Abstract:
A method of impacting liquid droplets onto a surface includes providing a series of liquid droplets, and directing the liquid droplets at a non-planar target surface to cause a shockwave in the droplets upon impact. An apparatus for impacting liquid droplets onto a surface includes a mechanism to produce a series of liquid droplets, and a mechanism to direct the liquid droplets at a non-planar target surface to cause a shockwave in the droplets upon impact. The non-planar target surface is shaped to intensify the shockwave in the droplets.

Description:
TECHNICAL FIELD 
     This invention relates to methods and apparatus for generating impacts between high velocity liquid droplets and a target in order to produce very high localised energies. 
     BACKGROUND 
     The development of fusion power has been an area of massive investment of time and money for many years. This investment has been largely centred on developing a large scale fusion reactor, at great cost. However, there are other theories that predict much simpler and cheaper mechanisms for creating fusion. Of interest here is the umbrella concept “inertial confinement fusion”, which uses mechanical forces (such as shock waves) to concentrate and focus energy into very small areas. 
     Much of the belief in inertial confinement fusion comes from observations of a phenomenon called sonoluminescence. This occurs when a liquid containing appropriately sized bubbles is driven with a particular frequency of ultrasound. The pressure wave causes the bubble to expand and then collapse very violently; a process usually referred to as inertial cavitation. The rapid collapse of the bubble leads to non-equilibrium compression that causes the contents to heat up to an extent that they emit light [Gaitan, D. F., Crum, L. A., Church, C. C., and Roy, R. A. Journal of the Acoustical Society of America 91(6), 3166-3183 June (1992]. There have been various efforts to intensify this process and one group has claimed to observe fusion [Taleyarkhan, R. P., West, C. D., Cho, J. S., Lahey, R. T., Nigmatulin, R. I., and Block, R. C.  Science  295(5561), 1868-1873 March (2002)]. However, the observed results have not yet been validated or replicated, in spite substantial effort [Shapira, D. and Saltmarsh, M.  Physical Review Letters  89(10), 104302 September (2002)]. This is not the only proposed mechanism that has led to luminescence from a collapsing bubble; however it is the most documented. Luminescence has also been observed from a bubble collapsed by a strong shock wave [Bourne, N. K. and Field, J. E.  Philosophical Transactions of the Royal Society of London Series A - Mathematical Physical and Engineering Sciences  357(1751), 295-311 February (1999)]. It is this second method that this invention relates to. 
     It has been proposed in U.S. Pat. No. 7,445,319 to fire spherical drops of water moving at very high speed (˜1 km/s) into a rigid target to generate an intense shock wave. This shock wave can be used to collapse bubbles that have been nucleated and subsequently have expanded inside the droplet. It is inside the collapsed bubble that the above-mentioned patent expects fusion to take place. The mechanism of shockwave generation by high-speed droplet impact on a surface has been studied experimentally and numerically before and is well-documented (including work by one of the present patent inventors, [Haller, K. K., Ventikos, Y., Poulikakos, D., and Monkewitz, P.  Journal of Applied Physics  92(5), 2821-2828 September (2002)].) 
     BRIEF SUMMARY 
     The present invention aims to provide an improvement on the aforementioned techniques and may also have other applications. When viewed from a first aspect the invention provides a method of impacting liquid droplets onto a surface comprising providing a series of liquid droplets, directing said liquid droplets at a non-planar target surface to cause a shockwave in the droplets upon impact, the non-planar target surface being shaped to intensify the shockwave in the droplet. 
     The invention also extends to apparatus for impacting liquid droplets onto a surface comprising means for producing a series of liquid droplets, means for directing said liquid droplets at a non-planar target surface to cause a shockwave in the droplets upon impact, wherein said non-planar target surface is shaped to intensify the shockwave in the droplet. 
     The Applicant notes that the scope of the present invention does not extend to the shockwave comprising an ultrasound shockwave. Thus, the scope of the present invention does not include the pocket of gas being collapsed through the process of sonoluminescence. 
     Thus it will be seen by those skilled in the art that in accordance with the invention, rather than a simple planar target being employed, the target is shaped in order to intensify the shockwave generated by the droplet impact. This can give much higher peak energy densities in the droplet than in the known techniques: for example modelling of a particular embodiment of the shaped target of the present invention can give a peak pressure inside the droplet approximately double that obtained with an otherwise identical droplet impacting a planar target. 
     The invention has many potential applications as will be mentioned again hereinbelow. However it is considered to be particularly applicable to droplets having cavities therein whereby impact of the droplet onto the target causes implosion of the cavities in the manner specifically described in U.S. Pat. No. 7,445,319. Accordingly, a set of embodiments comprises the steps of producing a pressurised liquid jet stream comprising a liquid and a nano-particle material, causing the break-up of said jet stream producing a plurality of droplets containing liquid and nano-particle material and irradiating the droplets with energy to produce and expand cavities within the droplets. Another set of embodiments comprises the steps of producing a pressurised liquid jet stream comprising a liquid and a volatile emulsion and causing the break-up of said jet stream, producing a plurality of droplets containing both said liquid and said volatile emulsion. The emulsion then causes bubbles to form in the droplets. 
     There are a wide variety of target surface shapes which can give rise to intensification of the shockwave in the droplets. The inventors have realised that an important criterion is that the surface is shaped such that the boundary of the contact patch between the droplet and the surface travels at a supersonic speed significantly beyond the initial moment of collision, e.g. for a 100 micron droplet moving at 1000 meters per second, the contact line remains supersonic for at least 20 nanoseconds after impact. The inventors have observed that in the case of a planar target surface, when the droplet initially impacts the target, the edge of the contact patch has a high instantaneous speed. It has also been observed that there is a very strong shock wave formed and that whilst the motion of the contact line between the surface of the droplet and the target remains supersonic in relation to the liquid&#39;s speed of sound, the shock remains pinned to the wall. During this time where the shock is pinned to the wall, it is being intensified. When the contact line speed falls below the speed of sound in the liquid, the shock begins to move outwards into the droplet to eventually strike the bubble nucleated within the droplet&#39;s interior. 
     By making the shape of the target surface conform to the shape of the droplet in accordance with the invention, the speed of propagation of the edge of the contact patch is maintained at a supersonic level for a longer time, leading to a stronger shockwave. 
     In a preferred set of embodiments, the target surface is concave which has the effect of focussing the energy and intensifying the initial formation of the shockwave. In some non-limiting examples, the surface could have an ellipsoid or paraboloid shape. The surface need not be continuously curved. For example, in one set of embodiments the target surface comprises a plurality of discrete portions, e.g. with a gradient discontinuity between them. The portions could themselves be partial ellipses, parabolas, and so on, but equally could be straight. A particular set of embodiments of surfaces made from discrete portions could be described as piecewise polynomial. 
     The target surface in accordance with the invention could be shaped such that there is a single contact patch generating a single shockwave (e.g. as would be the case with a shallow elliptic or parabolic shape). However, in a set of potentially advantageous embodiments, the surface is shaped such that there are a plurality of discrete initial contact points between the droplet and surface and therefore a plurality of discrete contact patches and shockwaves propagating into the droplet due to the interactions of the shock waves advancing into the droplet at a later time than the supersonic contact line regime. This can give rise to an even more intense concentration of energy inside the droplet. In a preferred set of embodiments in which the energy focussed into the droplet causes implosion of a cavity, this can increase the success of generating inertial confinement fusion. 
     The above-mentioned multiple contact points could arise from the discrete surface portions referred to above, with some or all of the portions corresponding to respective contact points. Additionally or alternatively the surface could comprise a plurality of protrusions to act as the multiple contact points. 
     The target may comprise a solid e.g. a metal, e.g. steel. Indeed any of the target materials disclosed in U.S. Pat. No. 7,445,319 would be suitable. However, it is not essential to use a solid target and it is envisaged that a liquid target could be employed. The required target surface shape could be achieved in a number of ways. For example, the surface of a volume of liquid could be excited with a suitable vibration (e.g. using ultrasound or another method) to generate a wave having the desired shape. Alternatively the desired shape could be achieved through the contact angle between a liquid and a solid surface with appropriately matched wetting properties. Of course, this latter example shows that the surface could comprise a combination of solid and liquid. 
     In a preferred set of embodiments, the methods described herein are employed to generate nuclear fusion reactions. The fuel for the reaction could be provided by the droplet, the vapour or gas inside the cavity within the droplet, the nano-particle, or the fuel could be provided by the target itself. Any of the fuels mentioned in U.S. Pat. No. 7,445,319 is suitable for use in the present invention. 
     The fusion reactions which can be obtained in accordance with certain embodiments of the invention could be used for net energy production (the long term research aim in this field), but the inventors have appreciated that even if the efficiency of the fusion is below that required for net energy production, the reliable fusion which is obtainable in accordance with embodiments of the invention is advantageous for example in the production of tritium which can be used as fuel in other fusion projects and is very expensive to produce with currently available technologies. The fusion can also be beneficial in giving a fast and safe neutron source which has many possible applications that will be apparent to those skilled in the art. 
     Moreover, it is not essential in accordance with the invention to produce fusion at all. 
     Certain embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating the impact of a droplet onto a planar target, shown for reference purposes only; 
         FIG. 2  is a close-up schematic diagram of an enhanced target in accordance with the invention; 
         FIG. 3  is a diagram similar to  FIG. 1  showing the progress of the shock when a curved target is employed; 
         FIG. 4  is a close-up schematic diagram of an alternative target shape in accordance with the invention; 
         FIG. 5  is a schematic diagram of another embodiment of the invention illustrating multiple shockwaves; 
         FIGS. 6 a  to 6 e    are various possible alternative shapes of the target surface in accordance with the invention; 
         FIG. 7  shows another possible embodiment of the target surface; and 
         FIGS. 8 a  and 8 b    show yet further alternative embodiments of the surface of the target which have protrusions. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows the basic mechanism behind the generation of focussed energy by impact of a droplet A onto a planar target B. This is described in greater detail in U.S. Pat. No. 7,445,319 and [Haller, K. K., Ventikos, Y., Poulikakos, D., and Monkewitz, P.  Journal of Applied Physics  92(5), 2821-2828 September (2002)]. In the left-hand part of  FIG. 1 , the droplet A is shown immediately prior to impact. In the central part of the diagram, it is shown that as the droplet A strikes the target B, a strong shockwave C is formed which begins to propagate back inside the droplet as indicated by the arrow D. During the initial moments after impact, the edge E of the contact patch between the droplet A and the target B moves outwardly faster than the route of propagation of the shockwave D, in other words the contact line moves at supersonic speed. At this point, the ends of the shockwave C remain in contact with the target B. However, the speed of the contact line soon falls and the shockwave C begins to propagate into the droplet as shown by the arrow D. For the case of a 100 micrometer droplet travelling at 500 meters per second and striking a flat target surface, this phase is completed after approximately 6 nanoseconds. As the shockwave propagates into the droplet, it causes a rapid implosion and therefore rapid heating of the cavity in the bubble (omitted from  FIG. 1  for clarity). This causes intense local heating which can be sufficient to generate a nuclear fusion reaction inside the droplet. 
       FIG. 2  shows, in close-up view, a target surface in accordance with a first embodiment of the present invention. In this embodiment, the target  2  has a curved, concave shape which forms part of an ellipse. As may be appreciated from  FIG. 2  and seen more clearly in  FIG. 3 , the elliptical shape of the target surface  2  conforms more closely to the shape of the droplet  4  which strikes it than a planar target does. It may be seen from this schematic diagram that the major axis of the ellipse is approximately twice the diameter of the droplet  4 . 
       FIG. 3  shows the stages of droplet impact and generation of the shockwave as for  FIG. 1  in the case of this curved target. As will be seen in this case, the surface  2  conforms better to the shape of the impacted droplet  4  which causes the edge of the contact patch  6  to travel faster than the front of the shockwave  8  for a longer period of time. For the case of a 100 micrometer droplet travelling at 500 meters per second and striking a surface shaped, in accordance with some embodiments of this invention, to resemble a sine wave with a period of 300 micrometers, the supersonic contact line regime is completed after approximately 12 nanoseconds 
     When the shockwave  8  propagates into the droplet as shown by the arrow  10 , the resulting pressure behind it is increased due to the intensification of the initial formation of the shockwave  8  by focusing the energy using the shaped surface. The resulting shape of the shockwave  8  as it propagates into the droplet is concave rather than convex (as in the flat target surface case) and this acts to further enhance the pressure behind the shockwave  8  as it strikes the cavity when compared to the flat target surface case. The increased pressure arriving at the cavity (again omitted for clarity) increases the temperature achieved during implosion and the energy available to induce fusion. The fact that the shockwave conforms better to the target bubble&#39;s surface is also beneficial. 
     A further embodiment of the invention is shown in  FIG. 4 . This is similar to the embodiment shown in  FIG. 2  except that the target surface  12  in this example is parabolic rather than elliptical. This embodiment gives a slightly greater degree of conformity than the elliptical shape shown in  FIG. 2 . 
       FIG. 5  shows an example of an embodiment in which the target surface is made up of three discrete surface portions  14   a  to  14   c . The individual surface portions  14   a  to  14   c  are flat rather than curved in this particular embodiment, although clearly the target surface as a whole is concave. As the diagram illustrates, when the droplet  2  impacts against the target surface  14 , there are three separate contact points which give rise to three corresponding shockwaves  16   a  to  16   c . The three shockwaves converge and overlap at a point  18  inside the droplet, reinforcing one another to give an even higher localised pressure and energy density. By optimisation of the surface design, the overlap point can be made to coincide with the cavity, thereby giving more violent implosion and higher peak temperatures inside the bubble. 
       FIGS. 6 a  to 6 e    show various possible alternative surface shapes which give rise to multiple contact points and thereby give the advantages described above. 
       FIG. 7  shows a yet further possible embodiment in which the target surface  20  comprises two disjointed surface portions  20   a ,  20   b  each of which is curved e.g. as an ellipse or parabola. This can be seen to combine the advantages of a curved surface, described with reference to  FIGS. 2 to 4 , with the advantages of multiple contact point, described with reference to  FIG. 5 . 
       FIGS. 8 a  and 8 b    show respectively two possible embodiments which illustrate another principle for combining the advantages of a curved target surface with multiple contact points and thus multiple shockwaves. In these embodiments, the target surface  22 ,  24  comprises a plurality of discrete protrusions  26 ,  28  from which shockwaves are generated. In these embodiments, the shockwaves produced by the discrete protrusions  26 ,  28  will combine with and reinforce those produced by impact with the rest of the surface  22 . In this manner the shape of the shockwave that advances into the bubble can be explicitly controlled by shaping the target accordingly. 
     In each of the embodiments described above, the diagrams shown are a vertical cross-section through a three-dimensional droplet and target surface and hence they depict embodiments that are rotationally symmetric. However, this is not essential to the invention. In particular the surface could comprise discrete surface portions in the rotational direction either instead of, or as well as in the vertical cross-section shown. In the latter case the target surface would be multi-facetted. Each facet could give rise to separate but converging shockwaves. By way of example, a non-rotationally symmetric embodiment of the ideas shown in  FIG. 8  could comprise an elliptical dish in the target surface with any arbitrary distribution of protrusions within it. 
     In all of the embodiments described, the apparatus can be used by firing a stream of very high velocity droplets e.g. of deuterated water containing nano-particles—such as gold—by producing a stream of liquid which is then broken up and irradiated by laser to create vapour cavities within the droplets. This procedure is as described in U.S. Pat. No. 7,445,319 and can be achieved using the apparatus described therein and in U.S. Pat. No. 7,380,918. In an exemplary implementation the droplets have a diameter of approximately 100 microns, travel at a speed of approximately 1 kilometer a second and are produced at a frequency of approximately 1 Megahertz. In numerical modelling of the experiment, this gives rise to a peak pressure of 7.5×10 8  Pascals which is sufficient to cause temperatures inside the collapsed bubble in excess of 150,000 Kelvin which can be sufficient for a nuclear fusion reaction of the deuterium atoms. In some non-limiting examples the resulting neutrons could be used in other processes, or could be absorbed by a neutron absorber for conversion of the kinetic energy of the neutrons to thermal energy and thus conventional thermodynamic energy generation. 
     However, there are many other ways of producing power. For example Boron-Hydrogen fusion could also be used. Boron-Hydrogen fusion results in Helium nuclei and the methods used to harness the energy from this reaction could be very different—e.g. moving charge could generate electricity directly. Moreover it is not essential for fusion to take place; the enhanced pressures and temperatures caused inside the collapsed bubble by the target shapes in accordance with the invention may be useful in other contexts to study other reactions under exotic conditions. 
     The invention is applicable in all such contexts as well as many others.