Directing Motion of Droplets Using Differential Wetting

Abstract

Apparatus for controlling motion of liquid droplets. A set of electrode pads is arranged to define one or more tracks over which liquid droplets may be induced to move over a sequence of the electrode pads. A surface over the electrode pads is dielectric, smooth, and slippery to the droplets. In some cases, the smooth surface is formed as a thin layer of a second liquid that is immiscible with the liquid of the droplets. The surface has wetting affinity to the liquid that can be individually varied in a controlled manner by application of voltage to respective electrode pads. A control is designed to alter the wetting characteristic of varying-wettability portions of the surface over respective electrode pads to effect induced motion of the droplets over the surface. The apparatus is designed with the smooth hydrophobic surface open, with no overlying or facing electrode or plate above the droplets.

The invention claimed is: 
     
         1 . Apparatus for controlling motion of liquid droplets, comprising:
 a set of electrode pads arranged in an array or in paths defining one or more tracks over which liquid droplets may be induced to move over a sequence of the electrode pads;   a surface over the electrode pads being dielectric, smooth to within 1 μm, the smooth surface being formed as a thin layer of a second liquid that is immiscible with the liquid of the droplets, an upper surface of the second liquid forming a liquid-liquid surface that is hydrophobic, having a slide angle for a 5 μl droplet of the liquid of no more than 5 degrees, and having portions whose wetting affinity to the liquid that can be individually varied in a controlled manner by application of voltage to respective electrode pads, the varying-wettability portions being arranged in an array or in paths defining one or more tracks over which liquid droplets may be induced to move over a sequence of the varying-wettability portions, the second liquid laid on a surface of an underlying solid substrate that is textured to hold the second liquid stable against gravity;   a control designed to alter the wetting characteristic of varying-wettability portions of the surface over respective electrode pads to effect induced motion of the droplets over the tracks, the wetting characteristic to be altered by controlling charging and discharging of the electrode pads in a desired sequence;   the apparatus being designed with the smooth hydrophobic surface open, with no overlying or facing electrode or plate above the droplets.   
     
     
         2 . Apparatus for controlling motion of liquid droplets, comprising:
 a set of electrode pads arranged in an array or in paths defining one or more tracks over which liquid droplets may be induced to move over a sequence of the electrode pads;   a surface over the electrode pads being dielectric, smooth to within 2 μm, having a slide angle for a 5 μl droplet of the liquid of no more than 5 degrees, and having a wetting affinity to the liquid that can be altered by application of voltage to the electrode pads;   a control designed to alter the wetting characteristic of portions of the surface over respective electrode pads to effect induced motion of the droplets over the tracks, the wetting characteristic to be altered by controlling charging and discharging of the electrode pads in a desired sequence.   
     
     
         3 . The apparatus of  claim 2 :
 the apparatus being designed with the smooth surface is open, with no overlying or facing electrode or plate.   
     
     
         4 . The apparatus of  claim 2 , further comprising:
 a second plate overlying or facing the surface over the electrode pads.   
     
     
         5 . The apparatus of  claim 2 :
 the surface over the electrode pads is textured to hold a thin layer of a second liquid that is immiscible with the liquid of the droplets, an upper surface of the second liquid forming a liquid-liquid surface that is slippery with respect to the liquid droplets.   
     
     
         6 . The apparatus of  claim 2 , wherein:
 the electrodes are printed on a substrate using printed circuit board technology.   
     
     
         7 . The apparatus of  claim 2 , wherein:
 the electrodes are manufactured using thin-film transistor (TFT), active matrix, or passive matrix backplane technology.   
     
     
         8 . The apparatus of  claim 2 , wherein:
 the surface is smoothed to within 1 μm by polishing.   
     
     
         9 . The apparatus of  claim 2 , wherein:
 the surface is smoothed to within 1 μm by applying a coating, the coating applied by at least one of spin coating, spray coating, dip coating, or vapor deposition.   
     
     
         10 . The apparatus of  claim 2 , wherein:
 the surface coating is of a material that is both dielectric and hydrophobic.   
     
     
         11 . The apparatus of  claim 2 , wherein:
 the surface is smoothed to within 1 μm by application of a sheet of a polymer stretched to remove wrinkles.   
     
     
         12 . The apparatus of  claim 2 , wherein:
 the slide angle is imparted to the surface by patterning or texturing to induce hydrophobicity.   
     
     
         13 . The apparatus of  claim 2 , wherein:
 the slide angle of a 5 μl droplet is no more than 3°.   
     
     
         14 . The apparatus of  claim 2 , wherein:
 the slide angle of a 5 μl droplet is no more than 1°.   
     
     
         15 . A method, comprising the steps of:
 introducing a liquid droplet onto a surface over a set of electrode pads arranged in an array or in paths defining one or more tracks over which the liquid droplet may be induced to move over a sequence of the electrode pads;   the surface being dielectric, smooth to within 2 μm, having a slide angle for a 50 droplet of the liquid of no more than 5 degrees, and having a wetting affinity to the liquid that can be altered by application of voltage to the electrode pads;   controlling the wetting characteristic of portions of the surface over respective electrode pads to effect induced motion of the droplet over the tracks, the wetting characteristic to be altered by controlling charging and discharging of the electrode pads in a desired sequence.   
     
     
         16 . The method of  claim 15 :
 the apparatus being designed with the smooth surface is open, with no overlying or facing electrode or plate.   
     
     
         17 . The method of  claim 15 :
 the surface over the electrode pads being textured to hold a thin layer of a second liquid that is immiscible with the liquid of the droplets, an upper surface of the second liquid forming a liquid-liquid surface that is slippery with respect to the liquid droplets.   
     
     
         18 . The method of  claim 15 , wherein:
 the electrodes are printed on a substrate using printed circuit board technology.   
     
     
         19 . The method of  claim 15 , wherein:
 the surface is smoothed to within 1 μm by polishing.   
     
     
         20 . Apparatus for controlling motion of liquid droplets, comprising:
 a smooth, hydrophobic surface, having portions with a wetting affinity to the liquid that can be varied in a controlled manner to induce droplets of the liquid to move over the surface;   a control designed to vary the wetting characteristic of portions of the surface to effect induced motion of the droplets over the tracks;   the apparatus being designed with the smooth hydrophobic surface open, with no overlying or facing electrode or plate above the droplets.   
     
     
         21 . The apparatus of  claim 20 , further comprising:
 a set of electrode pads arranged in an array or in paths defining one or more tracks over which liquid droplets may be induced to move over a sequence of the electrode pads, the varying-wettability portions being a dielectric surface over the electrode pads.   
     
     
         22 . The apparatus of  claim 21 , wherein:
 the electrodes are printed on a substrate using printed circuit board technology.   
     
     
         23 . The apparatus of  claim 21 , wherein:
 the electrodes are manufactured on a substrate using thin film transistor technology.   
     
     
         24 . The apparatus of  claim 21 , wherein:
 the electrodes are manufactured on a substrate using active matrix or passive matrix backplane technology.   
     
     
         25 . The apparatus of  claim 20 :
 the smooth, hydrophobic surface is smooth to within 1 μm, and has a slide angle for a 5 μl droplet of the liquid of no more than 5 degrees.   
     
     
         26 . The apparatus of  claim 20 :
 the smooth, hydrophobic surface is formed as a textured surface of a solid, designed to hold a thin layer of a second liquid that is immiscible with the liquid of the droplets, an upper surface of the second liquid forming a liquid-liquid surface that is slippery with respect to the liquid droplets.   
     
     
         27 . The apparatus of  claim 20 , wherein:
 the wettability of the varying-wettability portions of the surface is varied via application of light.   
     
     
         28 . The apparatus of  claim 27 , wherein:
 the varying-wettability portions of the surface operate by optoelectrowetting.   
     
     
         29 . The apparatus of  claim 27 , wherein:
 the varying-wettability portions of the surface operate by photoelectrowetting.   
     
     
         30 . The apparatus of  claim 20 , further comprising:
 the apparatus includes stations for one or more of the group consisting of dispensing, mixing, heating, cooling, application of magnetic field, application of electric field, addition of reagent, optical inspection or assay, and isolation or purification of proteins, peptides, or biopolymers.   
     
     
         31 . The apparatus of  claim 30 , further comprising:
 the apparatus includes stations for two or more of the group consisting of dispensing, mixing, heating, cooling, application of magnetic field, application of electric field, addition of reagent, optical inspection or assay, and isolation or purification of proteins, peptides, or biopolymers.   
     
     
         32 . The apparatus of  claim 20 , further comprising:
 an acoustic transducer configured to introduce to introduce liquid droplets into the apparatus.   
     
     
         33 . The apparatus of  claim 20 , further comprising:
 a microdiaphragm pump configured to introduce to introduce liquid droplets into the apparatus.   
     
     
         34 . The apparatus of  claim 20 , further comprising:
 one or more inkjet nozzles configured to introduce liquid droplets into the apparatus.   
     
     
         35 . The apparatus of claim  201 , further comprising:
 one or more syringe pumps configured to introduce liquid droplets into the apparatus.   
     
     
         36 . The apparatus of claim  201 , further comprising:
 one or more capillary tubes configured to introduce liquid droplets into the apparatus.   
     
     
         37 . The apparatus of claim  201 , further comprising:
 one or more pipette devices configured to introduce liquid droplets into the apparatus.   
     
     
         38 . The apparatus of  claim 20 , further comprising:
 the smooth, hydrophobic surface has one or more holes designed to introduce liquid droplets or reactants.   
     
     
         39 . A method, comprising the step of:
 introducing a liquid droplet onto a smooth, hydrophobic surface, having portions with a wetting affinity to the liquid that can be varied in a controlled manner, the varying-wettability portions being arranged in an array or in paths defining one or more tracks over which liquid droplets may be induced to move over a sequence of the varying-wettability portions;   varying the wetting characteristic of varying-wettability portions of the surface to effect induced motion of the droplets over the tracks;   the apparatus being designed with the smooth hydrophobic surface open, with no overlying or facing electrode or plate above the droplets.   
     
     
         40 . The method of  claim 39 :
 the varying-wetability portions overlay a set of electrode pads arranged in an array or in paths defining one or more tracks over which liquid droplets may be induced to move over a sequence of the electrode pads, the varying-wetability portions being a dielectric surface over the electrode pads.   
     
     
         41 . The method of  claim 39 :
 the smooth, hydrophobic surface being smooth to within 1 μm, and having a slide angle for a 5 μl droplet of the liquid of no more than 3 degrees.   
     
     
         42 . The method of  claim 39 :
 the smooth, hydrophobic surface being formed as a textured surface of a solid, designed to hold a thin layer of a second liquid that is immiscible with the liquid of the droplets, an upper surface of the second liquid forming a liquid-liquid surface that is slippery with respect to the liquid droplets.   
     
     
         43 . Apparatus for controlling motion of liquid droplets, comprising:
 a solid surface that is textured to hold a thin layer of a second liquid that is immiscible with the liquid of the droplets, an upper surface of the second liquid forming a liquid-liquid surface that is slippery with respect to the liquid droplets, having a slide angle for a 5 μl droplet of the droplet liquid of no more than 5 degrees, and having a wetting affinity to the droplet liquid that can be varied under control, the varying-wettability portions being arranged in an array or in paths defining one or more tracks over which the liquid droplets may be induced to move over a sequence of the varying-wettability portions; and   a control designed to vary the wetting characteristic of varying-wettability portions of the liquid-liquid surface to effect induced motion of the droplets over the tracks.   
     
     
         44 . The apparatus of  claim 43 :
 the varying-wetability portions being a surface arranged over a set of electrode pads arranged in an array or in paths defining one or more tracks over which liquid droplets may be induced to move over a sequence of the electrode pads, the wetting characteristic to be altered by controlling charging and discharging of the electrode pads in a desired sequence.   
     
     
         45 . The apparatus of  claim 43 :
 the apparatus being designed with the smooth hydrophobic surface open, with no overlying or facing electrode or plate above the droplets.   
     
     
         46 . The apparatus of  claim 43 , wherein:
 the second liquid is an oil that has wetting affinity for the solid and is immiscible with the liquid of the droplets.   
     
     
         47 . A method, comprising the step of:
 introducing a liquid droplet onto a liquid-liquid interface, being a liquid surface of a second liquid that is immiscible with the liquid of the droplets, formed on a solid surface that is textured to hold a thin layer of the second liquid, an upper surface of the second liquid forming a liquid-liquid surface that is slippery with respect to the liquid droplets, having a slide angle for a 5 μl droplet of the droplet liquid of no more than 5 degrees, and having a wetting affinity to the droplet liquid that can be varied under control, the varying-wettability portions being arranged in an array or in paths defining one or more tracks over which the liquid droplets may be induced to move over a sequence of the varying-wettability portions; and   varying the wetting characteristic of varying-wettability portions of the liquid-liquid surface to effect induced motion of the droplets over the tracks.   
     
     
         48 . The method of  claim 47 :
 the varying-wetability portions being a surface arranged over a set of electrode pads arranged in an array or in paths defining one or more tracks over which liquid droplets may be induced to move over a sequence of the electrode pads, the wetting characteristic to be altered by controlling charging and discharging of the electrode pads in a desired sequence.   
     
     
         49 . The method of  claim 47 :
 the apparatus being designed with the smooth hydrophobic surface open, with no overlying or facing electrode or plate above the droplets.