SYSTEM FOR ENHANCING STYLUS FEEDBACK
A stylus feedback-enhancing film for a touchscreen that includes an optically clear stylus interface layer and an optically clear substrate layer, the substrate layer having a second hardness lesser than the first hardness and a second thickness greater than the first thickness; wherein the substrate layer is coupled to the stylus interface layer and the touchscreen; wherein the stylus-feedback enhancing film resists lateral motion of the stylus across the touchscreen with a first frictional force when the stylus is slid across the touchscreen with no applied force and with a second frictional force, greater than the first frictional force, when the stylus is slid across the touchscreen with an applied force consistent with human writing; wherein the first frictional force is dominated by the adhesive effect and the second frictional force is dominated by the plowing effect.
1 . A stylus feedback-enhancing film for a touchscreen, the film comprising:
an optically clear stylus interface layer, comprising a first shear-thickening urethane polymer, the stylus interface layer having a first Shore hardness, a first thickness, and a first tensile strength at 50% strain; wherein the first Shore hardness is between 90 Shore A and 60 Shore D; wherein the first tensile strength is between 5-8 MPa;
an optically clear substrate layer, comprising a second shear-thickening urethane polymer, the substrate layer having a second Shore hardness lesser than the first hardness, a second thickness greater than the first thickness, and a second tensile strength at 50% strain less than the first tensile strength; wherein the second Shore hardness is between 80 Shore A and 50 Shore D; wherein the second tensile strength is between 2-4 MPa; wherein the substrate layer is coupled to the stylus interface layer on a first side of the substrate layer and coupled to the touchscreen on a second side of the substrate layer;
wherein the stylus-feedback enhancing film enhances writing feedback via application of a force resisting lateral motion of the stylus across the film; wherein the force results from elastic deformation of the film during writing using the stylus.
2 . The film of claim 1 , wherein the stylus interface layer has a first tensile set at 50% strain; wherein the substrate layer has a second tensile set at 50% strain lesser than the first tensile set; wherein the first tensile set is between 0 and 8%; wherein the second tensile set is between 0 and 5%; wherein the first and second tensile sets are measured according to ASTM D638 using a type V sample geometry.
3 . The film of claim 2 , wherein the first shear-thickening urethane polymer is a self-healing polymer.
4 . The film of claim 3 , wherein the film further comprises an oleophobic coating applied to the stylus interface layer; wherein the oleophobic coating is between 1 nm and 500 nm in thickness.
5 . A stylus feedback-enhancing film for a touchscreen, the film comprising:
an optically clear stylus interface layer, comprising a first shear-thickening polymer, the stylus interface layer having a first hardness and a first thickness;
an optically clear substrate layer, comprising a second shear-thickening polymer, the substrate layer having a second hardness lesser than the first hardness and a second thickness greater than the first thickness; wherein the substrate layer is coupled to the stylus interface layer on a first side of the substrate layer and coupled to the touchscreen on a second side of the substrate layer;
wherein the stylus-feedback enhancing film resists lateral motion of the stylus across the touchscreen with a first frictional force when the stylus is slid across the touchscreen with no applied force and with a second frictional force, greater than the first frictional force, when the stylus is slid across the touchscreen with an applied force consistent with human writing; wherein the first frictional force is dominated by the adhesive effect and the second frictional force is dominated by the plowing effect.
6 . The film of claim 5 , wherein both of the stylus interface layer and the substrate layer elastically deform in response to application by the stylus of the applied force consistent with human writing.
7 . The film of claim 6 , wherein the applied force consistent with human writing is between 0.1 Newton and 3.0 Newton.
8 . The film of claim 6 , wherein the first shear thickening polymer is a first urethane polymer and the second shear thickening polymer is a second urethane polymer.
9 . The film of claim 6 , wherein the stylus interface layer is between 1 micron and 100 microns thick; wherein the substrate layer is between 50 microns and 200 microns thick.
10 . The film of claim 6 , wherein the stylus interface layer has a first Shore hardness; wherein the substrate layer has a second Shore hardness lesser than the first Shore hardness; wherein the first Shore hardness is between 80 Shore A and 75 Shore D; wherein the second Shore hardness is between 60 Shore A and 65 Shore D.
11 . The film of claim 10 , wherein the first Shore hardness is between 90 Shore A and 60 Shore D; wherein the second Shore hardness is between 80 Shore A and 50 Shore D.
12 . The film of claim 6 , wherein the stylus interface layer has a first tensile strength at 50% strain; wherein the substrate layer has a second tensile strength at 50% strain lesser than the first tensile strength; wherein the first tensile strength is between 5 MPa and 15 MPa; wherein the second tensile strength is between 1 MPa and 10 MPa.
13 . The film of claim 12 , wherein the first tensile strength is between 5 MPa and 8 MPa; wherein the second tensile strength is between 2 MPa and 4 MPa; wherein the first and second tensile strengths are measured according to ASTM D638 using a type V sample geometry.
14 . The film of claim 6 , wherein the stylus interface layer has a first tensile set at 50% strain; wherein the substrate layer has a second tensile set at 50% strain lesser than the first tensile set; wherein the first tensile set is between 0 and 25%; wherein the second tensile set is between 0 and 20%.
15 . The film of claim 14 , wherein the first tensile set is between 0 and 8%; wherein the second tensile set is between 0 and 5%; wherein the first and second tensile sets are measured according to ASTM D638 using a type V sample geometry.
16 . The film of claim 6 ; wherein the substrate layer is a continuous layer; wherein the stylus interface layer is composed of a set of multiple discontinuous regions.
17 . The film of claim 16 , wherein each of the set of discontinuous regions has an area of 1 square millimeter or smaller; wherein each of the set of discontinuous regions is separated from a closest next discontinuous region by a distance of 10 microns.
18 . The film of claim 6 ; wherein the stylus interface layer has a first index of refraction at a visible wavelength; wherein the substrate layer has a second index of refraction at the visible wavelength; wherein the second index of refraction is within ten percent of the first index of refraction.
19 . A system for enhancing writing feedback comprising a stylus feedback-enhancing film for a capacitive touchscreen and a stylus;
the stylus feedback-enhancing film comprising:
an optically clear stylus interface layer, comprising a first shear-thickening polymer, the stylus interface layer having a first hardness and a first thickness;
an optically clear substrate layer, comprising a second shear-thickening polymer, the substrate layer having a second hardness lesser than the first hardness and a second thickness greater than the first thickness; wherein the substrate layer is coupled to the stylus interface layer on a first side of the substrate layer and coupled to the touchscreen on a second side of the substrate layer;
the stylus comprising a stylus capable of registering motion on the capacitive touchscreen and having a hard polymer stylus tip;
wherein the stylus-feedback enhancing film enhances writing feedback via application of a force resisting lateral motion of the stylus across the film; wherein the force results from elastic deformation of the film during writing using the stylus.
20 . The system of claim 19 , wherein the stylus feedback-enhancing film and the stylus are configured to mimic feedback of a ballpoint pen writing on cotton paper.
21 . The system of claim 19 , wherein the stylus feedback-enhancing film and stylus together have a dynamic scratch coefficient of friction between 0.03 and 0.40; wherein the stylus has a polyoxymethylene tip 1.5 mm in diameter; wherein the dynamic scratch coefficient of friction is measured using 300 g of applied force.
22 . The system of claim 21 , wherein the stylus feedback-enhancing film and stylus together have a dynamic scratch coefficient of friction between 0.07 and 0.15.
23 . The system of claim 19 , wherein the stylus feedback-enhancing film deforms between 10 and 200 microns in response to an application of 300 g of applied force by the stylus; wherein the stylus has a rigid tip 1.5 mm in diameter.
24 . The system of claim 23 , wherein the stylus feedback-enhancing film deforms between 25 and 75 microns in response to an application of 300 g of applied force by the stylus.