondence of the hexagons having a lateral separation of 200 micrometers . since pdms is more viscous , liquid filament and unstable liquid bridges have been formed [ 30 ], as clearly visible in fig5 ( a ). in fact , the separation distance d = i50 micrometer was again over the critical distance given by equation i . after the bridges collapsed , 400 pl of pending drops have been clearly distributed in the exact position corresponding to the geometry of the ppln sample ( see fig5 ( a )). the system according to the invention can be utilized in different configuration so as to permit the drops distribution and their patterning . in the basic configurations a single or multiple “ dispenser gun ” can be obtained from one or more thank drops , preliminary obtained by processes of wettability modelling ( spatial modulation of poling or thermal stimulus ) on the lower substrate . nevertheless , additional configurations can be provided , in which the substrate to be modelled is not the same dielectric polar functionalized crystal . in fact , the substrate on which the liquid is laid down could be a dielectric plate ( so as to avoid field perturbations ) or film 160 inserted between the glass 110 and ln 120 , in order to intercept the liquid drops as shown in fig5 ( b ). in this case , a geometric design may be obtained as formed by the separated drops even with the substrate shift , as shown in figure . in principle , every geometrical figure can be realized . besides , a system limit is the immediate interruption of the liquid emission is not possible , since the electric field slowly extinguishes with the sample cooling . an additional configuration may be provided in order to overcome this disadvantage . the glassy plate 110 may house through micro - pores 190 and micro - tubes 180 to dose the distribution liquid in the reservoir drops . besides , the liquid may be sucked from aspiration ducts if one wishes the immediate interruption of the distribution action . the actual level of technology will allow , by means of computer and control electronics , the total implementation of this integrated configuration of the method according to the invention for distributing and modelling a liquid material . again , another form of realisation provides spreading the liquid on ln and heating it thereon in order to realise the distribution on the plate . further , a different form of realisation provides that the two starting and destination surfaces are both pyroelectric surfaces , so as to create a reciprocal distribution of the liquid between the two surfaces . this may be useful in mixing processes . the z - cut ln crystals , commercially available , show uniform polarisation . the spontaneous polarisation of ln crystals can be reversed by a polarisation process with electric field ( efp ) [ 25 ], thus allowing the fabrication of periodically polarised ln crystals ( ppln ). an external tension that goes over the coercive field of the material ( about 21 kw / 1 mm ) has been necessary to invert the ferroelectric domains and the inversion selectivity is usually assured by an appropriate model of resist generated by photo - lithography . in the upper image of fig5 ( a ), the image obtained at the optical microscope of two ppln samples produced by the same inventors by “ electric field poling ” ( efp ) is shown . the samples consist in a square matrix of bulk reversed domains with a period of about 200 micrometers along both the x and y directions . the pyroelectric coefficient sign is reversed according to the inverted ferroelectric domains [ 29 ]. in the experiments previously described , a co 2 cw laser having an emission power of 10 w at a wave length of λ = 10 . 6 micrometer has been utilized . the power modulation in the range of 0 - 100 % has been possible thanks to an external tension of 0 - 5 v . the laser beam diameter has been of 4 mm . the beam can be focused through suitable lens so as to obtain the desired beam dimension in the diffraction range of about half wave length ( 5 micrometer ). the laser has been assembled on a x - y translational stage . the sphere head was about 25 cm apart from the ln sample . a simple tip has been utilized for heated welding / soldering . the diameter was of 1 mm . the maximum operative temperature was 250 ° c . the temperature has been measured by a temperature probe ( thermocouple ) in order to have a preliminary indication of the maximum temperature reached on both ln crystal faces . as lower glassy plate , a microscopy plate of 1 mm thickness has been used . different modalities have been utilized for preparing liquid film and reservoir drops , as described in the following : 1 ) almond oil as liquid film on glass : some microliter drops have been spread on glassy substrate ( experiments as shown in fig1 ( e ), 2 ( a ), 3 ( c )); 2 ) or alternatively almond oil as sessile drop on a clean plate : in order to have a sessile drop with contact angle of about 40 ° ( experiments as shown in fig4 ( a )); 3 ) almond oil as sessile drop on a pdms covered plate : in order to have a sessile drop with relatively high contact angle , a glassy substrate has been covered with a pdms ( sylgard ) layer spread with a covering at 5000 rpm for 60 s ; the pdms link process has been obtained through a heated plate at constant temperature of 100 ° c . for 10 min . afterwards , one or more drops of different volume ( depending on the specific experiments ) have been spread on a surface . the typical contact angle has been measured to be 30 ° ( experiments as shown in fig2 ( b ), 3 ( a ), 3 ( b ), 4 ( b )); 4 ) pdms on glass : few microliter drops of pdms polymer have been deposited and spun on glass at 1000 rpm for 5 s ; this preparation has been adopted in the experiments illustrated respectively in fig5 ( a ) and 1 ( f ). the measurement equipment is composed by a white lamp , a neutral density filter and a lens of images formation for visualising in a high rate cmos camera the process of taylor cone deformation and the drops ejection . the cmos camera , capturing 125 frame each second at the 1280 × 1024 resolution with pixel area of 12 × 12 micrometer , is utilized with two different lens for comprehending the fundamental dynamic of this process . when the thermal stimulus is applied an infrared laser source , the lens focus of image formation is 100 mm and the magnification is m = 1 . 4 , while , by utilising a heated tip source , in contact with the ln crystal , f = 25 . 4 mm and m = 2 . 2 . the method according to the present invention is applied in many industrial , scientific and technological fields ; the nano - drops production is , in fact , of principal interest , for example , in all the sectors that treat ink - jet printing , distribution or deposition of organic , inorganic or biological inks , micro - fluids , electrospray , drug delivery , and combinatory chemistry . this method not only resolve some existing problems in continuously developing fields , such as the ink - jet printing one , but provides at the same time the possibility to explore completely different sectors such as the biological ones . the creation of protein microstructures is for example a field in which , still today , a lot of difficulties are present due to the use of highly sophisticated robotic equipments , these problems can be overcome by utilising the described method that introduces a new concept of deposition , eliminating completely the contact with the sample . among the advantages of the invention method , with respect to the other traditional deposition techniques , there is , indeed , the possibility to deposit solution or liquid suspension drops of micrometric dimension without contact with the substrate . in this manner , it is possible to obtain , with a micrometric resolution , surfaces having a defined geometry . the particularity of the method , according to the invention , is the great flexibility that characterises the configuration lacking in electrodes in which the electric forces are generated by the heating induced by the utilized source directly on the interested substrate . the lack of fixed electrodes , high tension generators and complex circuits , allow to easily modify the applied forces on the basis of the different requirements . 1 ) todd m . squires and stephen r . quake microfluidics : fluid physics at the nanoliter scale . rev . mod . phys . 77 , 977 - 1026 ( 2005 ) 2 ) b . s . gallardo , v . k . gupta , f . d . eagerton , l . i . jong , v . s . craig , r . r . shah , n . l . abbott , electrochemical principles for active control of liquids on submillimeter scales . sci . 283 , 57 - 60 ( 1999 ). 3 ) d . e . kataoka , s . m . troian , patterning liquid 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